Source mechanisms and rupture processes of the 26 December 2006 Pingtung earthquake doublet as determined from the regional seismic records

摘要

The 26 December 2006 Pingtung earthquake doublet (both about M_w 7.0) were the two largest events with magnitudes close to 7.0, following the 1999 Chi-Chi earthquake. The locations and focal mechanisms of the Pingtung earthquake sequence provide good opportunities for scientists to investigate plate movement and deep tectonic structure in southern offshore of Taiwan. However, precise hypocenters and source mechanisms of this sequence are not definite due to poor station azimuthal coverage and a lack of information from historical seismicity in this area. Here we use regional ground motion full waveform records to investigate source rupture processes and determine fault planes of the Pingtung earthquake doublet. The finite fault inversion result of the first event shows two large asperities occurred near the epicenter. The rupture mainly originated from the south then propagated northward on an eastern dipping fault plane. The second earthquake is a bilateral rupture along NW-SE direction having a large asperity close to the epicenter which released most of its seismic energy within the first 15 seconds. The slips of the two earthquakes show compensative patterns when mapped on the earth's surface. We show that the Pingtung earthquake doublet are large normal and strike-slip faulting which dips toward the east and west, respectively. The mechanisms determined from inversion results infer that the first event might relate to the bending of the South China Sea Plate during its subduction to the Philippine Sea Plate. The second event could happen at a pre-existing weak zone that was triggered by transferred stress after the first event.