High-quality focal mechanisms within the Taiwan Seismic Network are determined utilizing first motions, SH/P amplitude ratios, and hypocentral locations obtained in a joint tomographic inversion. The small-to-moderate (2.7 ≦ ML ≦ 5.7) events, ranging in depths from 3 to 43 km, occurred in the area where mountain building is taking place. The data used were recorded between March 1991 and July 1994. Of the 97 earthquakes studied, the focal mechanisms of 23 of these determined on the basis of a 3D velocity model are remarkably different from the routine 1D results. In order to correct for the effect of attenuation on P- and S-wave amplitudes, QP and QS are determined using the spectral ratio method. QP ranges from 289 to 330 and QS from 201 to 301. The SH/P amplitude ratios are found to be very effective as constraints for choosing the best solutions among those consistent with the polarities. The resulting focal mechanisms show a high degree of variability across the island, but in terms of tectonic stress regimes, they show consistent patterns. They provide a glimpse of the mechanical processes under the Taiwan orogen.

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