GO2-82-581, Forwards Info Re Max Average Rate of strike-slip Displacement on RAW & Re-Evaluation of Seismic Velocity Measurements, Per NRC Request

Forwards Info Re Max Average Rate of strike-slip Displacement on RAW & Re-Evaluation of Seismic Velocity Measurements, Per NRC Request

ML17276B550
Person / Time
Site:	Columbia
Issue date:	07/07/1982
From:	Bouchey G WASHINGTON PUBLIC POWER SUPPLY SYSTEM
To:	Schwencer A Office of Nuclear Reactor Regulation
Shared Package
ML17276B551	List: GO2-82-581, Forwards Info Re Max Average Rate of strike-slip Displacement on RAW & Re-Evaluation of Seismic Velocity Measurements, Per NRC Request ML17276B552
References
GO2-82-581, NUDOCS 8207280168
Download: ML17276B550 (10)

Text

REGULATORY

%FORMATION DISTRIBUTION S'M (BIDS)

AOCESSION NBR;8207280168 DOC ~ DATE: 82/07/07 NOTARIZED:

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'AUTH BYNAME AUTHOR AFFILIATION BOUCHEY<G,D, Nashington,Public

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"-RBC IP BYNAME RECIP IENT AFFILIATION SCHKENCERgA ~

Licensing Branch 2

SUBJECT; Forwards info re max average rate of strike slip displacement on RAN '8, "Re Evaluation of 'Seismic Veloci,ty Measur ementsr" per NRC requests DI'STRIBUTION 'CODE:

B001S

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Washington Public Power Supply System P.O. Box 968 3000 George Washington Way Richland, Washington 99352 (509) 372-5000 July 7, 1982 G02-82-581 SS-L-02-CDT-82-082 Docket No. 50-397 Mr. A. Schwencer, Chief Licensing Branch No.

2 Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C.

20555

Subject:

NUCLEAR PROJECT NO.

2

RESPONSE

TO INFORMAL REQUESTS FOR INFORMATION P

Enclosed are sixty (60) copies of the information on Maximum Average

'ate of Strike-Slip Displacement on RAW and Reevaluation of Seismic Velocity Measurements which were requested informally by the NRC.

Very truly yours, G.

D. Bouchey Deputy Director, Safety and Security CDT/jca Enclosure cc:

R Auluck -

NRC WS Chin

- BPA R

Feil

- NRC Site 8207280168 820707 PDR ADOCK 05000397 A

PDR

I 0

MAXIMUM AVERAGE RATE OF STRIKE-SLIP DISPLACEMENT ON RAW The data regarding the amount and timing of strike-slip displace-ment on RAW do not allow estimates of the present slip rate to be made with a high degree of confidence.

However, it is possible to place constraints on the average (long-term) lateral slip rate on the basis of assumptions regarding the known geologic relationships.

No individual stratigraphic or geomorphic markers have been shown to be laterally offset across RAW.

The observations that suggest a lateral component of strain along RAW are:

(l) horizontal straie that are present on fault surfaces within the Wallula fault zone, and (2) the northwest orientation of RAN relative to the inferred north-south orientation of the maximum compressive stress in the central Columbia Plateau.

The tectonic models that propose strike-slip displacement along RAW postulate a zone of dextral shear that is at the surface along the Wallula fault zone to the southeast and that becomes either deeper (Laubscher, Appendix 2.5-0) and/or broader and more diffuse (Davis, Appendix 2.5-N) to the northwest beneath the structures of CLEW.

There-fore, any assessment of the lateral slip along RAN northwest of the Wallula fault zone must be based on an assumed relationship between the surface deformation and inferred dextral strain at depth.

A major problem in assessing fault slip rates is using rates averaged over a long period of time to estimate present-day slip.

The assumption that slip rates are constant over long periods of time may not be correct; detailed studies of fault slip rates show temporal variations over intervals of thousands of years (e.g.,

Sharp, l980).

For the Columbia Plateau, some studies suggest that the rate of deformation has been declining during the Quaternary period (Davis, Appendix 2.5-L) and geodetic data suggest deformation has either stopped or is continuing at a low

rate (Savage et al., 1981).

Therefore, the present-day slip rate may be considerably less than that based on the deformation that has occurred during the past 14 m.y., which is the interval during which the Columbia River Basalts have been deformed.

In light of the uncertainties presented in the previous discus-sion, several estimates of the maximum average rate of strike-slip displacement along RAW are presented below.

A prominent north-south gravity anomaly has been identified (Weston Geophysical, 1981) that crosses CLEW without any apparent lateral deflection at about longitude 120'W.

Weston Geophysical (1981) estimates

that, based on the resolution of the gravity data, the cumulative offset of the anomaly can be no more than 3 km.

The anomaly is believed to be associated with a basement feature that predates the initial deposition of the Columbia River basalts (about 14 m.y. ago).

The slip rate that results from a 3

km dis-placement during a

14 m.y. period is 0.2 mm/yr.

Constraints, on the maximum cumulative offset may also be esti-mated by relating the pattern and amount of surface deformation to inferred dextral strain at depth.

For example, Laubscher (Appendix 2.5-0) develops structural and kinematic arguments for the Columbia Plateau that limit the lateral component of strain across CLEW/RAW to less than 2 to 3 km.

Assuming displacement along CLEW/RAW coincided with the initiation of Columbia River basalts about 14 m.y. ago (Davis, 2.5-N), the maximum average slip rate is 0.1-0.2 mm/yr.

Bentley (1980) and Laubscher (Appendix 2.5-0) present data on amounts of crustal shortening across the east-west trending folds of the Yakima fold belt. If it is assumed that all of the north-south shortening across the Yakima folds within CLEW occurs along the north-west striking CLEW/RAW as strike-slip faulting, then the cumulative strike-slip displacement can be estimated.

At longitude 120'W, the displacement is calculated to be 3.2 km (shortening 4.5 km) based on Bentley's data, and 1.5 km

(shortening 2.1 km) based on Laubscher's data.

Again, assuming a

14 m.y. time period, the resulting maximum average slip rates are 0.2 mm/yr and 0.1 mm/yr, respectively.

The following table summarizes the slip rate estimates:

Slip Rates Techni ue Data Sli Rate Reference N-S Gravity Anomaly (3

km 0.2 mm/yr Kinematic Analysis 2-3 km 0.1-0.2 mm/yr Horizontal Shortening 3.2 km 0.2 mm/yr Horizontal Shortening 1.5 km 0.1 mm/yr Weston Geo-physical (1981)

Laubscher (1981)

Bentley (1981)

Laubscher (1981)

Re ferences

Davis, G. A., 1981, Late Cenozoic tectonics of the Pacific North-west with special reference to the Columbia Plateau:

WNP-2, FSAR, Appendix 2.5N, 47 p.

Laubscher, H. P.,

1981, Models of the development of Yakima deformation:

WNP-2 FSAR, Appendix 2.5-0, 69 p.

Sharp, R.'.,

1980, Variable rates of late Quaternary strike slip on the San Jacinto fault zone, southern California:

U.S.

Geological Survey Open-File Report 80-95, 37 p.

Savage, J. C., Lisowski, M., and Prescott, W. H., 1981, Geodetic strain measurements in Washington:

Journal of Geophysical

Research,

v. 86, no. B6,'p.

4929-4940.

Weston Geophysical Corporation, 1981, Compilation and interpreta-tion of gravity in Washington,

Oregon, and adjacent parts of British Columbia and Idaho:

WNP-2 FSAR, Appendix 2.5-L, 28 p.