Seismicity, active faults, stress patterns, and rupture processes in the Hualien region, Taiwan, investigated using the 1990 Hualien earthquake sequence

Abstract

The Hualien mainshock–aftershock sequence from December 1990 to January 1991 was used to investigate seismicity, active faults, stress patterns, and complex rupture processes in the Hualien region of eastern Taiwan. A combined data set, compiled from observations by the regional seismic network (Taiwan Telemetered Seismograph Network, TTSN) and a dense aftershock monitoring array (Hualien Temporary Seismic Network, HTSN), provided high-quality seismic data. Using these data, earthquake source parameters could be well constrained by the joint hypocenter determination (JHD) and double-difference relocation methods. These two relocation methods presented similar results and produced improved estimations of earthquake location. Station corrections determined by JHD showed significant lateral velocity variations across the Yuehmei fault. The relocated seismicity indicated that a previously known active fault, the Chimei fault, extends to offshore. Earthquake relocations also revealed a northwest dipping seismicity probably related to the Chimei fault. In addition, three sets of focal mechanism solutions are presented and their discrepancies are attributed to the complex rupture process. At first, earthquakes were initiated by thrust motions. Later, the rupture pattern changed to strike–slip motions. Variations in slip may reflect the fault geometry or rupture boundary conditions in the region. The observed stress patterns are considered to relate not only to the regional collision and mountain-building processes, but also to active faults in local structures.