ML20133G106
| ML20133G106 | |
| Person / Time | |
|---|---|
| Issue date: | 08/30/1984 |
| From: | Brocoum S Office of Nuclear Reactor Regulation |
| To: | Reiter L Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20132B198 | List:
|
| References | |
| FOIA-85-363 NUDOCS 8508080555 | |
| Download: ML20133G106 (7) | |
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AUG 3 01984 MEMORANdUMFOR:
Leon Reiter, Acting Chief Geosciences Branch Division of Engineering FRCM:
Stephan Brocoum, Leader Geology Section Geosciences Branch, DE
SUBJECT:
USGS BRIEFING ON THE RESULTS OF ONGOING NRC-FUNDED RESEARCH Attached is a sumary of the May 9,1984 meeting with the USGS in Reston in which the USGS presented progress reports on each of the NRC funded research tasks. The meeting was attended by S. Brocoum, I. Alterman, A. K.
Ibrahim, J. Kimball, P. Sobel and R. McMullen of the Geosciences Branch, each of whom contributed to this sumary.
g-Stephan Brocoum, Leader Geology Section Geosciences Branch, DE
Attachment:
As stated cc: w/ attachment L. Beratan
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M, en E. Zurflueh GSB Staff 8508080555 850624 PDR FOIA BELL 85-363 PDR
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GEOLOGICAL SURVEY BRIEFING ON THE RESULTS OF ONGOING NRC-FUNDED RESEARCH A briefing was given on May 9, 1984 at Reston, Virginia, by USGS researchers involved in NRC funded programs for representatives of the NRC earth sciences staff.
The meeting was to update the staff on recent developments in the research programs. A list of attendees and an agenda are attached as Enclosures 1 and 2, respectively.
R. Stewart described modifications that he has made in the Charleston earth-4 quake monitoring network to improve its efficiency, and the results of research that he has been doing there.
Currently, seismicity is cccurring in an area where there are gravity and magnetic anomalies and a substantial amount of relief on the basement surface.
Two mafic plutons and a NE-SW, down to the NW F
fault, have been identified from the geophysical data.
One of the areas of earthquake concentration is near the margin of one of the plutons, almost directly below Summerville, S.C., and near the NW edge of the 1886 epicentral ellipse.
A magnitude 3 earthquake in November, 1983, was strongly felt in Summerville, which is about 7 km north of the 1886 epicenter and directly above one of the plutons.
However, 10 km south of the epicenter, the earthquake was not felt at all.
The presence of the pluton is believed to have contributed to the earthquake being felt in Summerville.
Ray tracing has also been used to suggest that the presence of the pluton had a strong, positive effect on the ground motions.
This also suggests the plutons may affect hypocenter locations and fault plane solutions.
G. Gohn described in detail several liquefaction features that were formed during the 1886 Charleston earthquake and similar features that are interpreted to have formed during an earlier (pre-historic) earthquake.
One 1886 sandblow described was that which had been trenched by Talwani and Cox in June,1983.
Most of the features are located in the 10 Mile Hill area, within which the Charleston airport is located.
The general characteristics of the sand blows were described, and it was pointed out that the features that can be observed 08/29/84 i
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in cross section are mostly those that formed during stabilization after the earthquake.
Among the typical features are:
a circular surface lense, spread out over a surficial organic stained soil - black sandy clay; a small, rimmed crater, or caldera through which the liquefied sand was extruded; a funnel-shaped pipe and/or dikes of sand that extend downward to a source area, usually about 8 to 10 feet below ground surface.
Within the sand pipe are rotated blocks of soil from abcve or from the sides of the pipe.
The soil within the pipe is stratified in thin saucer shaped (plan and cross section) lamina that formed as the material filled the blow-out hole.
After this description, Gohn presented slides taken along a drainage canal cut, the whereabouts of which he would not disclose except to say the ditch is on the edge of the 1886 meizoseismal area.
The canal wall exposed about 8 feet of section through the Pleistocene soil.
Several liquefaction features were found alor.g this cut similar to the one described above.
However, on one feature, differences were observed that were interpreted to support a pre-1886
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origin for it.
The funnel was capped by a layer of humate soil, which obviously had accummulated after the event that caused the sand blow, and sand dikes from a later event cut across it.
The thin dikes of the second (later) event were projected downward to a source at depths between 15 and 20 feet.
This was done by comparing the mineralogy of the sand in the dikes and the known stratigraphy at shallow depths.that was derived from the study of Quaternary deposits that has been going on in the area since 1973.
Con ~ tent of mica was the most diagnostic correlating evidence.
Carbon-14 dating was performed on the organic material affected by the first generation (earlier) sand blow.
The age fell between 1600 and 2400 years ago.
Apparently the micaceous younger sand dike was forced upward in a parabolic pattern, struck the bottom of the very competent humate soil but was unable to break through it.
It did, however, separate and surround a block of the humate soil.
Anotner section of the dike cut through the first generation sand blow.
P. Thenhaus prcsented five seismic source zones used in the most recent Alger-missen probabilistic assessment of eastern U.S. seismicity.
He compared the 08/29/84 2
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first, which is the eastern decollement model, with:
(a) the reactivation of listric normal faults in the Great Basin; (b) the Ando model for eastern U.S.;
and (c) the Hatcher megashear model.
He concluded that evidence supports back-slipping along a detachment surface originally fermed by compressive thrusting.
The de'tachment source zone extends westward to the most western mappable thrust fault and eastward down to the Moho off the eastern coastline.
The second model, or Mesozoic rift source area, extends from the western margins of the western-most Triassic-Jurassic basins on the Piedmont out'onto the continental slope to the east.
The area is enlarged in the third model to include Iapetus (Early Paleozoic) rift zones mapped by R. Wheeler of the USGS.
This area would extend westward to the same boundary as the detachment source zone.
A fourth source zone model takes into account areas in eastern U.S. where uplift or subsidence are actively going on, such as Coastal Maine and the Adirondack Mountains.
The fifth source zone was defined by geomorphic consid-erations after the technique of John Hack (i.e., areas of epeirogenic, tectonic
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movement that has an effect on drainage patterns, etc).
D. Perkins briefly discussed the consequences of these models as reflected in the seismic proba-bility analysis.
He indicated that the USGS is. experimenting with attenuation models also.
R. Archuleta is supervising the installation and monitoring of several accele-rometers placed in drilled holes in Mammoth Lakes in the vicinity of the epicenters of four magnitude 6 carthquakes that occurred in 1980. The purpose of the experiment is t'o attempt to determine if thvre is attenuation of the higher frequencies (greater than 10 hertz) in ground motion in the upper 100 feet of soil or if it is source related.
The soil is mostly glacial till and is about 110 feet thick, overlying highly competent metamorphic rock.
Instruments have been placed at ground surface, at a depth of 120 feet in one hole at the soil / rock interface, and at a depth of 167 feet in rock in another hole. The accelerometers are calibrated up to 2.0 g and are flat in response j
to 100 hertz.
Shear wave velocities in the till range from 200 to 250 meters per second, and are 1600 meters per second in the bedrock.
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Archuleta then presented the results of his research on the 1980 Irpinia (Southern Italy) magnitude 6.9 earthquake.
The main shock occurred on a NW striking, NE dipping normal fault with a calculated 3 to 31/2 meters of dip slip displacement.
A magnitude 6.2 aftershock occurred 40 seconds later on a conjugate fault that strikes NE.
The events were well recorded on strong motion instruments and many accelerations of 0.3 g/0.2 g were recorded.
The USGS is developing maps of slip as a function of time and modeling velocity records.
The earthquake hypocenter was located at a depth of 16/18 km where the motion was dip slip.
Closer to ground surface slip was oblique with a right-slip component. An important initial finding to come out of this research is that str'ess drops during normal faulting may not be lower than those on reverse or strike-slip as previously thought.
It is a mystery though that there were no accelerations recorded above 0.3 g, but the lower accelera-tions may be because these events are deeper than most California earthquakes.
Substituting for Mark Zoback, Archuleta discussed the bore-hole breakout
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technique for determining the. direction of maximum horizontal compressive stress.
These breakouts are due to hydraulic fractures and occur in the direction of minimum stress.
Bore hole calipur tools and televiewers are being used to measure bore hole breakout in pre-existing holes in central and eastern United States.
He presented examples in the Illinois Basin in SE Illinois and SW Indiana where the maximum compressive stress is oriented N 71 E, and in NE Colorado where it is oriented N 9 W.
Significant changes in Zoback's original stress map of the U.S. are expected as a result of plotting bore hole breakouts in pre-existing holes in the eastern U.S., particularly in southeastern New York and northern New Jersey.
D. Boore talked on the subject of estimating high frequency ground motion in the eastern U.S.
He discussed his technique which uses-filtered random time series and an assumed source spectral shape, and has compared results of this to western U.S. attenuation equations.
The comparison is very good.
Application to the eastern U.S. will depend on whether eastern earthquake sources are the same as western sources, and how 'requency-dependent attenuation functions are l.
f modeled.
Currently the USGS is studying spectral shape as a function of magnitude and distance.
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Boore gave J. Boatwright's talk regarding the New Brunswick Earthquake studies.
They have found that at Miramichi there are pronounced site effects and high I
frequencies, which are different from results obtained in the western U.S.
There was also a significant difference in the stress drops between thar.
calcul~ated from the January 1982 events and the July 1983 events, and they are attempti,ng to determine the cause of this difference.
This work will assist the NRC in determining whether western U.S. strong motion records can be used to assess eastern U.S. earthquakes.
S. Hartzel spoke about subduction earthquakes in the Pacific Northwest in place of T. Heaton.
He stated that seafloor spreading adjacent to the Juan de Fuca plate has been taking place at the rate of 6 cm/ year for the last 15 million years, based on the age of basalts comprising the plate.
The convergence rate between the Juan de Fuca and North American plate is calcu-lated to be 3/4 cm per year.
Using a relationship developed by Ruff and Kanamori, a magnitude of 8.4 may be possible along the subduction zone, based on rate per year plotted against the age of the plata, and a comparison with other plates around the Pacific Ocean.
Data was also presented from studying other subduction zones around the Circum-Pacific belt.
The tectonics of the Washington-Oregon coastai area is thought to be similar to the southwestern Japan area, where there-have been several magnitude 8 earthquakes in the last 50 years and where it is relatively aseismic now.
The Washington-Oregon coastal area has not experienced large thrust earthquakes historically.
Young plates like southwestern Japan, Chile, and Juan de Fuca are composed of younger crust which is more-bouyant and provides better coupling with the overriding plate, their dips are shallow, there are no pronounced deep trenches, there are records of great thrust events (evidence for great thrust events in Washington and Oregon is lacking), and there is no back-arc spreading.
Relatively old oceanic plates subduct at steep dips, and they are characterized by well-pronounced, deep trenches, and back-arc spreading.
He stated that prior to the great Chilean earthquake of the 1950's, there had been relatively little seismicity, like Washington-Oregon today.
However, the convergence rate of the Juan de Fuca and North American plates is 1/3 of that of the plates in the region of Chile.
N. Ratcliffe presented essentially the same discussion he gave for the NRC staff on February 13, 1984.
He opened his talk by stating that faults are so 08/29/84 5
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numerous in the region that it is not likely earthquakes and faults will be shown to be related on a.one-to-one basis.
In spi.te of all the work that has been done in this region, no geologic evidence of post-Pleistocene movement on Mesozoic faults has been found, either in overlying Pleistocene strata or in the fabric within the faults themselves.
Earthquakes that are occurring in the area are as numerous in areas underlain by Precambrian rocks with no Mesozoic faults as they are in areas underlain by Mesozoic rocks and cut by Mesozoic faults.
In his view, the earthquakes are related to mylonite zones in the basement rock surrounding and beneath the Mesozoic basins.
Four seismic reflection lines have been run in the region:
three oriented NW-SE, perpendicular to the trend of the Mesozoic Basin, and one NE-SW, parallel to the basin.
Precambrian faults are shown to be low angle thrusts at depth that dip to the southeast at an angle of about 35.
Mesozoic faults, with the exception of the Thiells fault (Newark Basin border fault in southeastern New York), which dips southeast at an angle of.70*, are also low angle thrusts at depth.
A swarm of earthquakes that occurred in the Jersey Highlands in 1976 appear to be concentrated in a block of Precambrian rocks that lies between the Schooley Mountain Thrust on the bottom and the Flemington fault (Newark Basin border fault in that area) on the top.
Both of these faults are east dipping, low angle thrust faults.
Ratcliffe urged that records taken of this swarm of earthcuakes be analyzed to determine precise hypocentral depths and focal mechanism solutions.
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