Seismic Amplitude Measurements Suggest Foreshocks Have Different Focal Mechanisms than Aftershocks

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Science  07 Jul 1978:
Vol. 201, Issue 4350, pp. 56-59
DOI: 10.1126/science.201.4350.56


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.