SUMMARY
Conventional earthquake location methods depend critically on the correct identification of seismic phases and their arrival times from seismograms. Accurate phase picking is particularly difficult for aftershocks that occur closely in time and space, mostly because of the ambiguity of correlating the same phase at different stations. In this study, we introduce an improved Source‐Scanning Algorithm (ISSA) for the purpose of delineating the complex distribution of aftershocks without time‐consuming and labour‐intensive phase‐picking procedures. The improvements include the application of a ground motion analyser to separate P and S waves, the automatic adjustment of time windows for ‘brightness’ calculation based on the scanning resolution and a modified brightness function to combine constraints from multiple phases. Synthetic experiments simulating a challenging scenario are conducted to demonstrate the robustness of the ISSA. The method is applied to a field data set selected from the ocean‐bottom‐seismograph records of an offshore aftershock sequence southwest of Taiwan. Although visual inspection of the seismograms is ambiguous, our ISSA analysis clearly delineates two events that can best explain the observed waveform pattern.
On 26 December 2006, two M L = 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 SW to NE beneath southern Taiwan. Most of the relocated earthquakes are distributed just above the HVPZ or near and along the bottom of 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.
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