Earthquake Swarm Recorded by an Ocean Bottom Seismic Array in Southwest Offshore of Taiwan in October, 2005

We conduct a detailed analysis of the seismic records for 406 marine earthquakes in SW offshore of Taiwan recorded by a small OBS array during 7 12 October 2005. The spatial distribution of the events, which included a complete main shock-aftershock sequence, show concentration in a vertical chain reaching 40 km in depth and that the deeper events slightly turns to the NE toward the Taiwan island. We use these recorded data to construct a 1-D layered velocity model for the local lithosphere. The model puts the Moho at around 13 km in depth and plate seismogenic thickness at about 40 km for the studied area. Our observations reveal that the local seismogenic structure is consistent with the character of the oceanic-origin crust and a non-subducting related mechanism. We also examine the polarizations of P-, Swave first arrivals and P-to-SV ratios in the waveform to obtain focal mechanism solutions for a total of 26 events. However, their solutions particularly show a non-unique deforming comportment for the strain releases, implying a strain rebound corresponding to the back-and-forth motion and also affected by the complexity of the pre-existing structure in the region.