Fault-plane Identification and Focal Mechanism of the 2003 Chengkung Earthquake Sequence, at East Taiwan

Two subduction systems are taken place around Taiwan Island respectively to the north and to the south. In between, the arc-continent collision proceeds at east Taiwan along the Longitudinal Valley fault. The Chengkung Earthquake, MW 6.5 December 10 2003 at the southern part of the Longitudinal Valley fault, is a typical thrust reflecting release of stress/strain accumulated in the eastern collision framework. Such earthquake sequence contains a seismicity distribution roughly covered a region of 40 km by 40 km, which compose of a thrust fault dipping to the east. According to the focal mechanism of the main shock and the spatial distribution of aftershocks, the mainly crustal movement in the Chengkung earthquake can therefore be recognized as a rupturing with an east-dip plane and thrusting toward the west. However, 15 larger aftershocks occurred 7 days after the main shock showed a more complicated tectonic setting around this area. In the offshore area, three aftershocks illustrated normal-fault-type deformation, which is rarely observed in this area. On land, the larger aftershocks exhibited oblique thrust deformation along the Longitudinal Valley fault. In order to better understand the seismotectonic setting in this earthquake, a waveform inversion was applied to refine the focal mechanism solutions and identify the fault plane from one or multiple seismic recordings at short epicentral distance. Kanamori et al., (1990) and Singh et al., (1997) showed clear examples of local events for which source parameters could be well constrained by using near-field waves, even with a single station. However, when surface ruptures are not observed, as in the case of too small events or blind faults, the fault plane may be undetermined. The method used in this study includes the effect of source finiteness directly in the inversion process, allowing us to invert sparse, near-field data for focal mechanism and fault plane determination simultaneously. Seven parameters, including the strike, dip, rake, and dislocation, were explored with a grid search and minima of the misfit error between the observed and calculated seismograms. The seismograms considered here are in displacement integrated from three-component strong-motion record, organized by the Central Weather Bureau of Taiwan. We have identified three groups of aftershocks showing nearly identical focal mechanisms in the Chengkung sequence, which have initially compiled by the BATS (Broadband Array in Taiwan for seismology), Academia Sinica of Taiwan. Each of these groups have been examined their spatial attitudes of fault plane and refined their focal mechanisms. Our results show a segmented rupture at the southern part of the Longitudinal Valley fault, and a deformation accommodation as function of space can be inferred in the Chengkung sequence. In comparison with the historical earthquake taken place in the late 1951 at the almost location, the Chengkung sequence is probably the reactivated rupture along the same fault segment, suggesting that the temporal and spatial rupture pattern was repeated.