ML19347D292

Forwards Phase 2 Corrected Acceleration,Velocity & Displacement Traces for Three Largest Strong Motion Recordings from Accelerograph at Reservoir.Describes Methods Used to Calculate Stress Drop & Source Parameters

ML19347D292
Person / Time
Site:	Monticello
Issue date:	02/24/1981
From:	Fletcher J INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
To:	Jackson B Office of Nuclear Reactor Regulation
References
NUDOCS 8103110753
Download: ML19347D292 (19)

Text

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.L UNITED STATES

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DEPARTMENT OF THE INTERIOR g

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GEOLOGICAL SURVEY a

OFFICE OF EARTHQUAKE STUDIES Branch of Ground Motion and Faulting 345 Middlefield Road, Mail Stop 77 r-o y;

d Menlo Park, California 94025 E

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h3 February 24, 1981 W'-

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$BobJMkson NucleTr Regulatory Commission Geoscience Branch, Office of Engineering Washington, DC 20555

Dear Bob:

Enclosed are the phase II corrected acceleration, velocity, and

. displacement traces for what I believe are the three largest strong motion recordings from accelerograph at the Monticello reservoir.

Identifying the earthquakes that wrote these accelerograms is possible with some certainty for the record recovered during.the sunner of_1978 and the second record on the film recovered during the fall of 1978. Each of these records had relatively high peak accelerations and moments, and short S-wave minus -trigger times such that their identification with the two Mag =2.7 events during the proper time period appears relatively unanbiguous: Aug. 27,1978_10.23 UTC _ and Oct. 27, 1973 16.27 UTC

- respectively. The first record on the film recover ed during the fall is most likely the Oct. 27, 1978 07.26 UTC earthquake from a comparison of the magnitude and hypocentral distance of that earthquake with the moments and S wave minus trigger time of the accelero' grams.

The phase II output was calculsted at 500 pts./sec._ with the high and. low pass filtering at 2 and 50 Hz respectively. A Butterworth filter is used_ in both cases and.the roll-off is 24 db/ octave for

.the high-pass case and 48 db/ octave for the_ low-pass case.

Although the accelerations are 'quite high the displacements are all less than 1.5.mm reflecting the high frequency content of these records.

The table accompanying the data-shows the source parameters from the spectra using the Brune model. The estimates of the stress drop from' the two horizontal components vary by up;to a factor of

-20.

A factor of 5 ~is usually quoted as the likely uncertainty so that this kind of scatter -is unusual. As-the site is the same for NOB 11.0 h h

all records the variation appears to be due to multiple arrivals or higher-mode waves that are sensitive to the angle of i ncidence.

The 180 component for the Aug. 27, 1978 event is simple and does not exhibit any appreciable energy in secondary arrivals and is similar to theoretical pulses calculated in homogeneous half spaces. Thus its stress drop may reflect the stress drop at the source at least as far as the Brune model is valid.

In most of the work we are doing at the Geological Survey, when we try to calculate stress drop we try to average using at least five to eight stations for one event. Then an average moment is combined with an average source radius to get the stress drop.

Another method is to use the root-mean-square of the accelerations (Arms) that appears to be directly attributable to the faulting proass as noted in recent articles by Hanks and McGuire. The stress drop determined from Arms is more stable because an average is used rather than discreet points. As a consequence it is not clear that the resulting stress difference is equivalent to the Brune stress drop.

The pertinent data is given below for the stress determined for Arms for the 6 components:

l 2

3 Event Arms Fc T*

Ao Aug. 27, 1978 1023 180 102 8.5 1.55x16-3 30 90 82 8.5 1.55x10-3 24 Oct. 27, 1978 07.26 180 76 15 1.26x10-3 22 90 63 15 1.26x10-3 18 Oct. 27, 1978 16.27 180 56 5.5 5.4x10-4 2.7 90 87 5.5 5.4x10-4 4.3 fcm/sec 2

Hz

-3bars The average of the corner frequencies on each record was used as I believe the corner frequency in this case is used to estimate the duration of faulting at the source. A Q of 300 is assumed in computing T*.

Thus when one uses a stress drop formula to get moment one must be careful whether an average or peak value is needed.

Clearly more digital strong motion instruments need to be deployed around seismically active areas in the eastern United States if source parameters are going to be used in determining

. seismic risk.

2

I intend to publish the source parameters in the letter in my paper on the Monticello earthquakes, Sincerely,

(},ji)'~l{sdchrv J. B. Fletcher cc: Andrew Murphy, NRC Phyllis Sobel, NRC

-Robin McGuire, Fugro Inc.

Pradeep Talwani John Filson Roger Borcherdt Ted Algermissen 3

Source Parameters for 3 Monticello Events o - 2.9 gm/cc Mo = 4 wos3 noR/ Red s - 3.4 km/sec Re6 = 0.85 r = 2.34s TwTc-r=7 Mo 15 r4 s

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' August 27, 1978 10.23 180*

8.2 x 10-4 12.23 1.6 2.2 x 1020 103 88

,90*

2.3 x 10-3 4.86 1.6 6.2 x 1020 261 15 October ?7, 1978 07.26-180*.

4.4 x 10-4 24.0

.1.3 9.6 x 1019-54 267 90*

1.5 x 10-3 6.0 1.3-3.3 x 1020 217 14 October.27, 1978 16.27-

-180*

6.6 x 10-3 6.0 0.56 15.2 x 1020 217 26 90*

6.5 x'10-3 5.0 10.56 6.~1 x 1020 261 15 i

- 1 Lcm-sec-3. kilometers 5 meters

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