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Science 7 July 1978:
Vol. 201. no. 4350, pp. 56 - 59
DOI: 10.1126/science.201.4350.56
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Articles

Seismic Amplitude Measurements Suggest Foreshocks Have Different Focal Mechanisms than Aftershocks

ALLAN LINDH 1, GARY FUIS 2, and CONSTANCE MANTIS 3

1 U.S. Geological Survey, Menlo Park, California 94025, and Geophysics Department, Stanford University, Stanford, California 94305
2 U.S. Geological Survey, Seismological Laboratory, California Institute of Technology, Pasadena 91125
3 U.S. Geological Survey, Menlo Park, California 94025

The ratio of the amplitudes of P and S waves from the foreshocks and aftershocks to three recent California earthquakes show a characteristic change at the time of the main events. As this ratio is extremely sensitive to small changes in the orientation of the fault plane, a small systematic change in stress or fault configuration in the source region may be inferred. These results suggest an approach to the recognition of foreshocks based on simple measurements of the amplitudes of seismic waves.

Submitted on December 28, 1977
Revised on March 14, 1978


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