Waveforms and spectra of preshocks and aftershocks of the 1979 imperial valley, California, earthquake: evidence for fault heterogeneity
We have compared digitally-recorded waveforms of M/sub L/ 2.0--2.8 earthquakes that occurred in two small areas along the Imperial fault before and after it broke in the M/sub L/ 6.6 Imperial Valley earthquake on October 15, 1979. Eight preshocks (1977--1979) from a 4 1/2 by 1 1/2 km area centered 4 km SE of the mainshock epicenter have strikingly similar waveforms over the entire record length (approx.30 s), with an average peak cross correlation between seismograms of 0.74. The seismograms are well correlated at frequencies up to at least 4 Hz. This implies similar source mechanisms and hypocenters with 1/4 of the 4-Hz wavelengths, i.e., <200--400 m. Five aftershocks from the same area show an average peak cross correlation between seismograms of only 0.23. Any associated changes in mechanism must be small because they are not reflected in the first motion data. Analysis of frequency content of these events using bandpass-filtering techniques showed no systematic temporal changes in spectral shape. Ten preshocks and 24 aftershocks from a 1 1/2 by 2 km source area centered along the fault 16 km NW of the mainshock epicenter were also studied. First motion data suggest that all of the aftershocks and a swarm of six preshocks on December 7--9, 1978, were associated with the main fault but that four earlier preshocks were not. The six preshocks on December 7--9, 1978, were tightly clustered, as evidenced by the strong similarity of the waveforms (most peak cross correlations > or =0.6). During this swarm the 8- to 16-Hz spectral amplitude increased relative to the 1- to 2-Hz spectral amplitude over the whole record length by about a factor of 3, suggesting a systematic increase in stress drop.
- Research Organization:
- Seismological Laboratory, California Institute of Technology, Pasadena, California 91125
- OSTI ID:
- 6250897
- Journal Information:
- J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 87:B13; ISSN JGREA
- Country of Publication:
- United States
- Language:
- English
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