The role of the west-dipping collision boundary fault in the Taiwan 2022 Chihshang earthquake sequence

On 17-18 September 2022, an earthquake sequence with a moment magnitude of 6.6 foreshock and a 7.0 mainshock occurred in southeast Taiwan along the Longitudinal Valley. Several surface breaks and collapsed buildings were observed after the event and one person died. The focal mechanisms of the foreshock and mainshock both had a west-dipping fault plane, which is different from the known active east-dipping boundary fault between the Eurasian Plate and the Philippine Sea Plate. Joint source inversions were performed to better understand the rupture mechanism of this earthquake sequence. The results show that the ruptures mainly occurred on a west-dipping fault. In the mainshock, the slip originated from the hypocenter and propagated toward the north with a rupture velocity of approximately 2.5 km/s. The east-dipping Longitudinal Valley Fault also ruptured, which could be passive and dynamically triggered by the significant rupture on the west-dipping fault. Most importantly, this source rupture model together with the occurrence of large local earthquakes over the past decade strongly supports the existence of the Central Range Fault, which is a west-dipping boundary fault that lies along the north to south ends of the Longitudinal Valley suture.