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The Relationship of Small Earthquakes to Strain Accumulation Along Major Faults in Southern California


Pechmann, James Christopher (1983) The Relationship of Small Earthquakes to Strain Accumulation Along Major Faults in Southern California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WC53-1X39.


Fault-plane solutions for recent small (ML ≤ 4.6) earthquakes in the central Transverse Ranges, California, were determined using an azimuthally-varying crustal model. The dominant type of faulting observed is reverse faulting on east-striking planes, which suggests a regional stress field characterized by north-south compression. Some strike-slip faulting also occurs. There is some indication that strike-slip earthquakes may be more common than reverse-slip earthquakes during episodes of crustal dilatation (Sauber et al., 1983). The rate of north-south crustal shortening attributable to small earthquake deformation during 1974-1976 is two orders of magnitude smaller than the 0.3 parts per million per year north-south contraction measured at the surface by Savage et al. (1978). The scatter in earthquake hypocenters and general inconsistency of focal mechanisms with geologically determined motions on nearby major faults indicate that the small earthquakes in this region are not associated with large-scale block movements along major fault systems. Rather, they appear to represent fracturing along random minor zones of weakness in response to the regional stress field, or alternatively, small-scale block movements that are below the resolution of this study. Earthquakes in the San Gabriel Mountains north of the Santa Susana-Sierra Madre-Cucamonga frontal fault system tend to concentrate near the eastern and western ends of the range where good evidence for late Quaternary movement along the frontal faults has been found. Seismicity is markedly lower north of the central section of the frontal fault system where evidence for late Qlaternary movement is lacking.

Digitally-recorded waveforms of ML 2.0-2.8 earthquakes that occurred in two small areas along the Imperial fault before and after it broke in the ML 6.6 Imperial Valley earthquake on October 15, 1979, were examined and compared. Eight preshocks (1977-1979) from a 4½ by 1½ km area centered 4 km southeast of the mainshock epicenter have strikingly similar waveforms over the entire record length (~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 within ¼ 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 ½ by 2 km source area centered along the Imperial fault 16 km northwest of the 1979 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 ≥ 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. Groups of like events are also present among the aftershocks in this data set. The average peak correlation for pairs of aftershocks, 0.43, is almost the same as that for pairs of preshocks, 0.45, if all 10 preshocks are included. However, several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations ≥ 0.6.

A search was undertaken for earthquakes with similar waveforms within two small (< 5 km) areas along the San Jacinto fault zone northwest of the Anza gap, but none were found. Focal mechanisms for most of the earthquakes in these two study areas agree very well with the faulting observed geologically at the surface. This, together with the concentration of hypocenters near the San Jacinto fault zone, suggests that small earthquakes are occurring along the main faults traces here, in contrast to the situation in the central Transverse Ranges.

The highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities. The observations above are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Geophysics
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Ingersoll, Andrew P.
Thesis Committee:
  • Kanamori, Hiroo (chair)
  • Allen, Clarence R.
  • Clayton, Robert W.
  • Harkrider, David G.
  • Ingersoll, Andrew P.
Defense Date:24 March 1983
Funding AgencyGrant Number
Record Number:CaltechTHESIS:02052013-085739683
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7462
Deposited By: Benjamin Perez
Deposited On:05 Feb 2013 17:27
Last Modified:09 Nov 2022 19:20

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