On 26 December 2006, two ML = 7.0 events occurred offshore south of Pingtung; one is associated with a normal-faulting and the other with a strike-slip faulting. The area where these earthquakes were located is not usually expected to have large earthquakes.We deployed 11 short period OBSs over the source zone for one week and recorded a series of aftershocks which were also recorded on land at the CWB network stations. The joint dataset made it possible for us to perform a 3-D velocity tomography and earthquake relocation in this region, where the velocity structures were not well known and location of earthquakes with only land data was uncertain.
The tomographic results show a prominent high Vp perturbation zone (HVPZ) that we consider as the uppermost mantle of the subducted plate dipping from SWtoNE beneath southern Taiwan.Most of the relocated earthquakes are distributed just above the HVPZ or near and along the bottomof a relatively low velocity subducted crust. Our results show that the subducted and bent Eurasian plate off SW Taiwan could have been unbent and become an upwards concave geometry for the upper 30 km. The main shock is near the bottom of the inflected surface. The distribution of the earthquake sequence generally displays in a NW-SE direction, coinciding with the plate convergence orientation between the Philippine Sea Plate and Eurasian Plate. This orientation also follows a relatively low Bouguer gravity anomaly stripe that is due to a heavy loading of the Taiwan orogen on the east-dipping Eurasian Plate. Considering that the hypocenter of the first main-shock is near the bottom of the aftershocks, we suggest that the first normal faulting earthquake was caused by an unbending effect in the subducting crust and this event triggered the release of accumulated energy between the Philippine Sea Plate and Eurasian Plate. Thus, we suggest that the rupture surface of the Pingtung earthquake sequence had propagated upwards and northwestward in the direction of plate convergence.