Source geometry and slip distribution of the April 21, 1935 Hsinchu-Taichung, Taiwan earthquake

Abstract

The disastrous Hsinchu-Taichung earthquake (magnitude M GR = 7.1) occurred on April 21, 1935. The earthquake was known to be associated with two faults: the Tuntzuchiao oblique right-lateral strike-slip fault and the Shihtan reverse fault. Preliminary studies for this earthquake have been done. At present, however, the source geometry and slip distribution are still not clear. In this study, the geodetic observations of coseismic ground deformation for this earthquake are modeled using the finite-element method to infer its source parameters and to deduce the mechanism of its fault rupture. We determined the length, width, and offset of the fault, the seismic moment, and the stress drop of this shock series and proposed that the Hsinchu-Taichung earthquake sequence was actually associated with three main faults—the Shihtan fault, the Tuntzuchiao fault and the fault of the July 17 aftershock. The Shihtan fault is a high-angle reverse fault of a curved plane geometry, with a dip angle decreasing continuously with depth. The major fault slips occurred in the central part of the fault plane. The fault length, width and average slip are 25 km, 11 km and 1.94 m, respectively. The Tuntzuchiao fault is a high-angle, right-lateral, strike-slip fault dipping southeast. The fault length, width and average slip are 15 km, 6.2 km and 1.9 m, respectively. The July 17 aftershock was associated with a pure left-lateral strike-slip fault, dipping at 60° toward southwest having a fault length of 12.5 km, a width of 5.8 km, and a uniform slip of 1.0 m. The estimated seismic moments of these faults are 3.2 × 10 26 dyne cm for the Shihtan fault, 1.1 × 10 26 dyne cm for the Tuntzuchiao fault, and 4.4 × 10 25 dyne cm for the July 17 aftershock. The stress drops are 162 bar for the Tuntzuchiao fault, 90 bar for the Shihtan fault, and 88 bar for the July 17 aftershock. Our results indicated that the Hsinchu-Taichung earthquake is a special case of complex fault geometry and heterogeneous slip distribution. Furthermore, from the fault rupture pattern and regional stress direction, we deduced that the motions on these faults were consistent with those under the regional tectonic stress in the SE-NW direction, and the ruptures of the Tuntzuchiao fault and these aftershocks along the Houlung River, including the July 17 aftershock, seem to indicate conjugate faulting.