Focal mechanism determinations of the 1991 Chiali earthquake (M-L=5.7) sequence

The focal mechanisms of the 1991 Chiali earthquake sequence were investigat ed by considering the first motion data and available strong motion wavefor ms. Due to the sparse distribution of the first-motion data, the first-moti on solutions are multiple for the earthquake sequence. By considering the w aveform mechanism, which is determined by considering the amplitude ratio o f P, SV and SH waveform, as the constraint, we chose the optimum first-moti on mechanisms of the earthquake sequence. Either the optimum first-motion m echanisms or the waveform mechanisms of most of the aftershocks show thrust faulting mechanism. Except for the event E-3, the waveform mechanisms of t he earthquake sequence have a northeast-southwest striking plane similar to the distribution of aftershock sequence, while the optimum first-motion me chanisms of the earthquake sequence show more complex focal mechanisms. Com pared with the aftershock distribution in consideration of the waveform mec hanisms of the earthquakes, the fault plane with NE-SW strike and NW dippin g plane is suspected as the preferred fault plane. Since the location of th e earthquake is near the transition between the extension mechanism of sout hwestern Asian passive margin and the compression mechanism due to Luzon ar e collision, the thrust faulting mechanism of the Chiali earthquake might i ndicate an ongoing compression mechanism due to are-continental collision i n such an extensional environment. The b value of about 0.7 in this region implies a relatively stable tectonic environment compared with other region s in Taiwan. The larger ratio of the logarithm of cumulative seismic moment of aftershocks to that of the seismic moment of the mainshock for the 1991 Chiali earthquake (about 1) also suggests a smaller stress drop for this e arthquake is compared with other moderate size earthquakes in the Chianan r egion.