abstract

The ratios of the amplitudes of SV to P, as recorded on vertical component seismographs near the earthquake, provide a means of determining the focal mechanism. The observed ratios are compared with values calculated on the basis of dislocation theory, with the effects of transmission across boundaries and incidence on the free surface approximately accounted for by plane-wave coefficients. A search is made for the strike and dip of the fault that provides a fit to as many of the stations as possible, for the direction of fault slip assumped. The direction of slip may also be treated as a free parameter to be determined, but the theoretical results are especially simple for pure strike-slip or pure dip-slip faulting. The technique has been applied to the analysis of events in three settings, in which pure thrusting, pure strike-slip, and pure normal faulting could be assumed. The utility and the limitations of the method are illustrated by these examples. The important limitations are that the method cannot distinguish between conjugate mechanisms or show the sense of slip on the fault. It also has a weak resolution of strike for some commonly occurring mechanisms, so that the solution may depend on the readings at only a few stations.

The method has been used to pick out a group of foreshocks to an Aleutian Islands earthquake and to provide a focal mechanism for an event in the Rhine graben, at a place where such information is lacking and needed. A study of 15 central California earthquakes revealed no changes in fault plane orientation or in relative attenuation of P and S waves during 1 yr prior to a magnitude 5 earthquake.

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