The role of a hidden fault in stress triggering: Stress interactions within the 1935 Mw 7.1 Hsinchu-Taichung earthquake sequence in central Taiwan

Hidden faults often lead to misestimation of earthquake hazard. In western Taiwan where many blind faults are located, how the stress interactions correlate with potentially significant earthquakes and what is the role of blind faults in earthquake triggering are the important questions for earthquake hazard assessment. Given sequential rupture of active faults with an unexposed fault segment involved, the 1935 Mw 7.1 Hsinchu-Taichung earthquake sequence in central Taiwan provides an exceptional case to study. This destructive earthquake sequence took ~. 3000 lives and was composed of four M > 6 earthquakes occurred within three months. The Mw 7.1 mainshock and a subsequent Mw 6.8 event that occurred 12. s later caused surface ruptures on the Tuntzuchiao fault (TTCF) and Shihtan fault (STF) which are separated by 25. km. These two ruptures were characterized by different types of faulting: thrust-faulting on the STF and right-lateral faulting on the TTCF fault. About 24. min later, an Mw 6.0 event occurred ~. 45. km north of the mainshock. Three months later on July 17, the last event, Mw 6.2, occurred 30. km northwest of the mainshock. In this study we revisit the data available for the 1935 sequence in an effort to place constraints on the fault models. Using five different fault models, a series of Coulomb stress calculations are conducted to understand whether static stress transfer advances slip during the subsequent events. We propose that key features of the 1935 Hsinchu-Taichung earthquake triggering can be explained by the existence of an unexposed fault segment in between the segments of TTCF and STF. We also show that the 1935 earthquake sequence may play an important role in activating post-1935 earthquake activity along the Sanyi-Puli seismic zone, where the sequential ruptures of the 1935 events encourage ruptures of a NW-SE-trending seismic zone at later times.