Reconnaissance of the 2022 Guanshan and Chihshang earthquakes in eastern Taiwan

Two significant earthquakes struck eastern Taiwan on consecutive days in mid-September of 2022. The first, the Guanshan earthquake, occurred on the night of September 17 (local time) with a local magnitude (ML) of 6.4, and the second, the Chihshang earthquake, occurred on the afternoon of September 18 with a mainshock of ML of 6.8. The strong motion of these two events resulted in a series of ground surface ruptures, 2 collapsed buildings, two collapsed bridges, and more than 100 partially damaged structures along the Chihshang and Yuli faults around the Longitudinal Valley in eastern Taiwan. The observed response spectra at the damage sites of the Chihshang earthquake were significantly larger than the design response spectra of the 1997 Taiwan building code. Near-fault velocity pulses with a maximum value of 132 cm/s were observed along the faults, destructively impacting the damaged structures. The details and causes of the structural damage are presented in this article according to the findings of the on-site reconnaissance and ground motion analysis. Finally, the behavior of selected structures that have a structural health monitoring system at the time of these two destructive earthquakes is also evaluated.