GO2-15-137, Response to the Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Seismic Reevaluations

Response to the Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Seismic Reevaluations

ML15267A780
Person / Time
Site:	Columbia
Issue date:	09/24/2015
From:	Swank D Energy Northwest
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
GO2-15-137
Download: ML15267A780 (58)

Text

David A. Swank Columbia Generating Station P.O. Box 968, PE04 Richland, WA 99352-0968 Ph. 509-377-2309 l F. 509-377-2354 daswank@energy-northwest.com September 24, 2015 GO2-15-137

Reference:

10 CFR 50.54(f)

U.S. Nuclear Regulatory Commission ATTN: Document Control Desk 11555 Rockville Pike Rockville, MD 20852

Subject:

COLUMBIA GENERATING STATION, DOCKET NO. 50-397 RESPONSE TO THE REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS

Reference:

1) Letter, GO2-15-045, dated March 12, 2015, D. A. Swank (Energy Northwest) to NRC, "Seismic Hazard And Screening Report, Response to NRC Request for Information Pursuant to 10 CFR 50.54(f) Regarding Recommendation 2.1 of the Near-Term Task Force Review of Insights From the Fukushima Dai-Ichi Accident"

2) Letter dated August 18, 2015, NRC to M. E. Reddemann (Energy Northwest) "Columbia Generating Station - Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Seismic Reevaluations"

Dear Sir or Madam:

By Reference 1, Energy Northwest submitted the Seismic Screening Evaluation Report for the Columbia Generating Station (Columbia). By Reference 2, the Nuclear Regulatory Commission (NRC) requested additional information related to the Energy Northwest submittal. The attachment to this letter contains the requested information.

No new commitments are identified in this letter.

If you have any questions or require additional information, please contact Ms. L. L.

Williams at (509) 377-8148.

RESPONSE TO THE REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Page 2 of 2 I declare under penalty of perjury that the foregoing is true and correct.

Executed on the,;ir-r~ay of Sef *-Jewt b~v- ,2015 Resp:ctf~y, /)

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D. A. Swank Assistant Vice President, Engineering

Attachment:

As stated cc: NRC Region IV Administrator NRC Senior Resident lnspector/988C NRC NRA Project Manager C Sonoda - BP A/1399 (email)

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 1 of 56 NRC RAI #1:

The Hanford sitewide Senior Seismic Hazard Analysis Committee (SSHAC) Report describes the local seismic sources' (both areal and faults) maximum magnitude (Mmax) values to be used in the Probabilistic Seismic Hazard Analysis (PSHA) calculations. The distribution of Mmax for Zone B was developed considering the largest observed magnitudes within the zone and ranges from 6.5 to 7.5.

Given the limited knowledge regarding the seismic history of the region and observational evidence that at least one historical earthquake occurred in Zone B with possibly a magnitude larger than 7.0 (i.e. the 1872 Lake Chelan Earthquake) and consistent with the 50.54(f) letter and the Screening, Prioritization and Implementation Details (SPID)2 guidance, please provide the following information:

a. Additional detail for your basis that the lower weights for Mmax values of 7.25

[0.09] and 7.5 [0.01] are appropriate for your site and, in general, whether this distribution of Mmax adequately captures the potential for large earthquakes in Zone B.

Energy Northwest Response to RAI #1:

As discussed in Section 8.3.4.6 of the Hanford PSHA report (PNNL,2014), the basis for the Mmax estimate for Zone B is a combination of the size of the largest earthquake in the historical record and the fact that fault sources are not identified separately within the source zone. Based on these two lines of evidence, the SSC TI Team concluded that the assessed Mmax distribution is representative of the center, body, and range of technically defensible interpretations. This response summarizes the rationale for the assessment and provides additional detail, as needed.

Size of the 1872 Lake Chelan Earthquake As summarized in Section 4.4.1.1 of the PNNL (2014) report, the definitive study that established the location and magnitude of the 1872 earthquake is that of Bakun et al.

(2002). They concluded that the event occurred on a shallow crustal fault (non-intraslab) on the east side of the Cascade Range and the moment magnitude M was estimated to be 6.5-7.0 at the 95% confidence level. They employed a method that was developed by Bakun and Wentworth (1997) for using earthquake intensity information to assess the location and moment magnitude of the earthquake. Bakun and Wentworth (1997) developed an objective method for analyzing seismic intensity data that results in an intensity magnitude, MI that is calibrated to equal moment magnitude, M (Hanks and Kanamori, 1979). The method provides objective uncertainties, empirically tied to confidence levels, for M and for source location, and it works well with historical earthquakes for which only a limited number of intensity observations are available (Bakun et al., 2002). Bakun et al. (2002) tested and calibrated this method using intensity data for 12 twentieth-century Pacific Northwest earthquakes for which there is also good epicentral location and instrumental M data. They then applied these results to determine permissible source locations and magnitudes for the 1872 earthquake from its intensity assignments. From this analysis Bakun et al. (2002) conclude that the

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 2 of 56 epicentral region was near Lake Chelan and the epicenter was near the south end of Lake Chelan, Washington.

With a central estimate of M 6.75, the 1872 earthquake is the largest historical earthquake east of the Cascades and the largest historical crustal earthquake (non-intraslab) in the State of Washington.

Faults within Zone B With the establishment of the likely location of the 1872 earthquake, work has been done to attempt to identify the causative fault. Bakun et al. (2002, p. 3253) noted that the epicentral region of the 1872 event lies at the boundary of the North Cascades and Columbia Plateau geologic provinces near the northern edge of the Yakima fold belt .

Geology of each province is permissive of young deformation. They note that the region is likely undergoing north-south contraction and that the apparent absence of a surface scarp suggests that the 1872 earthquake may have occurred on a blind fault.

(p. 3254). Their review of the faults in the region is the following (p. 3250):

The most significant fault in this part of the North Cascades is the Entiat fault, which appears to have no Neogene slip. It extends from Wenatchee more than 160 km to the northwest. It truncates the Eocene Chumstick Formation and is overlapped by the latest Eocene to early Oligocene Wenatchee Formation, indicating that activity was largely late Eocene.

Linearity of the Entiat fault (Fig. 10), features within the fault zone (Laravie, 1976), and the regional Eocene tectonic pattern (Haugerud et al., 1994) suggest that it was a regional strike-slip fault. At its northern end, the Entiat fault is intruded by, and fails to offset, the 20-22 million year-old Cloudy Pass batholith (Tabor et al., 1988). The geologic evidence suggests that there has not been sustained Neogene displacement on the Entiat fault, though perhaps young displacement on its southern part should not be ruled out.

Other recognized faults within this part of the North Cascades are also unlikely sources for the 1872 earthquake. The southern Ross Lake fault is plugged by the 48 million year- old Cooper Mountain batholith. The Mad River thrust appears to be have moved during or prior to the Late Cretaceous (100-65 Ma) regional metamorphism and has no signs of late Cenozoic activity. The remaining mapped faults are too short to generate a M 6.5-7.0 event.

Reported subsequent to the conclusion of the Hanford PSHA, Sherrod (2015) has analyzed LiDAR data to reveal a northwest-side-up scarp along the north side of Spencer Canyon, ~7 km southwest of Entiat, WA. The location of this site (labeled as

1. 35) and preliminary observations supporting the suspected Quaternary deformation designation were included in the inventory of paleoseismic sites (Appendix E-4 to the Hanford PSHA report). The scarp is about 6 km long with a maximum height of 2.4 m.

Based on the age of geologic deposits in exploratory trenches; Sherrod (2015)

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 3 of 56 concludes that the scarp likely formed at the time of the 1872 earthquake. This interpretation would confirm the location of the earthquake as being within the region specified by Bakun et al. (2002) and would provide evidence that it was related to displacement on a shallow reverse fault (Sherrod, 2015). However, the limited length of the identified scarp does not provide additional information that might be used to independently assess Mmax for Zone B.

Much of the area within Zone B lies within the rugged eastern slope of the Northern Cascades province, where it is difficult to identify geologically-young deposits or geomorphic surfaces. Likewise, the Columbia Plateau was subject to catastrophic Pleistocene flooding that removed most of the Quaternary geologic record, which might have recorded young faulting and deformation. With these caveats in mind, no Quaternary or Holocene faults are mapped within Zone B in the USGS Quaternary Fault and Fold Database (USGS, 2006). In the PSHA project for the Mid-Columbia Dams (JBA et al., 2012) consideration was given to the earthquake potential of the Pinto and Badger Mountain structures, which are anticlinal structures that appear to be related to faults. Although both faults lack definitive evidence of Quaternary deformation, both inferred faults are assigned non-zero probabilities of activity, P(a) in the Mid-Columbia PSHA based primarily on the possibility that they may have been associated with the 1872 earthquake and by drawing analogies to the faults of the Yakima Fold Belt (YFB) to the south. Because of the proximity of the faults to the dam sites in the Mid-Columbia study, they are included as fault sources in that study and were assigned the following probabilities of activity: Badger Mountain fault P(a) = 0.5; Pinto fault P(a) = 0.8. The assessed Mmax distributions for the two faults in that study were the following:

Badger Mountain Fault 6.3 [0.2]

6.7 [0.6]

7.2 [0.2]

Pinto Fault 6.2 [0.2]

6.6 [0.6]

7.0 [0.2]

Because of the long distance of these potential fault sources to the Hanford sites, they were not included in the Hanford PSHA. However, from the standpoint of their maximum magnitudes, the assessed Mmax values for the faults lie clearly within the central values of the Mmax distribution assessed for Zone B in the Hanford PSHA. Likewise, it was concluded by the SSC TI Team that, although local fault sources may exist within Zone B that have not been mapped, the extent of faulting and deformation within the zone is much less developed than the region to the south in the YFB. From the standpoint of the Mmax for Zone B, this means that the chances for unrecognized longer through-going faults having the dimensions of the faults within the YFTB Zone (lengths of few tens to several tens of kilometers, structural relief of hundreds of meters) is judged to be very low.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 4 of 56 Technical Justification for Zone B Mmax Distribution The Mmax distribution for Zone B that is given in the logic tree for the source is the following:

6.5 [0.2]

6.75 [0.5]

7.0 [0.2]

7.25 [0.09]

7.5 [0.01]

The two key components of the technical justification for the distribution by the SSC TI Team are the consideration of the maximum historical earthquake within the zone and consideration of faults that are not included as separate fault sources in the SSC model.

The Bakun et al. (2002) study of intensity data for the 1872 Lake Chelan earthquake concluded that the event occurred in the continental crust (not within the Juan de Fuca oceanic slab) and in the region of Lake Chelan within the zone of small-magnitude ongoing instrumental seismicity. Based on the use of the well-accepted approach to the use of intensity data (Bakun and Wentworth, 1997) calibrated using 12 earthquakes in the Pacific Northwest, Bakun et al. concluded that the event had a moment magnitude of 6.5-7.0 at the 95% confidence level.

The 1872 earthquake is not only the largest historical earthquake in Zone B, it is the largest earthquake east of the Cascades and the largest crustal earthquake in Washington state. As such, the SSC TI Team concludes that its magnitude provides a strongly defensible basis for the estimate of Mmax for Zone B. Accordingly, the range of the moment magnitude estimates of 6.5 to 7.0 is represented in the distribution and the central estimate of 6.75 is given higher weight because it is the central estimate from the Bakun et al. analysis.

In addition to the largest historical earthquake, the SSC TI Team also considered the potential for faults within the zone whose dimensions might imply that the Mmax might be larger than the historical maximum. As discussed, the causative fault for the 1872 earthquake is unclear but Sherrod (2015) has identified a ~6 km-long scarp that he concludes was associated with the event. It is not clear if the scarp is related to a previously unmapped fault or through-going fault zone. Other candidate faults in the epicentral region do not appear to display evidence of young deformation (Bakun et al.,

2002).

As part of the Mid-Columbia Dams PSHA, the Badger Mountain and Pinto faults were identified as potential fault sources. The maximum inferred lengths of these faults based on the maximum lengths of the folds with which they are associated is 38 km for the Badger Mountain and 30 km for the Pinto fault. Assuming that these faults are seismogenic, the maximum magnitude distributions assessed for the faults ranges from 6.3 to 7.2 for the Badger Mountain and 6.2 to 7.0 for the Pinto fault. Because of the distance of these faults to the Hanford site, they are not included as fault sources, but

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 5 of 56 their assessed Mmax values lies well within the distribution assessed for Zone B in the Hanford PSHA.

Zone B includes portions of the northern Cascades and Columbia Plateau provinces and no Quaternary faults have been recognized within the zone (USGS, 2006).

Although neither of the regions is conducive to the preservation of evidence for geologically-young deformation, the SSC TI Team concludes that there is ample geologic evidence to preclude the presence of unrecognized faults having the dimensions of faults within the YFB (tens of kilometers long, hundreds of meters of structural relief). Such faults exist within the Columbia Plateau to the south of Zone B and are clearly expressed geologically in the same Columbia River Basalt units that exist within Zone B. It is believed that if such faults exist within Zone B with comparable amounts of structural relief, they would have been mapped.

It is concluded that the existence of the 1872 earthquake confirms that seismogenic faults capable of generating moderate-to-large earthquakes exist within Zone B, but the lack of definitive evidence for the recency or mapped continuity of faults precludes the use of fault rupture dimensions within the zone to estimate Mmax. Possible exceptions to this conclusion are the Badger Mountain and Pinto faults, but their assessed Mmax distributions based on maximum inferred rupture lengths lies well within the Mmax distribution assessed for Zone B. The SSC TI Team concluded that it is very unlikely that faults having the lengths and continuity of the YFB exist within Zone B. As shown in Figure 8.93 of the Hanford PSHA report, most of the Mmax values for the fault sources within the YFTB zone lie within the range of M 7 to 7.5. To account for the remote possibility that such faults exist within Zone B but as yet have not been mapped, low probability is given to the upper values of the Mmax distribution (7.25 [0.09], 7.5 [0.01]).

The expected rupture length associated with a M 7.5 earthquake is several tens of kilometers and it would be expect to occur on a major fault zone, so very little weight is given to this alternative value of the distribution.

Given the available seismological and geological data, and the accepted approaches to assessing Mmax for seismic source zones, the SSC TI Team concluded that the Mmax distribution assessed for Zone B captures the center, body, and range of technically defensible interpretations.

From the standpoint of hazard significance, Figure 10.44 of the report shows that the contribution from Zone B to the total hazard at the CGS site is not significant at annual frequencies of exceedance less than 10-3 for all structural periods. Given the broad range of Mmax values that is already included in the Mmax distribution, it is very unlikely that changes in Mmax would have any effect on hazard at the CGS site.

References Bakun WH, RA Haugerud, MG Hopper, and RS Ludwin. 2002. The December 1872 Washington State Earthquake. Bulletin of the Seismological Society of America 92(8):3239-3258.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 6 of 56 Bakun WH, and CM Wentworth. 1997. Estimating earthquake location and magnitude from seismic intensity data. Bulletin of the Seismological Society of America 87: 1502-1521.

Haugerud RA, EH Brown, RW Tabor, BJ Kriens, and MF McGroder.1994. Late Cretaceous and early Tertiary orogeny in the North Cascades. Geologic Field Trips in the Pacific Northwest, DA Swanson and RA Haugerud (Editors), Geological Society of America 1994 Annual Meeting, Department of Geological Sciences, University of Washington, Seattle: 2E1-2E53.

Hanks T and H Kanamori. 1979. A moment magnitude scale. Journal of Geophysical Research 84: 2348-2350.

JBA (Jack Benjamin & Associates), URS Corporation Seismic Hazards Group, Geomatrix Consultants, Inc., and Shannon & Wilson. 2012. Probabilistic Seismic Hazard Analyses Project for the Mid-Columbia Dams. Final Report prepared for the Public Utility Districts of Chelan, Douglas, and Grant Counties. Chelan County Public Utility District, Wenatchee, Washington.

Laravie JA. 1976. Geological field studies along the eastern border of the Chiwaukum graben, central Washington, M.S. Thesis, University of Washington, Seattle, 56 pp.

PNNL (Pacific Northwest National Laboratory). 2014. Hanford Sitewide Probabilistic Seismic Hazard Analysis. Report prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830, and Energy Northwest, Report PNNL-23361, Pacific Northwest National Laboratory, Richland, Washington 99352.

Sherrod BL. 2015. Lidar identifies source for 1872 earthquake near Chelan, Washington (abstract). Seismologic Research Letters, v. 86(2B): 608.

Tabor RW, DB Booth, JA Vance, AB Ford and MH Ort.1988. Preliminary geologic map of the Sauk River 30 by 60 minute quadrangle, Washington, U.S. Geological Survey Open-File Report. 9-692, scale 1:100,000, 50 pp.

USGS (U.S. Geological Survey). 2006. Quaternary fault and fold database for the United States, accessed 23 August 2015, from USGS web site:

http//earthquakes.usgs.gov/regional/qfaults/.

NRC RAI #2:

The SSHAC report discusses the process used to estimate slip rates for mapped active faults, but the information provided does not provide sufficient detail for the staff to relate subsurface geometry of faults in the Yakima Fold and Thrust Belt to their surficial topographic expression or to confirm the slip rate estimates for individual faults.

Consistent with the 50.54(f) letter and the SPID guidance, please provide the following information:

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 7 of 56

a. Discuss how changes in fault dip with depth, tapered slip, and variation in fault geometries were represented in the final models for determination of slip rates in SSHAC Report Section 8.4.3.4.

b. Clarify how the results of advanced elastic dislocation models were used to constrain the slip rates.

c. Provide a quantitative comparison of the differences between the modeled topographic profiles resulting from the structural relief to net slip conversions with observed topographic profiles transecting the fault(s) being analyzed.

d. Discuss in more detail the steps followed to estimate slip rates specifically for the Rattlesnake Mountain, Saddle Mountain, Umtanum Ridge, and Yakima Ridge faults.

Energy Northwest Response to RAI #2a:

As described in Section 8.4.3.6 of the Hanford PSHA report (PNNL, 2014) long-term average dip slip rates for reverse faults beneath the Yakima folds were derived by using trigonometric relations between structural relief on uplifted rocks in the hanging walls and the dip of the faults, and by considering the period of time over which the deformation has occurred. Net slip was then determined from the dip-slip values using net slip factors that are dependent on the style of faulting assigned to the fault. A summary of the methodology is further outlined in the Response to RAI #2c The assessment of the dip of the fault and uncertainties in the dip are a key part of the slip rate estimation. For a given fault (or fault segment) dip angle , the magnitude of structural relief (SR) and the duration of deformation (t), the long-term average dip slip rate is given by:

Slip Rate = (SR)/[(sin ) t] (1)

Specifically, Section 8.4.3.4 in the PNNL (2014) report referenced in this RAI, describes the methodology for deriving the dip angle of faults beneath the Yakima Fold Belt (YFB) anticlines for use in equation (1) to calculate long-term average dip slip rates. The following discussion summarizes the steps involved in the derivation of .

The approach assumes that the YFB anticlines have developed in the hanging walls of planar reverse faults. As noted in the Responses to RAI #2B and #3, other kinematic models that assume folding occurs by kink-band migration (i.e., Suppe, 1985) do not fit the observations and were not used. The key geometric assumptions of the planar fault model are that the faults dip in the same direction as the backlimb of the fold, and they terminate downward at the base of the seismogenic crust vertically beneath the synformal hinge at the base of the backlimb. These assumptions are informed by an analysis performed by WLA (2000), which showed that simple elastic dislocation models using planar faults can produce folds with wavelengths and amplitudes similar to those of the folds within the YFB.

The primary data used in the derivation are topographic profiles of the YFB anticlines extracted from a 10 m digital elevation model (DEM). From analysis of a topographic

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 8 of 56 profile across a given fold (or segment of a fold), the synformal hinges at the bases of the forelimb and backlimb of the fold are identified and mapped. In plan view, these synformal hinges define the up-dip and down-dip extent, respectively, of the modeled planar fault beneath the fold. For convenience, we refer to the area in plan view bounded by these hinges as the fold polygon. The width of the polygon normal to fold trend is equivalent to the wavelength of the fold. As discussed in Section 8.4.3.2 of PNNL (2014), individual folds were investigated by analyzing multiple topographic profiles perpendicular to the fold axes. From these multiple profiles an average width for a fault (or fault segment) was calculated. Ridge profiles extracted along the fold trends at the highest elevations of the individual folds were used in conjunction with regional geologic maps and knowledge of the geomorphology to evaluate potential rupture segments. Many of the ridge profiles show bell-shaped profiles indicative of horizontal slip gradients that are very similar to displacement profiles commonly associated with coseismic ruptures. This pattern of tapered slip along individual folds or segments of longer folds was considered in the assessment of likely magnitudes of characteristic ruptures that could occur on the faults in future ruptures (see Section 8.4 3.8 of PNNL, 2014).

With the plan extent (average width) of the fault beneath the fold thus defined, the fault dip is given by:

= tan-1 (seismogenic thickness/polygon width) (2)

The methodology outlined in Section 8.4.3.4 of PNNL (2014) explicitly addressed two primary sources of epistemic uncertainty in :

1) Thickness of the seismogenic crustUncertainty in due to uncertainty in the thickness of the seismogenic crust per equation (2) above was captured in the seismic source model by adopting a weighted range of crustal thicknesses (13 km, 16 km, and 20 km.

2) Horizontal variation in the down-dip termination of the faultIn performing elastic dislocation modeling for a given fold, WLA (2000) initially assumed the down-dip termination of the fault lay vertically beneath the synformal hinge at the base of the fold backlimb. Using a forward modeling approach, WLA (2000) systematically varied the fault dip from this starting model to find the best-fit fault geometry for a given fold. WLA (2000) found that in some cases the down-dip termination for the best-fit fault was shifted toward the forelimb of the fold, resulting in a more steeply dipping fault than the starting model. In other cases, the down-dip termination of the best-fit model was shifted away from the fold forelimb, resulting in a less steeply dipping fault than the starting model. To capture additional uncertainty in fault dip suggested by these elastic modeling results, the seismic source model included three alternate fault geometries (see PNNL [2014] Figure 8.77 attached to this response):

• Geometry 1-The dip was derived by using the average width of the fold polygon to define the base of the backlimb, and by assuming that the fault

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 9 of 56 terminates against the base of the seismogenic crust vertically beneath this point. This is equivalent to the starting geometry used by WLA (2000) in their forward modeling approach.

• Geometry 2-The fault dip was derived from the maximum width of the polygon that characterizes the plan fold dimensions. For a given thickness of seismogenic crust, the fault dip angle derived from the maximum polygon width will be less than the dip based on average polygon width in Geometry 1 above.

• Geometry 3-The fault dip was derived from 60% of the average width of the polygon that characterizes the plan fold dimensions. For a given thickness of seismogenic crust, the dip angle derived in this manner will be greater than the dip based on average polygon width in Geometry 1 above.

To summarize, the seismic source model assumes that the Yakima anticlines are asymmetric fault-propagation folds developed above reverse faults. The faults are assumed to be simple planar surfaces, and the dip is assumed to be uniform to the base of the seismogenic crust. This assumption is consistent with the results of elastic dislocation modeling presented by WLA (2000), as well as the geometry of well-documented seismogenic reverse faults in California as illuminated by aftershocks of moderate-magnitude earthquakes (e.g., Eaton, 1990; Carena and Suppe, 2002; McLaren et al., 2008). Uncertainty in fault geometry is captured by ranges in dip that reflect uncertainty in the thickness of the seismogenic crust and the precise relationship of the down-dip termination of the fault to the backlimb of the overlying anticline.

References WLA (William Lettis & Associates, Inc.). 2000. Down-Dip Geometry of Blind Thrust Faults Beneath the Syrian Arc Fold Belt. Basic Data Report No. 20, Shivta-Rogem Site Investigation, prepared for the Israel Electric Corporation, Ltd. WLA, Walnut Creek, California.

Carena S. and J Suppe 2002. Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California. Journal of Structural Geology, 24(4), p. 887-904.

Eaton, JP,., 1990. The earthquake and its aftershocks from May 2 through September 30, 1983. In, in Rymer, MJ.and WL Ellsworth (Editors). The Coalinga, California, Earthquake of May 2, 1983. United States Geological Survey Professional Paper 1487,

p. 113-170.

McLaren, MK., Jl.Hardebeck, N van der Elst, JR.Unruh, GW BawdenGWBawden, and JL Blair. 2008. Complex faulting associated with the 22 December 2003 Mw 6.5 San Simeon, California, earthquake, aftershocks, and postseismic surface deformation.

Bulletin of the Seismological Society of America, 98(4):1659-1680.

PNNL (Pacific Northwest National Laboratory). 2014. Hanford Sitewide Probabilistic Seismic Hazard Analysis. Report prepared for the U.S. Department of Energy under

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 10 of 56 Contract DE-AC05-76RL01830, and Energy Northwest, Report PNNL-23361, Pacific Northwest National Laboratory, Richland, Washington 99352.

Energy Northwest Response to RAI #2b:

Elastic dislocation modeling was not used to directly constrain slip rates for Yakima Fold Belt (YFB) faults. Rather, the results of elastic dislocation modeling performed by WLA (2000) were used to inform development of fault models for the YFB (see Response to RAI #2a). Specifically, the WLA (2000) modeling results provided a basis for estimating the location and dip of planar reverse faults beneath the Yakima anticlines from aspects of the fold geometry that can be assessed from digital topographic data (see response to RAI #2c). The fault dip inferred from this approach was used along with structural relief and the timing of fold deformation to derive net slip rates for the seismic source model.

As summarized in Section 8.4.3.4 of the Hanford PSHA report (PNNL, 2014), WLA (2000) employed elastic dislocation modeling to evaluate potential fault geometries beneath the Syrian Arc fold belt in Israel. WLAs use of elastic modeling was motivated by the observation that some well-known kinematic models for fault-propagation folding based on migration of kink bands (e.g., Suppe, 1985) are intrinsically incapable of reproducing the very long backlimbs and low amplitudes exhibited by the Syrian Arc folds. WLA (2000) used elastic models to: (1) test the hypothesis that the observed wavelength and amplitude of the asymmetric Syrian Arc anticlines could be produced by elastic folding above a simple planar reverse fault; and (2) derive fault geometry for use in a seismic hazard model. The elastic modeling software used by WLA (2000) is described in Erikson (1987).

As summarized in Section 8.4.3.4 of the Hanford PSHA report (PNNL, 2014), the YFB structures are similar to the Syrian Arc folds in that the anticlines have low amplitudes relative to their long backlimbs. Given this similarity, and the success of the elastic models in replicating the geometry of the Syrian Arc folds (WLA, 2000), the YFB anticlines were assumed to have formed as fault-propagation folds above blind, planar reverse faults that extend to the base of the seismogenic crust.

In addition to providing a geo-mechanical basis for inferring simple planar faults beneath the YFB anticlines, the elastic modeling results of WLA (2000) also informed the methodology used to derive the geometry of the faults for inclusion in the SSC model. In performing elastic modeling, WLA (2000) assumed a starting model in which the synformal hinges at the bases of the forelimb and backlimb of a fold define, in plan view, the up-dip and down-dip extent, respectively, of the model fault beneath the fold. These parameters, along with the thickness of the seismogenic crust, determine the location and dip of a planar fault beneath the fold. The same relationships between fault geometry, fold width and seismogenic thickness used by WLA (2000) were employed to develop models for planar faults beneath the Yakima folds (see Response to RAI #2a).

To summarize, the results of elastic dislocation modeling performed by WLA (2000) to evaluate the Syrian Arc anticlines were reviewed and used to inform modeling of planar faults beneath the Yakima folds. The fault geometries inferred for the Yakima folds,

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 11 of 56 including a distribution in dip values, were used along with structural relief and timing of deformation to evaluate slip rates for the seismic source model. No elastic modeling was performed for the Yakima folds to directly evaluate fault geometry, or indirectly constrain slip rates.

References Erickson L. 1987. Users Manual for DIS3D: A Three-Dimensional Dislocation Program with Applications to Faulting in the Earth. Geomechanics, Applied Earth Science Department, Stanford University, Stanford, California.

PNNL (Pacific Northwest National Laboratory). 2014. Hanford Sitewide Probabilistic Seismic Hazard Analysis. Report prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830, and Energy Northwest, Report PNNL-23361, Pacific Northwest National Laboratory, Richland, Washington 99352.

Suppe J. 1985. Principles of Structural Geology. Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 537 p.

WLA (William Lettis & Associates, Inc.). 2000. Down-Dip Geometry of Blind Thrust Faults Beneath the Syrian Arc Fold Belt. Basic Data Report No. 20, Shivta-Rogem Site Investigation, prepared for the Israel Electric Corporation, Ltd. WLA, Walnut Creek, California.

Energy Northwest Response to RAI #2c:

The topographic profiles used in this study to calculate net slip rates for the YFB faults were not modeled; rather, they were extracted from a regional 10m DEM in ArcGIS. The profiles were used in a multi-stage analysis, along with other data and observations, to derive net slip rates on the YFB fault sources. Based on careful evaluation of multiple field and borehole observations, the SSC TI team assessed that the topographic relief of the YFB structures as expressed in the profiles can be used as a proxy for structural relief. The structural relief, fault geometry, and ages of various geologic units were used as inputs to calculate net slip rates for individual faults in the YFB, as described in Sections 8.4.3.2 to 8.4.3.6 of the Hanford PSHA report (PNNL, 2014). This process is outlined below. Note that figures and tables referred to in this response are from the Hanford PSHA report (PNNL, 2014), and the PSHA report numbering is retained.

Copies of the referenced figures are provided as attachments to this document to aid the reader.

Topographic Data Analysis We analyzed topographic data in conjunction with a three-dimensional (3-D) hydrogeologic model of the Columbia Plateau (Burns et al. 2011) and structure contour maps on the top of basalt (Myers et al. 1979) to evaluate the geometry of the YFB anticlines and derive models for the location and dip of underlying thrust faults. This analysis consisted of the primary activities summarized below and discussed in detail in Section 5 of the Hanford PSHA Report Appendix E:

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1. Topographic AnalysisUsing GIS software (ArcGIS) to visualize data from a 10-m DEM, we characterized the geometry of individual YFB structures by measuring the length of the anticlines and their widths (i.e., wavelengths), and by evaluating topographic relief normal to the crests of the folds (i.e., fold amplitude). The measurements were performed by extracting and analyzing numerous two dimensional (2-D) topographic profiles across the folds using a standard ArcGIS tool. Profiles acquired normal to the trend of the folds allowed us to determine the locations of synformal hinges at the base of the forelimb and backlimb of each fold, and to accurately measure the full width or wavelength of the structure, as well as topographic relief across the fold crest. Profiles acquired along the fold axis allowed us to assess lateral variations in topographic relief and identify discrete structural or geometric reaches of the fold that potentially indicate variations in underlying thrust fault geometry and/or activity.

2. Comparison of Topographic Relief to Structural ReliefThe Burns et al. (2011) hydrogeologic model is a 3-D representation of the first-order stratigraphic and structural framework of the Columbia Plateau. The model is derived from surface geology and data from more than 13,000 wells, and depicts uplift, folding, and faulting of Miocene Columbia River Basalt (CRB) units across the major YFB structures. The Burns et al. model is available as an interactive web-based tool (http://or.water.usgs.gov/proj/cpras/index.html) that allows users to define and extract arbitrary 2-D cross sections from the model and directly measure the relief of key stratigraphic contacts (e.g., the tops of the Saddle Mountains Basalt and Wanapum Basalt) across individual Yakima folds. We used this tool to compare structural relief on the CRB units across the folds to topographic relief and test the hypothesis that the two are similar, thus establishing a basis for using topographic relief as a proxy for assessing long-wavelength variations in structural relief. Locally, where erosion and deposition related to cataclysmic floods have significantly modified the surface expression of the folds, a top-of-basalt structure contour map (Myers et al. 1979) that incorporated additional information from subsurface data also was used to assess structural relief.

Derivation of Source Geometry and Long-Term Slip Rate We used the fold geometry and estimates of structural relief from the topographic analysis to derive fault geometry beneath the folds and estimate the long-term average reverse separation rate. The analytical approach and assumptions involved in this analysis, as well as the derived fault geometry and long-term average slip rate, are described below.

i) Results of Structural Relief Analysis Using the topographic analysis, structural relief information was derived and tabulated for each major structure within the YFB study area. Multiple relief measurements were collected along each fold in order to capture the range of uncertainty and best estimates for average (mean) and maximum values (Figure 8.72). Results of the structural relief analysis are provided in Table 8.9. The average (mean) result of this analysis was used

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 13 of 56 in conjunction with the dip (and its uncertainty) of the modeled reverse fault beneath each YFB structure to calculate a range of dip slip values for each fault source. The dip slip values then were used to calculate net slip values, depending on the sense of slip of a given fault as described below and in Section 8.4.3.3 of the Hanford PSHA report (PNNL, 2014).

ii) Sense of Slip, Factors for Net Slip The YFB comprises predominantly reverse faults that have uplifted basement rocks and overlying Columbia River Basalt (CRB). From a tectonic perspective, the YFB may not be considered a classic fold and thrust belt due to its orientation being perpendicular to, and its location being far from, a convergent plate margin. The fan-shaped morphology and sub-parallel orientation of YFB structures departs from the pattern of imbricate thrust stacking and large culminations exhibited by typical fold-and-thrust belts.

Regardless, the faults within the YFB have produced large-scale, fault-related anticlines that have hundreds of meters of vertical relief, indicating that the fold belt is in a compressional environment with predominantly reverse movement accommodating north-south shortening.

Assessing the style of faulting for individual fault sources within the YFB is challenging due to the lack of paleoseismic data and site-specific kinematic indicators. Fault-specific kinematic information was used where available to assess the style of faulting of individual fault sources. In the absence of fault-specific data, the sense of slip was assessed for fault sources based primarily on regional evidence from geologic mapping and topographic data. As indicated on the Washington State Geologic Map, major faults in the YFB are reverse faults. This mapping in conjunction with observations of major compressional structures made during the Quaternary Geologic Studies (Appendix E of PSHA Report (PNNL, 2014)) and topographic analysis as part of this study provide strong evidence for reverse motion along the YFB faults. As further discussed in Hanford PSHA Report Appendix E, analyses of focal mechanisms from small background earthquakes in eastern Washington indicate that the modern seismotectonic environment of the YFB generally is characterized by approximately north-south crustal shortening. Table 8.10 presents the weights assessed by the SSC TI Team for the style of faulting for all of the fault sources in the SSC model.

The dip angle of thrust or reverse faults beneath YFB anticlines was derived using a simple model to relate the fault geometry to the plan dimensions of the folds (see Response to RAI #2a). The model assumes that the anticlines have developed in the hanging walls of blind or emergent thrust faults. The faults are assumed to extend from where they are mapped at the surface (or, if the fault is blind, from beneath the synformal hinge at the base of the forelimb of the anticline), and dip in the same direction as the backlimb of the anticline. Because the majority of the Yakima folds are asymmetric, north-vergent anticlines, the associated underlying thrust or reverse faults dip south. The key geometric assumption of the model is that the faults terminate downward at the base of the seismogenic crust vertically beneath the synformal hinge at the base of the fold backlimb (Figure 8.75). For the faults that lie within the YFTB

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 14 of 56 source zone, the SSC TI Team adopted 13 km, 16 km, and 20 km as alternative models for the seismogenic thickness (see Sections 8.3.2 and 8.3.3 of PNNL [2014] for discussion). This model for deriving fault dip is informed by an analysis of the Syrian Arc fold belt in Israel for the Shivta nuclear power plant site characterization by William Lettis & Associates, Inc. (WLA 2000) as further discussed in the Response to RAI # 2b.

Figure 8.74 shows a diagram of the process used to calculate net slip, which is the total amount of slip occurring on a fault that takes into account the style of faulting. With the assumption described in Section 8.4.3.2 of the PSHA Report (PNNL, 2014) that topographic relief can be used as a proxy for evaluating structural relief, the average structural relief value was used with a range of fault dip values to calculate a range of total dip slip values along each fault source. To account for the three styles of faulting (dip slip, oblique slip and strike slip), net slip factors were developed for this study (Table 8.11). The factors are based on reasonable assumptions for different types of faulting. For reverse faults, the dip slip is equal to the net slip and therefore has a factor of 1.0. For faults with an oblique slip, a 1:1 ratio of lateral to dip slip is assumed, which yields a net slip factor of 1.4. For faults with predominantly strike-slip displacement, 2:1 and 5:1 lateral to dip-slip ratios are assumed, resulting in net slip factors of 2.2 and 5.1, respectively (Table 8.11).

To calculate net slip for each fault source, the dip-slip value was multiplied by the net-slip factor. Because many of the fault sources are assigned weights for various styles of faulting, these faults have a range of associated net slip values that vary according to the assumed obliquity of displacement (i.e., for given structural relief and fault dip values, the net slip increases with obliquity).

iii) Net-Slip Rates The assessment of net slip discussed in Section 8.4.3.3 of the Hanford PSHA Report (PNNL, 2014) is the total amount of slip that has occurred on a fault given the style of faulting represented by a net-slip factor. The net slip was derived from the dip slip, which in turn was based on the dip of the fault plane and amount of structural relief accommodated on the plane. The net-slip value relies on the assumed style of faulting; therefore each fault segment has up to four net-slip values because there are four possible net-slip factors (1 for reverse, 1.4 for oblique, 2.2 and 5.1 for strike slip).

Further, the net slip is calculated for each of the three values of dip that reflect uncertainties in the topographic width used to assess the dip: the average width, maximum width, and 60% of the width as discussed in Section 8.4.3.4 of the Hanford PSHA Report (PNNL, 2014) (see Response to RAI #2a). Lastly, there are three possible seismogenic depths: 13 km [0.2], 16 km [0.5], and 20 km [0.3]. In total, there are nine weighted net-slip values for a given fault source segment in the SSC model. Together these net-slip values and their associated weights cover a defensible range of epistemic uncertainty built into the fault source characterization portion of the model.

To calculate a rate for a given fault source, the net slip is divided by a time period over which the deformation (i.e., the post-CRB deformation now reflected in the topographic relief of the YFB anticlines) occurred. Section 8.4.3.5 of the Hanford PSHA Report

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 15 of 56 (PNNL, 2014) describes the rationale behind the two end-member start times (10 Ma and 6 Ma) used to calculate the long-term slip rate for each fault segment. Taking into account the two starting times and the three options of dip, there are six net-slip rates calculated for each seismogenic depth. Given that there are three seismogenic depths in the model, a total of 18 net-slip rates are calculated for each fault type considered for each fault segment. The average net-slip rate for the entire fault is calculated as a length-average of all segments of the fault. The resulting net-slip rates are the fault-specific, long-term average slip rates for the YFB faults (see Section 8.4.3.6.2 of PNNL

[2014]). To capture the complexity and range of the net slip rates, they are presented as cumulative distribution function (CDF) plots in Figure 8.83 (PNNL, 2014). The CDF is a slip-rate probability function that takes into account all factors in the logic tree that contribute to slip rate as well as their relative weights.

Summary In summary, the range in net slip values for the individual fault sources captures a broad range of uncertainty in both the geometry and sense of slip of each of the individual faults or fault segments. As noted above, topographic profiles used in the analysis were not modeled, but rather were extracted from a publicly available DEM. Results of elastic dislocation modeling of folds in the Syrian fold belt as described by WLA (2000) were used to inform and support the decision to model the YFB structures as planar reverse faults.

References Burns ER, DS Morgan, RS Peavler, and SC Kahle. 2011. Three-dimensional Model of the Geologic Framework for the Columbia Plateau Regional Aquifer System, Idaho, Oregon, and Washington. U.S. Geological Survey (USGS) Scientific Investigations Report 2010-5246, USGS, Tacoma, Washington.

Myers CW, SM Price, JA Caggiano, et al. 1979. Geologic Studies of the Columbia Plateau - A Status Report. RHO-BWI-ST-4, Rockwell Hanford Operations, Richland, Washington.

WLA (William Lettis & Associates, Inc.). 2000. Down-Dip Geometry of Blind Thrust Faults Beneath the Syrian Arc Fold Belt. Basic Data Report No. 20, Shivta-Rogem Site Investigation, prepared for the Israel Electric Corporation, Ltd. Walnut Creek, California.

PNNL (Pacific Northwest National Laboratory). 2014. Hanford Sitewide Probabilistic Seismic Hazard Analysis. Report prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830, and Energy Northwest, Report PNNL-23361, Pacific Northwest National Laboratory, Richland, Washington 99352.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 16 of 56 Figure 8.72: Map showing average and maximum vertical structural relief for fault segments

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 17 of 56 Table 8.9: Mean structural relief measurements for fault sources in the YFB.

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RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 19 of 56 Figure 8.75: Example showing projection of alternative dips given the geometry of the fold and alternative seismogenic depths.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 20 of 56 Figure 8.74: A diagram of the procedure used to calculate dip slip and net slip for fault sources.

The net-slip factor varies for each style of faulting. The net slip was used to calculate long-term slip rates.

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RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 23 of 56 Energy Northwest Response to RAI #2d:

Long-term (post-Columbia River Basalt [CRB]) slip rates for Yakima fold belt (YFB) fault sources, including the Rattlesnake Mountain, Saddle Mountain, Umtanum Ridge, and Yakima Ridge faults, were calculated using the methodology described in the Response to RAI #2c in which topographic relief, supplemented as appropriate by subsurface structure contour maps, is used to estimate structural relief. This assessment of structural relief then is used in combination with fault geometry and sense of slip to calculate a long-term average net slip rate for a fault. Quaternary deposits and surfaces with age constraints, and with sufficient extent to be useful datums, were available for only a few of the YFB structures: i.e., the Toppenish Ridge, Ahtanum Ridge, Rattlesnake Mountain, Manastash, and Umtanum faults.

As noted in Section 8.4.3.6.1 of the Hanford PSHA report (PNNL, 2014), due to the preliminary nature of both the dating results and terrace correlations, new information that could be used to constrain Quaternary uplift rates for the Manastash and Umtanum Ridge faults was used primarily as a check on the long-term post-CRB rates inferred from the structural analysis. Mapping and dating of the Yakima River fluvial terraces as summarized in Section 9 and Table 9-1 of Appendix E (PNNL, 2014) indicated that the differential incision rates (a proxy for uplift rate) measured across the Manastash and Umtanum folds were of a similar order of magnitude but slightly less than the long-term average vertical rates estimated from the structural analysis. Quaternary studies for the Rattlesnake Mountain fault conducted as part of the Hanford PSHA yielded information that was used to assess a Quaternary vertical separation rate that was explicitly included in the analysis for that fault source.

The data and steps in the approaches used to assess slip rate for the Umtanum Ridge, Yakima Ridge, Rattlesnake Mountain, and Saddle Mountain faults are outlined in this response. The characterizations of the segments of the Umtanum Ridge and Yakima Ridge faults within much of Pasco Basin are based on subsurface contour map data.

The approach and data sets used to assess these two faults were similar as described below. The Saddle Mountains and Rattlesnake faults used topographic data and field observations from the Hanford PSHA study to evaluate long-term slip rates, and Quaternary data where available to assess a shorter term slip rate. Note that figures and tables referred to in this response are from the Hanford PSHA report (PNNL, 2014),

and the PSHA report numbering is retained. Copies of the referenced figures are provided as attachments to this document to aid the reader.

Umtanum Ridge and Yakima Ridge Faults The Umtanum Ridge fault is subdivided into five characteristic rupture segments based on criteria discussed in Sections 8.4.2 and 8.4.3.8 of the Hanford PSHA report (PNNL, 2014). From west to east the segments include the Umtanum Ridge West (UR-W),

Umtanum Ridge Central (UR-C), Umtanum Ridge East (UR-E), Umtanum-Gable Mountain (U-GM), and Umtanum Ridge-Southeast Anticline (UR-SA) (Figure 8.43, PNNL, 2014). Similarly, the Yakima Ridge fault is subdivided into three characteristic segments, including Yakima Ridge West (YR-W), Yakima Ridge East (YR-E), and

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 24 of 56 Yakima Ridge Southeast (YR-SE). The segments vary in the amount of structural relief, which was a primary input used in calculating long-term net slip rates for the fault sources.

The western portions of both faults, particularly the segments including UR-W, UR-C, UR-E, YR-W, and YR-E, are associated with large anticlines that have well preserved structural relief. The slip rates along these segments were characterized by measuring topographic profiles to assess structural relief along multiple portions of the folds.

Sections 8.4.3.2 and 8.4.3.3 (PNNL, 2014) and the Response to RAI #2c discuss in detail the methodology for measuring the structural relief across the folds and steps for calculating net slip from structural relief. These methods apply to characterizing folds with clear topographic relief with little to no flood-related erosion, and should be referred to for fault segments UR-W, UR-C, UR-E, YR-W, and YR-E.

The area around the Hanford Site has experienced intense Pleistocene flood erosion and deposition, which has modified the topographic expression of the folds that are associated with the U-GM, UR-SA, and YR-SE fault sources. For the faults in this area, the TI team used the same methodology for calculating slip rate from structural relief, but adopted a different methodology for measuring structural relief due to the absence of reliable topographic information (i.e. for these structures, topographic expression does not equal structural relief). Existing structural contour maps of the top of the Saddle Mountains Basalt cover a large area of the Hanford Site (Myers et al. 1979).

Fecht et al. (1992) and Thorne et al. (2014) provide detailed information about the subsurface geometry and relief of buried structures at the site. Figure 8.71 (PNNL, 2014) shows the Thorne et al. (2014) structure contour map that indicates the locations and continuity of the buried anticline ridges. The 10m (~32 ft). contours provide detailed information on the structural relief of the anticlines and documentation of the eastern extent of the structures where they plunge out to zero structural relief at the ends of the faults (Figure 8.71, PNNL, 2014). The structure contour map was imported into an ArcGIS environment where topographic profiles were extracted from the structural contours and used to measure structural relief at several locations along the buried fault segments. Once the relief measurements were compiled, the same methodology as was used as was implemented for the topographically well-expressed faults sources. A detailed discussion of creating topographic profiles from the structural contour maps is provided in Section 5.2.3 of Appendix E of the Hanford PSHA report (PNNL, 2014).

The average structural relief across these fault segments, as measured from the structure contour map, is listed below:

U-GM = 160m UR-SE = 90m YR-SE = 65 m These relief measurements were used in conjunction with the fault type, fault dip estimates, and start time to produce a long-term-average slip rate as described in the Response to RAI #2c. The following data were used in the analysis to arrive at a long-term net slip rate:

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 25 of 56 Style of Faulting Reverse [1.0]

Saddle Mountain Start time for Slip Rates 6 Myr [0.4]

10 Myr [0.6]

The response to RAI #2c, and discussion in Section 8.4.3.6.2 of PNNL (2014), both present cumulative distribution functions (CDFs) for the net slip rate, which express the long-term slip rates for each of the fault sources (Figure 8.83, PNNL, 2014). The fault input values that were used to develop the CDF plots are provided in HID_alltables_20140317.xlsx (see Appendix D Supplemental Material of PNNL (2014)).

The CDF is a slip-rate probability function that takes into account all factors in the logic tree that contribute to slip rate as well as their relative weights. These factors include topographic width, fault dip, seismogenic depth, structural relief, and style of faulting.

The ranges in slip rates for the U-GM, UR-SA, and YR-SE fault sources as presented in the CDF plots are below:

U-GM 0.016 to 0.038mm/yr centering on 0.017mm/yr at 50% probability UR-SA 0.009 to 0.022mm/yr centering on 0.009mm/yr at 50% probability YR-SE 0.006 to 0.015mm/yr centering on 0.007mm/yr at 50% probability Saddle Mountains Fault The Saddle Mountains fault source was divided into two segments corresponding to the eastern and western portions of the structure (SM-E and SM-W). The anticline associated with the Saddle Mountains fault source generally is well preserved with various units of the CRB exposed at the crest and on the front limb of the anticline. In particular, the Elephant Mountain basalt (10.5 Ma) underlies the crest of the mountain and is present and exposed in the footwall of the Saddle Mountains thrust to the north, demonstrating that the structural relief on the Elephant Mountain basalt is equivalent to the topographic relief of Saddle Mountains at this location. Therefore, to estimate a long-term average slip rate, topographic profiles were used to estimate the average structural relief (Figure 8.65, PNNL, 2014). South of Saddle Mountains, on the backlimb of the fold, Plio-Pleistocene sediments and loess overlie the CRB, and therefore there is some uncertainty in the structural relief due to burial of the basalts. However, available well data in the area were used to construct detailed cross sections and better constrain the structural relief along the eastern and western portion of the anticline north of the Hanford Site (Figures 8.69 and 8.70, PNNL, 2014). This approach allowed for accurate measurements of total structural relief across the Saddle Mountains fault.

As discussed in Section 8.4.3.5 of the PSHA report (PNNL, 2014), observations during field reconnaissance for the study revealed a modest angular unconformity between the

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 26 of 56 Ringold Formation and the Elephant Mountain basalt on the backlimb of the Saddle Mountains anticline, and possible thinning of the Ringold Formation across the crest of the fold (see Appendix E to PNNL, 2014). These observations suggest that folding began prior to, and/or was coeval with, local deposition of the Ringold Formation.

Similar relationships are shown in a series of cross sections across the Gable Mountain structure developed by Bjornstad et al. (2010) from correlation of borehole data.

Specifically, these cross sections indicate uplift and folding was occurring, along with incision of the CRB units beneath the Hanford Site, prior to deposition of basal Ringold Formation strata between about 8.5 Ma and 5 Ma.

Proponent models of regional tectonics in eastern Washington, analyses of the direction and distribution of Pacific-North American plate motion since late Neogene time, and stratigraphic and structural relations in the Hanford area discussed in PNNL (2014) are consistent with uplift and folding of the 10.5 Ma Elephant Mountain Member of the CRB between about 10 Ma and 6 Ma (Section 8.4.3.5, PNNL, 2014). Given the stratigraphic and structural relations in the Hanford region indicating that uplift, folding, and incision of the Elephant Mountain Member occurred before deposition of the basal Ringold Formation (Bjornstad et al. 2010), as well as proponent models for progressive growth of the Yakima folds during eruption and deposition of the Saddle Mountains flows of the CRB (Reidel et al. 1983; Reidel 1984), the SSC TI Team assigned slightly higher weights to the older 10 Ma date for the start time of deformation.

As described in the Response to RAI #2c, the structural relief was used in conjunction with fault type and fault geometry to calculate a total net slip along the fault sources. In order to calculate a net slip rate, the net slip was associated with two weighted alternative start times of 10 Ma and 6 Ma, the technical bases for which are described in Section 8.4.3.5 of the Hanford PSHA Report (PNNL, 2014). To summarize, the inputs for structural relief, start time, and net slip rates for Saddle Mountain are provided below:

Mean Structural Relief SM-E = 320m SM-W = 335m Style of Faulting Reverse [1.0]

Saddle Mountain Start time for Slip Rates 6 Myr [0.4]

10 Myr [0.6]

The ranges in slip rates for the SM-E and SM-W fault sources are shown in the CDF plot in Response to RAI #2c (Figure 8.83, PNNL, 2014):

SM-E and SM-W 0.034 to 0.65mm/yr centering on 0.04mm/yr at 50% probability

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 27 of 56 Rattlesnake Mountain Fault The Rattlesnake Mountain fault source was modeled as a single fault source. It is a unique structure in the SSC model because it has the highest structural relief in the YFB, and Quaternary data are available to compare against the long-term average slip rate.

Long-term-average slip rate The long-term-average slip rate for the Rattlesnake Mountain fault source was calculated like the other topographically well-preserved structures in the YFB. The average structural relief was measured with a series of topographic profiles as described in the Response to RAI #2c. Figure 8.82 (PNNL, 2014) shows a topographic profile and ridge profile used in the analysis. A summary of the parameters used to calculate a long term average net slip rate are listed below:

Mean structural relief RM = 619m Rattlesnake Mountain Style of Faulting Reverse [0.9]

Oblique [0.1]

Rattlesnake Mountain Start time for Slip Rates 6 Myr [0.3]

10 Myr [0.7]

The ranges in slip rates for the long term net slip rate for the RM fault sources as presented in the CDF plots in Figure 8.83 (PNNL,2014):

RM 0.03 to 0.4mm/yr centering on 0.1mm/yr at 50% probability Quaternary slip rate As discussed in Section 8.4.3.6.1 of the PSHA Report (PNNL, 2014), Quaternary studies related to the Rattlesnake Mountain fault that were conducted as part of this study yielded information about the Quaternary rate of vertical separation that is explicitly included in the assessment of the slip rate and recurrence for this fault. Key conclusions and observations from these studies for the Rattlesnake Mountain fault are as follows:

Quaternary tectonic activity in the Rattlesnake Mountain study area is observed along at least two structures: the range-front fault and the gas field anticline. The rang-front fault has produced clear vertical separation of an alluvial fan surface (Qaf4) estimated to be middle Pleistocene (>380-800 ka) in age based on soil profile development calibrated based on Th/U-series dates for similar soils in nearby cataclysmic flood deposits. Younger inset alluvial fan surfaces (e.g., Qaf2 and Qaf1), however, are deposited across the

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 28 of 56 fault in a few places, and field observations indicate that they are not deformed. Examination of topographic profiles along the younger fan surfaces and their continuation as terraces in the canyon upstream of the range front reveals that the surfaces are continuous and undeformed across the fault.

Based on the estimated age of the Qaf2 fan the most recent surface rupture occurred prior to ~13 ka to as much as 70 ka ago. The Qaf4 surface is also clearly tectonically tilted down to the north and south on the forelimb and backlimb, respectively, of the gas field anticline. Cumulative vertical stratigraphic separation of the Qaf4 fan surface across both the range-front fault and gas field anticline ranges from 22-33 m with a preferred average of 25-30 m. The evidence of no measurable lateral offset of geomorphic features, combined with the evidence for vertical separation across a surface scarp at the range front and fold deformation on the gas field that is consistent with the presence of a blind thrust or reverse fault, demonstrates that the uplift of Rattlesnake Mountain in the Quaternary has been primarily been accommodated by reverse slip on a fault that includes both emergent and blind splays that likely merge at depth. (PNNL, Appendix E, p.vi).

The key data used to assess the Quaternary slip rate for the Rattlesnake fault source are further described below.

Based on field observations and desktop studies (see Section 7.3.3 of Appendix E, PNNL, 2014) the most suitable Quaternary deposit to use for evaluating total vertical stratigraphic separation across the entire Rattlesnake Mountain fault zone (including both the range-front fault and the gas field anticline) is a broad coalescing alluvial fan (bajada) (map unit Qaf4). The age of the Qaf4 fan is estimated to be middle Pleistocene

(>380-800 ka) based on comparison of the degree of soil profile development and clast weathering of this unit relative to nearby flood gravel deposits in the Badger Coulee area that have been dated using both thorium/uranium analysis of pedogenic carbonate rinds and magnetic polarity data (Baker et al. 1991; Appendix E, Tables 4.3 and 7.1).

The large extent of the Qaf4 fans suggest a long period of deposition, possibly throughout much of the marine oxygen isotope stage (MIS) 16, MIS 14, and MIS 12 glaciations and transitional periods to intervening interglacial periods. It is assumed that the final abandonment and initiation of soil profile development likely occurred toward the latter part of this time period. Subsequent stages are characterized by more extreme interglacial periods (e.g., MIS 11 and MIS 9) when conditions are judged to be less favorable for extensive fan deposition. Based on these observations and the best-estimated values from the thorium/uranium analyses used for calibration, a preferred age of between 425 and 600 ka is used to calculate a post-Qaf4 vertical separation rate.

Total cumulative vertical stratigraphic separation across both the range-front and gas field structures estimated from the reconstructed projections range from 20 +/- 3 m at Profile 4 to 30 +/- 3 m at Profile 3 (see Appendix E, Plate 2, PNNL, 2014), with an average value of approximately 25 m. Profiles 2 and 3, which span the central part of the emergent range-front fault, show similar cumulative offsets of 29-30 m.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 29 of 56 A cumulative distribution function (CDF) for a Quaternary vertical stratigraphic separation rate is estimated from the combined probability distributions for age and displacement of the Qaf4 fan surface. Based on the preferred ages and displacements outlined above a trapezoidal form was used to develop both probability distributions based on the following:

Age: minimum 380 ka preferred 425-600 ka maximum 800 ka Displacement: minimum 22 m preferred 25-30 m maximum 33 m The results range from a minimum vertical stratigraphic separation rate of 0.03 mm/yr to a maximum of 0.09 mm/yr (Figure 8.81). The Miller and Rice (1983) methodology used to define a set of discrete approximations that represent the CDF yields the following five-point distribution:

Weight Vertical stratigraphic separation rate (mm/yr) or (m/kyr) 0.10108 0.035 0.24429 0.042 0.30926 0.050 0.24429 0.059 0.10108 0.070.

Evidence of no measurable lateral offset of geomorphic features, combined with the evidence for vertical separation across a surface scarp at the range front and fold deformation on the gas field that is consistent with the presence of a blind thrust or reverse fault, demonstrates that the uplift of Rattlesnake Mountain in the Quaternary has been accommodated primarily by reverse slip on a fault that includes both emergent and blind splays that likely merge at depth (Appendix E, Section 7.3.3.1, PNNL, 2014). As discussed in Section 8.4.3.3 (PNNL, 2014), a small component of lateral slip is not precluded by these observations, and because the orientation of the Rattlesnake Mountain fault is favorable for some lateral slip, as suggested by the analysis of small-magnitude earthquake focal mechanism data (see PNNL, 2014, Appendix E, Section 6.0), oblique slip is given some weight [0.1]; based on the geologic evidence, reverse slip is given the majority of the weight [0.9].

The average Quaternary (post-Qaf4 fan) rate can be compared to long-term average post-CRB vertical separation rates based on the structural analysis and start times described in Sections 8.4.3.2, 8.4.3.5, and 8.4.3.6 of the Hanford PSHA report (PNNL, 2014). Based on the general location of the study area relative to the crestal profile of the entire Rattlesnake Mountain fold, it is reasonable to assume that the cumulative vertical separation of the Qaf4 fan in the central part of the study area is representative of the average post-middle Pleistocene separation (Figure 8.82, PNNL, 2014).

Summary In summary, characterization of fault sources associated with Rattlesnake Mountain, Saddle Mountain, Umtanum Ridge, and Yakima Ridge relied on measurements of

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 30 of 56 structural relief on the CRB as a key input for calculating a long-term net slip rate. This was accomplished along the eastern portions of the Umtanum Ridge and Yakima Ridge fault sources through the use of structure contour maps where the anticlines are partially eroded and buried. Structural relief measurements were performed at Saddle Mountain through the use of topographic profiles and geologic relationships from maps and boreholes. The structural relief for the Rattlesnake Mountain fault source was measured from topographic profiles of the CRB for long-term rates. The Quaternary slip-rate distribution outlined above is similar to but slightly lower than the long-term vertical separation rate based on post-CRB topography (619 m/6-10 Myr = 0.06-0.1 m/kyr) and the estimated rate (0.06 m/kyr) from the beginning of Saddle Mountains Basalt time based on thinning of basalt flows across the structure (Reidel et al. 1983).

The weighted combination of the long-term rate and the Quaternary slip rates are used to develop the full slip rate uncertainties given in the CDF for the Rattlesnake Mountain fault source. The comparison of the long-term slip rates based on post-CRB displacements with Quaternary displacements for those faults with such data (e.g.,

Rattlesnake Mountain), show that the rates are comparable. This provides confidence that the use of long-term rates for all of the fault sources is valid and technically defensible.

References Baker VR, BN Bjornstad, AJ Busacca, KR Fecht, EP Kiver, UL Moody, JG Rigby, DF Stradling, and AM Tallman. 1991. Quaternary Geology of the Columbia Plateau. In Quaternary Nonglacial Geology: Conterminous U.S., The Geology of North America, Volume K-2:215250, RB Morrison (ed.). Geological Society of America, Boulder, Colorado.

Bjornstad BN, PD Thorne, BA Williams, GV Last, GS Thomas, MD Thompson, JL Ludwig, and DC Lanigan. 2010. Hydrogeologic Model for the Gable Gap Area, Hanford Site. PNNL-19702, Pacific Northwest National Laboratory, Richland, Washington.

Fecht KR, SP Reidel, and MA Chamness. 1992. Contour Map of the Top of the Basalt of the Hanford Site. Scale 1:62500. Washington State University Open-File Map 2014-1.

(Contact Steve Reidel at Washington State University to obtain a copy.)

Miller AC and TR Rice. 1983. Discrete approximations of probability distributions.

Management Science 29:352362.

Myers CW, SM Price, JA Caggiano, et al. 1979. Geologic Studies of the Columbia Plateau - A Status Report. RHO-BWI-ST-4, Rockwell Hanford Operations, Richland, Washington.

PNNL (Pacific Northwest National Laboratory). 2014. Hanford Sitewide Probabilistic Seismic Hazard Analysis. Report prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830, and Energy Northwest, Report PNNL-23361, Pacific Northwest National Laboratory, Richland, Washington 99352.

Reidel SP, NP Campbell, KR Fecht, and KA Lindsey. 1994. Late Cenozoic structure and stratigraphy of south-central Washington. In R Lasmanis and ES Cheney (eds.),

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 31 of 56 Regional Geology of Washington State. Washington Division of Geology and Earth Resources Bulletin 80:159-180.

Reidel SP, RW Cross, and KR Fecht. 1983. Constraints on Tectonic Models. Chapter 5 (pp. 5.15.19) in DOE 1983, Preliminary Interpretation of the Tectonic Stability of the Reference Repository Location, Cold Creek Syncline, Hanford Site. RHO-BW-ST-19P, Rockwell Hanford Operations, Richland Washington.

Thorne PD, AC Rohay, and SP Reidel. 2014. Development of a Basin Geologic and Seismic Property Model Used to Support Basin-Effects Modeling for the Hanford Probabilistic Seismic Hazards Analysis. PNNL-23305, Pacific Northwest National Laboratory, Richland, Washington.

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RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 39 of 56 NRC RAI #3:

The recent paper by Casale and Pratt (2015) uses seismic reflection data to interpret faults in the subsurface in terms of a thin-skinned fault model that results in relatively high slip rates on the faults that comprise the Yakima Fold and Thrust Belt. Although the thin-skinned model was considered during the SSHAC process for the Columbia site, it was not adopted in the PSHA (SSHAC Report Section 4.1).

Consistent with the 50.54(f) letter and the SPID guidance, please provide the following information:

Discuss the potential significance to the PSHA at the Columbia site of any new information in Casale and Pratt (2015) that was not considered in the SSHAC process, especially with regard to the proposed range of slip rates presented in the paper.

Energy Northwest Response to RAI #3:

The Casale and Pratt (2015) paper seeks to test competing models for the subsurface fault geometry of the Yakima Fold Belt (YFB)specifically shallowly rooted versus deeply rooted fault systems. In particular the paper discusses seismic reflection data, borehole logs, and surface geologic data to test two proposed kinematic end-member models incorporating thick- and thin-skinned listric faults beneath the Saddle Mountains anticline of the YFB. The paper also discusses the use of the YFB as an analog to evaluate kinematic and geometric models of wrinkle-ridge structures on other terrestrial planets.

The key data set used to constrain possible end-member structural models is a proprietary seismic profile across the Saddle Mountains anticline (SMA) purchased by the U.S. Geological Survey (USGS) with limited publication rights. Dr. Pratt initially showed this line during the SSHAC Workshop 1 (July 23, 2012) presentation that described available seismic reflection data. He also presented this line and discussed preliminary interpretations of the line and limitations of the data and uncertainties for structural interpretations and modeling of the data during the SSHAC Workshop 2 (December 3, 2012). The schematic diagrams showing the two end-member models (published image shown on Figure 2) were presented to the TI Team during this workshop. A preprint (version 10A) of the Pratt (2012) paper, which included a figure showing the same uninterpreted and interpreted seismic line, also had been made available to the TI Team. Therefore, while there are some new or slightly different interpretations and more discussion of the interpretation presented in this paper, there are no new data that the SSC TI Team has not seen or evaluated as part of the development of the Hanford SSC model.

The estimated slip rates, particularly the high rates inferred for the thin-skinned model, are not considered to be technically defensible based on evaluation of the structural relationships and timing information presented in the paper. A discussion of the limitations of the data, non-uniqueness of structural interpretations, and inconsistencies in the end-member models presented by Casale and Pratt (2015) are discussed in the following sections.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 40 of 56 Limitations of the Available Seismic Reflection Data As noted by Casale and Pratt (2015, p. 746), the Columbia River Basalt Group (CRB) form notoriously difficult terrain in which to carry out subsurface imaging, and the profile is one of few in the region that shows reflectivity below a few kilometers depth. They also note in regard to processing of the seismic profile across the SMA that Despite extensive acquisition and processing efforts, subsurface reflections remained weak.

With regard to direct imaging of deep structure, the following limitations are noted:

• Seismic reflectors are not imaged below a depth of ~7 km.

• The thrust or reverse fault beneath SMA is not directly imaged by the seismic profile; in particular the location and geometry of the deep detachment, which lies below the limit of data interpretability, is not supported by any direct observations.

Kinematic Modeling and Limitations A main conclusion of the paper is that soling of the fault beneath the SMA into a decollement horizon, resulting in a listric geometry, is necessary to satisfy kinematic constraints. The statement on p. 749 that the combination of a shallow dip of the backlimb of Saddle Mountains anticline and a steep fault near the surface requires a listric fault geometry at depth is not correct. The fold profile can be reproduced with a planar, non-listric fault based on a consideration of elastic dislocation modeling conducted in analogous locations (see Section 8.4.3.4 of the Hanford PSHA report

[PNNL, 2014] and the Response to RAI #2b).

The resolution and continuity of reflectors at depth are poor as noted above, and can support multiple interpretations. In particular, the dip of reflectors associated with sub-basalt stratigraphic units may have been produced by completely different faults during a different tectonic regime, and not necessarily an earlier version of the same regime that produced the YFB as argued by the Casale and Pratt paper. In fact, Casale and Pratt do not cite existing papers (e.g., Campbell, 1989; Wilson et al., 2008) on pre-CRB deformation of the Columbia Plateau that provide alternative models for how the older layered rocks may have been tilted.

The modern topographic relief in eastern Washington, which has been shown to be a good proxy for post-CRB structural relief (Section 5.1 of Appendix E to PNNL [2014]),

provides the best constraint to use to assess the viability of alternative structural geometries and kinematic models for late Cenozoic faults.

Both of the models presented in the image shown in Figure 2 of the published paper (should be Figure 3; the images for Figures 2 and 3 appear to be reversed) assume perfectly horizontal layered bedding prior to fold deformation. This does not accurately represent the documented regional southward gradient of the CRB section across the Palouse Slope (Burns et al., 2011; also Figures 5.1, 5.2, and 5.3 in Appendix E to PNNL, 2014). This gradient must be included to accurately model any post-CRB deformation between Saddle Mountains and Frenchman Hills (per model 1, below)

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 41 of 56 based on changes in elevation of the base of the CRB. Additionally, the CRB is shown as maintaining uniform thickness over the Saddle Mountain anticline in both models, implying that all fold growth postdates the youngest CRB flows. This is not consistent with data in Reidel (1984) that documents Miocene fold growth and thinning of the CRB section over the crest of the SMA.

Despite statements in the paper (p. 748) that the two end-member models were tested by iterative kinematic forward modeling to make balanced restorable cross sections of the SMA, no restored models are presented to demonstrate that the fault geometries, displacements and pre-deformed stratigraphy are viable and admissible. As noted below, some model displacements cited in the text cannot be reconciled with the figures.

Model 1: Thin-skinned Model This model is described as the low-angle thrust, two-decollement kinematic model.

Inconsistencies in the post-CRB structural relief predicted by this model that are used to assess long-term slip rates are described as follows:

• The shallow detachment at the base of the CRB (~ 4 km depth) in model 1 ramps up to the surface where the Saddle Mountain fault is commonly mapped. Because the 10.5 Ma Elephant Mountain basalt is present at the crest of the Saddle Mountains anticline south of the river, and exposed in the bank north of the river, total structural relief of the upper CRB units across the fault is directly measured to be about 335 m (see Table 8.9 in PNNL, 2014). For a fault dipping 30°, as measured directly from Figure 2, total dip slip required to generate the observed relief is about 670 m. The estimated heave value (i.e., the horizontal component of fault motion) should be about 580 m, not 350-450 m as stated in the text. The text states that the upper fault may dip as steeply as 35°; if so, then maximum slip to generate the observed 335 m of structural relief is about 584 m and associated maximum heave is about 478 m. In either case, its not clear where the slip estimate discussed in the text comes from because its not consistent with the fault dip shown in the figure or discussed in the text.

• The paper states that 3.2 km of post-CRB heave are required on the lower (9-10 km) detachment. The basis for this estimate is not clear as the figure in the paper shows about 2 km of heave or less as measured by offset of the base of the CRB in the plane of the fault. As discussed previously, syn-CRB fold growth produced thinning of the CRB over the crest of the anticline (Reidel, 1984), which is not represented in the model. Evidence for the syn-CRB deformation described by Reidel (1984) can be inferred from inspection of the seismic reflection profile in the paper (Figure 3);

there is about 800 m to 1000 m of structural relief on the base of the CRB over the crest of the SMA relative to the region south of the anticline, in contrast to the 335 m of structural relief on the 10.5 Ma Elephant Mountain Member of the CRB observed over the anticline crest observed at the surface in the northern Saddle Mountains.

The modern structural and topographic relief of the Yakima folds in the Hanford region and central Columbia Basin is primarily expressed by uplift and folding of the

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 42 of 56 10.5 Ma Elephant Mountain Member (Reidel et al. 1994). Because this stratigraphic unit represents an originally sub-horizontal datum across a large area of the YFB (particularly the eastern Yakima belt and the Hanford Site), it represents a more appropriate datum for evaluating late Cenozoic deformation rates than the much older and complexly deformed base of the CRB. Casale and Pratt do not discuss how the measured heave on the basal CRB units relates to deformation of the youngest CRB units (in particular the Elephant Mountain Member) that are not imaged on this line.

• The lower fault in model 1 projects to a point on the surface about 9 km north of Saddle Mountains. There is no known geologic structure at this location. Although Casale and Pratt speculate that the lower thrust may form the Frenchman Hills (p.

748), the surface projection of this lower thrust is about 9 km south of the Frenchman Hills. If it is assumed that there is 3.2 km of post-CRB heave on a 25° fault (as shown in the thin-skinned kinematic model), then there should be about 1.49 km of structural relief on the CRB north of the Saddle Mountains. For comparison, the estimate of structural relief on the CRB across the Frenchman Hills anticline, which is the largest and most likely structure on which to resolve this amount of slip, is about 155 m (average) to 180 m (maximum) (see Figure 8.72 of PNNL, 2014).

Model 2: Thick-skinned listric-reverse model.

This model is described as the listric reverse fault, single decollement kinematic model.

A dip of about 54° is used to model the upper portion of the fault; this dip is consistent with the interpretation of an absence of a fault cutting the BN1-9 borehole. Casale and Pratt (2014 p. 746) note that this geometry, in contrast to the shallow dips of Model 1, is more consistent with potential field modeling that indicates more steeply dipping, deeply penetrating faults forming the YFB anticlines.

Casale and Pratt state that timing and near-fault total slip were determined using the near-fault reflectors in the deepest part of the section and the base-CRBG reflectors and surface geometry in the shallow part of the section. (p. 749). Some of the same concerns and issues regarding the lack of documentation and displacements used to estimate long-term slip rate noted for model 1 above also pertain to this model.

Specifically, the paper states that about 600-800 m of total decollement heave (translated directly as slip on the dipping ramp) occurred on the fault after deposition of the CRB. If this heave is accommodated as slip on the 54° ramp, then this predicts about 826 m to 1,101 m of structural relief on upper CRB units like the Elephant Mountain Member, which is greater than the observed 335 m of relief on this unit across the Saddle Mountains.

Discussion Most of the slip rate estimates in the Casale and Pratt discussion section fall in the hundredths (0.01) mm/yr range, which is similar to the range of dip slip values used to model the Saddle Mountain thrust fault and many other structures in the YFB for the

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 43 of 56 Hanford PSHA model (PNNL, 2014). The Casale and Pratt post-CRB rates represent long-term averages of estimated displacements of the CRB units over the past ~8.5 Ma:

these rates are very similar to rates expected for several hundred meters of structural relief accumulating over 6-10 million years using the Hanford PSHA structural analysis approach (PNNL, 2014, Sections 8.4.3.2 and 8.4.3.5).

Higher dip-slip rates of tenths (0.1) of mm/yr suggested by Casale and Pratt (2015) stem from two questionable assumptions. The first assumption giving rise to estimated higher rates of 0.1 to 0.23 mm/yr, is that the amount of slip needed to lift the Ringold Formation found on the crest of the SMA from the base of the anticline occurred after 3.5 Ma. As outlined in Section 8.4.3.5 of the PSHA report (PNNL, 2014, p. 8.122-8.124) the following observations indicate that folding was concurrent with deposition of the Ringold Formation:

• Observations cited in Appendix 2.5N of the Energy Northwest Final Safety Analysis Report (1998) indicate that bedding dips gradually decrease upsection within the Ringold Formation, suggesting that regional deformation and folding was occurring during the late Miocene while the Ringold was being deposited.

• Observations during field reconnaissance for the PSHA study (see Appendix E of PNNL, 2014) reveal a modest angular unconformity between the Ringold Formation and the Elephant Mountain basalt on the backlimb of the Saddle Mountains anticline, and possible thinning of the Ringold Formation across the crest of the fold, suggesting that folding began prior to, and/or was coeval with, local deposition of the Ringold Formation. Similar relationships are shown in a series of cross sections across the Gable Mountain structure developed by Bjornstad et al. (2010) from correlation of borehole data. Specifically, these cross sections indicate uplift, folding was occurring, and incision of the CRB units beneath the Hanford Site prior to deposition of basal Ringold Formation strata between about 8.5 Ma and 5 Ma.

Also, the higher rate for the thick-skinned listric fault (0.23 mm/yr) is based on both the 800 m of heave (which, as noted above, is in excess of what can be inferred from the structural relief), and a conservative age for the onset of deformation (3.5 Ma).

The second exception comes from the use of 3.2 km from the deeper fault in model 1, which yields the long-term rate to be as much as 0.4 mm/yr. As discussed above, the origin of 3.2 km of slip is unknown because it is not discernable in the model (Fig 2a/3a). More importantly, however, there are no data to support this inferred offset of the CRB on a deeper fault between the Saddle Mountains and Frenchman Hills. It appears to be based exclusively on the assumption that the deep tilted reflectors below the CRB are in the hanging wall of an inferred late Cenozoic listric fault. If it is assumed that this deformation would be expressed at the surface as relief in the CRB (like every other active structure in the YFB), then the absence of any deformation or topography between the Saddle Mountains and Frenchman Hills is evidence against this model and the associated slip rate. Even if is assumed that the fault really exists and is somehow structurally linked to the Frenchman Hills, then the observed structural relief across

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 44 of 56 Frenchman Hills anticline is an order of magnitude less than that predicted by the model.

A primary conclusion of the paper is that both end-member models require decollement slip between 7 and 9 km depth. There is no direct evidence of this postulated decollement, and alternative models incorporating planar faults are capable of replicating the observed fold geometry. The lower decollement in the thin-skinned model predicts the presence of a significant fold in CRB in the region between the Saddle Mountains and Frenchman Hills where available mapping and subsurface data show clear evidence for the absence of such a structure. The listric geometries inferred for both the thin-skinned and thick-skinned kinematic models and inferred amounts of slip on the decollement(s) produce amounts of post-CRB topographic relief on SMA that is clearly in excess of what is observed. For these reasons, the listric fault geometries and subhorizontal decollements presented in this paper are not considered to be technically defensible interpretations.

References Burns, ER, DS Morgan, RS Peavler, and SC Kahle. 2011. Three-dimensional model of the geologic framework for the Columbia Plateau Regional Aquifer System, Idaho, Oregon, and Washington. U.S. Geological Survey (USGS) Scientific Investigations Report 2010-5246, USGS, Tacoma, WA. 44 p.

Casale G and TL Pratt. 2015. Thin- or thick-skinned faulting in the Yakima Fold and Thrust Belt (WA)? Constraints from kinematic modeling of the Saddle Mountain anticline. Bulletin of the Seismological Society of America 105(2A):745-752.

doi:10.1785/0120140050.

PNNL (Pacific Northwest National Laboratory). 2014. Hanford Sitewide Probabilistic Seismic Hazard Analysis. Report prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830, and Energy Northwest, Report PNNL-23361, Pacific Northwest National Laboratory, Richland, Washington 99352.

Pratt TL. 2012. Large-scale splay faults on a strike-slip fault system: The Yakima folds, Washington State.: Geochim. Geophys. Geosyst. 13: Q11004. doi:

10.1029/2012GC004405.

Campbell, NP. 1989. Structural and stratigraphic interpretation of rocks under the Yakima fold belt, Columbia Basin, based on recent surface mapping and well data. In Volcanism and Tectonism in the Columbia River Flood-Basalt Province, SP Reidel and PR Hooper (eds.), Special Paper 239, pp. 209-222. Geological Society of America, Boulder, Colorado.

Wilson, MS, TS Dyman, and SM Condon. 2008. Evaluation of Well-Test Results and the Potential for Basin-Center Gas in the Columbia Basin, Central Washington. In Geologic Assessment of Undiscovered Gas Resources of the Eastern Oregon and Washington Province. U.S. Geological Survey Digital Data Series DDS-69-O, ch.4, 12 pp.

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RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 46 of 56 NRC Request #4:

The SHSR describes the process used in site response calculations. The response states that "At some frequencies, the calculated site amplification for high base rock amplitudes is less than the minimum value of 0.5 recommended by the SPID (EPRI, 2013a). The 0.5 limit is not imposed here in the calculation of the surface hazard because the intended purpose of this report is to obtain the best estimate of the mean and fractile levels of the seismic response for plant risk assessment with no added conservatism." The SHSR states that mean amplification values for a frequency of 100 Hertz that range from 0.339 at a rock spectral acceleration corresponding to an annual exceedance frequency of 1 E-4 per year to 0.179 at an annual exceedance frequency of 1 E-5 per year.

Consistent with the 50.54(f) letter and the SPID guidance, please provide the following information:

a. Additional detail to describe your basis, including historical records from sites with reasonably analogous characteristics, that such de-amplification is plausible.

b. Further justification for not implementing the 0.5 limit, and demonstrate the impacts of not implementing the 0.5 limit on the final ground motion response spectra values and the control point hazard curves, which will be used for the seismic risk evaluation.

Energy Northwest Response to RAI #4a:

Site amplification is dependent on the definition of the rock or reference condition.

Section B-5.1.4.1 of the EPRI SPID recommends a minimum site amplification of 0.5 and cites EPRI (1993) and Abrahamson and Silva (1997) as justification. There is no recommendation or discussion of a minimum site amplification in Abrahamson and Silva (1997). Site amplification is discussed within Chapter 6 of the EPRI (1993) report. In this Chapter, a minimum value of 0.60 is recommended. At the time, site conditions were based on simple descriptions (e.g., rock, shallow soil, and deep soil) and a conservative minimum amplification of 0.6 was recommended (Section 6.4.1 of EPRI, 1993). EPRI (1993) noted that this lower bound is in agreement with empirical site amplification observed in Western North America (WNA) ground motions, which were obtained from sites where the conditions are significantly different than the reference hard-rock conditions for the CGS site.

Figure 1 shows the EPRI (1993) comparison of empirical WNA site amplification with respect to WNA rock reference to analytical site response with an Eastern North America (ENA) hard-rock reference. The VS for the WNA rock is not given. However, empirical GMPEs developed at the time often used simple site classes of rock and soil (e.g., Abrahamson and Silva, 1997; Sadigh et al., 1997) and the sites in the rock class for those GMPEs have estimated average VS30 values in the range of 520 to 550 m/s (Abrahamson and Silva, 2008; Chiou and Youngs, 2008). This suggests that the average VS for the WNA rock sites in the EPRI (1993) comparison is in the range of 500 to 600 m/s. The ENA hard-rock velocity used by EPRI (1993) for the comparison shown

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 47 of 56 on Figure 1 was 1,830 m/s. The reference condition at the CGS site (Vs = 3,000 m/s and 0 = 0.020 s) is at a larger velocity than the velocity of the EPRI (1993) ENA hard-rock.

The analytical hard-rock site response analysis predicted greater deamplification at high frequencies than the empirical soft-rock site amplification. A minimum of about 0.3 is observed in the hard-rock site amplification for the case shown on Figure 1. EPRI (1993) noted that the lower analytical amplification factors at higher frequencies and higher loading levels may be due to the greater high frequency content for the ENA input hard-rock motions.

In the active tectonic portion of WNA, the definition of rock ranges from 550 to 1170 m/s. For these sites with typical velocity profiles, studies comparing analytical and empirical site amplification have consistently found analytical site response analysis provide similar estimates of amplification to that observed in ground motion data. As an example, Seyhan and Stewart (2014) compared empirical site amplification using the NGA-West2 database to analytical site amplification developed by Kamai et al. (2014).

Seyhan and Stewart (2014) computed intra-event residuals defined as the natural-log difference of the spectral acceleration of the observed ground motion relative to the spectral acceleration of the ground motion predicted on rock (Vs = 760 m/s), shown in Figure 2 below. The nonlinearity of the two datasets (analytical and empirical) was examined by Seyhan and Stewart (2014) by comparing the f2 term derived from the two datasets. The comparison between the empirical NGA West2 dataset and the Kamai et al. (2014) results are presented in Figure 3. Note that in this figure the Kamai et al.

(2014) study is abbreviated as KEA14. Seyhan and Stewart (2014) conclude:

The simulation-based slopes are comparable to the data-based slopes, except for 5%

damped pseudo-spectral accelerations (PSAs) at T=0.5 to 3.0 s, where the data exhibit more nonlinearity than is evident from the simulations. These variations in slopes may reflect differences between the average soil properties (VS profiles and nonlinear relationships) at the NGA-West2 sites and those used in simulations, or could result from limitations of the1-D equivalent-linear analysis method.

While the conclusion is generally positive, there is some indication that between 0.5 and 3.0 s the f2 term from the NGA West2 empirical dataset is lower than the analytical simulations, which would result in more nonlinear behavior than predicted by the analytical results.

The level of high frequency content in ground motions is now typically parameterized in terms of the site kappa, 0. Al Atik et al. (2014) demonstrated that scaling a ground motion prediction equation from a 0 from 0.04s (typical of WNA rock sites) to a 0 of 0.02 s would result in an increase in the spectral accelerations at 20 Hz by a factor of about 2.5, as shown in Figure 4. These results indicate that ground motions for sites with 0 of 0.02s is expected to have significantly higher frequency contents at frequencies above 10 Hz than motions at typical WNA rock sites. These high frequency motions would then be affected to a greater extent by soil damping, leading to higher levels of site deamplification at high frequencies.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 48 of 56 Site amplification with values less than 0.5 is not limited to equivalent linear site response analysis methods. Hashash et al. (2008) computed site amplification for the Mississippi Embayment using time-domain nonlinear site response analyses. The spectral ratios for PGA, shown in Figure 5, clearly fall below a value of 0.5. Note, that these ratios are relative to the B/C boundary rock condition (760 m/s), and which is softer than the hard-rock horizon at the CGS site.

The CGS rock condition (defined by Vs and 0) differs from typical soft-rock site conditions where strong ground motion data has permitted testing of analytical models.

Thus, it is not expected that the analytical site amplification computed for this hard-rock site or other sites like it will behave like a typical soft-rock site. Significant deamplification in the high frequency of the acceleration response spectrum, which are observed in the site amplification, are caused by shifting the peak from high frequencies (associated with a high velocity and low 0) to lower frequencies (associated with lower velocities and higher 0). This behavior is expected due to the characteristics of the input motion and site properties.

In conclusion, the analytical results developed by number of researches for sites where recorded motions are available using both equivalent linear and nonlinear methods demonstrate the adequacy of the analytical methods and points to the areas where differences are noted. The deamplification at high frequency has not been identified as a deficiency in these methods. For site conditions similar to CGS with a high velocity contrast between the rock and soil, no comparative data exists at high intensities.

However, given the good comparison between the predictions using analytical methods and empirical observation (e.g. Seyhan and Stewart, 2014) for typical WNA sites, the applicability of the method and adequacy of the results for CGS site is considered valid.

Lower amplification for soil sites compared to a high velocity, low 0 hard-rock reference site that observed empirically for soft rock WNA sites is consistent with current understanding of the processes that control site amplification.

References Abrahamson, N.A. & Silva, W.J. (1997). Empirical response spectral attenuation relations for shallow crustal earthquakes. Seism. Res. Lett., 68(1),94-127.

Abrahamson, N.A., & Silva, W.J. (2008). Summary of the Abrahamson & Silva NGA ground-motion relations. Earthquake Spectra, 24(1), 67-97.

Al Atik, L., Kottke, A.R., Abrahamson, N.A., & Hollenback, J. (2014). Kappa () Scaling of GroundMotion Prediction Equations Using an Inverse Random Vibration Theory Approach. Bulletin of the Seismological Society of America, 104(1), 336-346.

Chiou, B. S.-J., and Youngs, R. R. (2008). An NGA model for the average horizontal component of peak ground motion and response spectra, Earthquake Spectra 24(1),

173215.

Electric Power Research Institute [EPRI] (1993). Guidelines for Determining Design Basis Ground Motions, Report No. TR-102293s-V1-V5.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 49 of 56 Hashash, Y.M., Tsai, C.C., Phillips, C., & Park, D. (2008). Soil-column depth-dependent seismic site coefficients and hazard maps for the upper Mississippi Embayment. Bulletin of the Seismological Society of America, 98(4), 2004-2021.

Kamai, R., Abrahamson, N. A., & Silva, W. J. (2014). Nonlinear horizontal site amplification for constraining the NGA-West2 GMPEs. Earthquake Spectra, 30(3),

1223-1240.

Sadigh, K., Chang, C.-Y., Egan, J.A., Makdisi, F.I., and Youngs, R.R. (1997).

Attenuation relationships for shallow crustal earthquakes based on California strong motion data, Seismological Research Letters, 68(1), 180-189.

Seyhan, E., & Stewart, J.P. (2014). Semi-empirical nonlinear site amplification from NGA-West2 data and simulations. Earthquake Spectra, 30(3), 1241-1256.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 50 of 56 Figure 1: Comparison between empirical WNA site amplification and analytical ENA site amplification presented in Section 6.5 of EPRI (1993, Figure 6-38).

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 51 of 56 Figure 2: Empirical site amplification reported by Seyhan and Stewart (2014).

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 52 of 56 Figure 3: Comparison by Seyhan and Stewart (2014) of the empirical (NGA-West 2 data) to the Kamai et al. (2014, labeled KEA14 in the figure) data.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 53 of 56 Figure 4: Spectral scaling factors computed by Al Atik et al. (2014).

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 54 of 56 Figure 5: Spectral ratios computed by Hashash et al. (2008) as a function of thickness of the Mississippi Embayment.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 55 of 56 Energy Northwest Response to RAI #4b:

A sensitivity analysis was performed to evaluate the effect of maintaining the minimum amplification values of 0.10 and 0.50 on the results. Figure 4 shows the results corresponding to no minimum requirements imposed to two other cases, where the minimum of 0.10 and to 0.50 are imposed. The primary case (no minimum imposed) is the same as the results when the minimum of 0.10 imposed. However, when the minimum of 0.50 is imposed, the results deviate from the primary case starting from 10 Hz with differences increasing with increased frequency.

Conclusion The literature survey clearly indicates the applicability of the analytical methods for site response analysis with satisfactory correlation with recorded motion. The studies by various researches using both equivalent linear and nonlinear methods confirm this observation while pointing to reasons for differences observed. The site amplification at high frequency has not been identified as a deficiency in the analytical methods. In addition the trend in the results for high frequency response from low rock velocity and high kappa sites to high velocity low kappa regions is reasonable in terms of frequency shifting of the peak response and the resulting deamplification in high frequency.

Furthermore, it should be noted that calculation of GMRS is intended for seismic PRA and not for design. The calculation must be intended to develop mean responses without being conservatively biased. Given the available information regarding the minimum amplification and the intended use of the results, we plan to maintain the GMRS with no minimum amplification imposed for seismic PRA application.

RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ASSOCIATED WITH NEAR-TERM TASK FORCE RECOMMENDATION 2.1, SEISMIC REEVALUATIONS Attachment Page 56 of 56 Figure 4: Sensitivity of GMRS to Maintaining Minimum Spectral Amplification Values.