Abstract

The western margin of the Philippine Sea plate, extending from Luzon, Taiwan, to Ryukyu, represents one of the most tectonically active plate boundaries on our planet. This region is characterized by ongoing subduction and arc-continent collision processes, which together profoundly shape the regional tectonic framework and generate frequent earthquakes. Given its high seismic hazard, it is essential to document seismogenic faults and investigate their frictional properties using multiple geophysical data. This lecture will delve into this topic by characterizing the seismogenic faults of two M7-class earthquakes that occurred within the western margin of the Philippine Sea plate in 2022, using geodetic observations such as Global Navigation Satellite Systems (GNSS) data and SAR-based satellite imagery. The first earthquake is the Mw 7.0 Northwestern Luzon earthquake occurred in Philippines, which is the largest crustal earthquake in northern Luzon since 1990. However, no coseismic surface ruptures were found after the earthquake, indicating the earthquake source occurred on an unmapped blind fault. By integrating geodetic data, we imaged the oblique coseismic slip at depths of 11-22 km beneath the Philippine fault zone in northern Luzon, where the seismogenic layer could be as thick as 30 km. Another seismic event we will discuss is the 2022 Chihshang earthquake sequence occurred in the Longitudinal Valley suture zone, eastern Taiwan. The sequence was initiated by a Mw 6.5 foreshock followed by the Mw 7.0 mainshock 17 hours later. These earthquakes were accompanied by seismic ruptures on the west-dipping Central Range fault, whose activity has been debated for decades. By leveraging multiple geodetic data, we mapped the coseismic slip and early afterslip on the Central Range fault and the adjacent Longitudinal Valley fault in detail. These results potentially revise our conventional view of the fault geometry along the plate boundary in eastern Taiwan. Finally, we will discuss how 25-year-long GNSS data helped us to track shallow slow slip events (SSEs) beneath the southern Ryukyu forearc. The inferred forearc SSE area likely correlates with the subducted topography. The proximity of the inferred SSE area and the source of the 1771 Yaeyama tsunami earthquake implies that they may be spatially complementary. To fully characterize the slow earthquake phenomena in the southern Ryukyu forearc, as well as the crustal deformation mechanisms in the western margin of the Philippine Sea plate, we highlight the need for the continued development of seafloor geodesy in this region.