Publication Date:
1976-01-01
Description:
Seismic data recorded at Jamestown, California, provide P- and SV-wave spectral corner frequencies for 18 trans-Sierra Nevada events in the range 3.2≤ML≤4.0. These include most of the available earthquakes from 06 July 1974 to 31 August 1975. The ratio R of P- to SV-wave corner frequency was 0.96±0.17 for these events, which sample a wide range of source azimuths and focal mechanisms. Assumptions of a flat far-field source spectrum and frequency-independent attenuation in the 1-20 Hz band lead to not less than 480. Many recent source theories predict factor-of-two or more variation of R with azimuth, and significant deviation of its average value from unity; we conclude that such source models do not apply to these earthquakes. Previous studies of R by Molnar et at. (1973), Stump (1974), and Bakun et al. (1975), also based on excellent data, find different results. We interpret this discrepancy not as a contradiction, but as an indication that R may be an extremely useful source-model discriminant, its variation indicating that different models need be applied to different source regions. R may lead to a clearer understanding of the physical processes associated with earthquakes; for example, the only source models that predict a value of unity for R at all azimuths are those in which the corner frequency measures the time duration of motion at the source, thus indicating that this duration is as long as the rupture propagation time across the source. © 1976, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.
Print ISSN:
0022-3743
Electronic ISSN:
1884-2305
Topics:
Geosciences
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