SOURCE PARAMETERS OF AFTERSHOCKS OF THE 1991 COSTA-RICA AND 1992 CAPE-MENDOCINO, CALIFORNIA, EARTHQUAKES FROM INVERSION OF LOCAL AMPLITUDE RATIOS AND BROAD-BAND WAVE-FORMS

Source parameters of aftershocks of the 22 April 1991 (Mw = 7.7) Costa Rica and the 25 April 1992 (Mw = 7.1) Cape Mendocino, California, ear thquakes are determined using a grid search inversion of P, SH, and SV amplitude ratios recorded by sparse local networks of three-component broadband and short-period stations. The inversion procedure consists of computing synthetic seismograms for three fundamental fault orient ations for all source-receiver pairs over a range of source depths; ca lculating the complex envelopes of the observed and synthetic seismogr ams to determine peak amplitudes of P, SH, and SV waves; combining the fundamental fault amplitudes for all possible values of strike, dip, and rake, at 10 degrees increments; and determining the best fault ori entation and depth as the one that yields the smallest misfit between observed and synthetic P/SH, P/SV, and SV/SH amplitude ratios. The amb iguity in the sense of motion on the nodal planes, arising due to the use of amplitude ratios, is resolved by examining P-wave polarities. T he sensitivity of source parameters to uncertainties in earthquake loc ation and crustal structure is explored. For events with good station coverage, focal mechanism determinations are stable for a wide range o f assumed values of crustal structure, earthquake location, and depth. Source parameters for many of the largest events (M > 3.4) are also d etermined by inversion of broadband displacement waveforms using a sim ilar grid-search technique. Comparable results were obtained using bot h broadband waveforms and amplitude ratios. Focal mechanism solutions for 20 aftershocks of the Costa Rica earthquake reveal a complicated f aulting geometry, indicating active thrust, normal, and strike-slip fa ults in the back-arc of Costa Rica. The 1992 Cape Mendocino earthquake occurred at the intersection of the North American, Gorda, and Pacifi c plates. While the mainshock was associated with underthrusting of th e Gorda plate beneath the North American plate, fault plane solutions for 70% of the 38 largest aftershocks indicate that these events resul t from either motion between the Gorda and Pacific plates or from inte rnal deformation within the Gorda plate.