Analysis of active fault morphotectonic behavior and seismic deformation during the September 2022 earthquakes in eastern Taiwan

Taiwan situated on an active orogenic between the Eurasian plate and the Philippine Sea plate with an annual contraction rate of >8 cm, possesses therefore high seismicity and frequent geological hazards. Furthermore, many of the seismogenic faults pass across dense population areas and cause severe damages. In September 2022, several large earthquakes occurred in Longitudinal Valley, caused more than 100 casualties and damage roads, bridges and houses. However, exception of a few apparent surface cracks, the location, total length and actual deformation of the faults remain incomplete in most areas. In order to better constrain deformation and faulting behavior and potential threats, we focus on the Longitudinal Valley fault and Central Range fault in eastern Taiwan. To better estimate ground deformation around the active fault, large-area high-resolution geoinformatic datasets before and after the earthquake are critical. In this study, we use DMC aerial images, taken in April and September 2022, to produce DTM and orthomosaic images. Based on paired orthoimages before and after the earthquake, the particle image velocimetry (PIV) method was used to calculate the horizontal ground deformation. Vertical displacements near the fault were estimated from digital terrain models (DTM) of differences (DoD) pre- and post-earthquake. The results showed that the maximum horizontal displacement was greater than 2-3 meters and was been verified on field.