Characteristic repeating earthquakes in an arc-continent collision boundary zone: The Chihshang fault of eastern Taiwan

Creeping crustal faults often generate a large number of microearthquakes, and less commonly, they may also produce major earthquakes that rupture the brittle crust. The Chihshang fault in eastern Taiwan is characterized by such behavior and has been known to undergo 2-3 cm/yr surface creep, making it one of the most active creeping thrust faults known in the world. It gives an excellent opportunity to study how a creeping fault can generate large earthquakes. However, the understanding of fault behavior at depth in this area has been limited due to sparse sampling from seismic and geodetic stations. In this study we determine and evaluate a population of repeating earthquakes to improve the understanding of deep fault deformation. We propose a repeating sequence identification scheme in the region where the station coverage is sparse and one-sided. Using this identification method, we found 30 M 2-3 characteristic repeating sequences under the northern half of the Chihshang fault at 7-23 km depth. Slip estimates from these sequences indicate an average slip rate of ∼ 3 cm/yr, which is consistent with the rate inferred from surface geodetic measurements. We infer that the 30-km-long Chihshang fault is creeping along its northern half and locked in the south, consistent with the occurrence of the 2003 M_L 6.4 earthquake on the southern fault section.