The Chichi earthquake sequence, Taiwan: results from source parameter and stress tensor inversions

The main shock of the Chichi earthquake took place along the north–south-trending Chelungpu fault between the coastal plain and western foothills of Taiwan. It consisted of five major sub-events with the rupture propagating from south to north. The largest seismic moment is located 30–35 km north of the initial rupture. Waveform inversion results indicate a low-angle thrust fault dipping to the east. The centroid depth is 11–12 km. The seismogenic patterns of aftershocks indicate that the Chichi earthquake sequence can be explained by westward block thrusting on the Chelungpu fault, with strike-slip motions at the northern and southern tips. A deeper seismogenic zone lies 15 km directly beneath the Chichi main fault plane, indicating that the deformation associated with the Chichi event involves deep crust and thus cannot be entirely explained by detachment at shallow crust levels. The result of stress tensor inversion is remarkably consistent with that derived from regional background seismic data before the Chichi sequence, and with the palaeostress pattern reconstructed from geological evidences. It is inferred that the stress regime corresponding to the regional collision in Taiwan is the same as that responsible for the occurrence of the Chichi earthquake sequence. Consequently, we propose to consider the Chichi earthquake sequence as a typical manifestation of the mountain-building process in Taiwan.