The southern Ryukyu subduction zone is one of the potential sources for tsunamigenic earthquakes. Despite a great seismic risk, the deformation pattern remains poorly known, primarily due to the absence of seafloor constraints. With GNSS‐acoustic measurements over years, we characterize the convergence rate across this margin growing from 92 mm/yr offshore eastern Taiwan to 123 mm/yr near the Gagua Ridge. The new data suggest the subduction interface is capable of hosting Mw 7.5–8.4 earthquakes. The orientations of seafloor movement and P‐axes in the Nanao Basin are both subnormal to the trench, notably deviate from the direction of plate convergence. By considering the combined effect of plate convergence and backarc rifting, different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip‐partitioning. The trench‐parallel component is likely accommodated in part by earthquakes near Taiwan, lower plate deformation, and strike‐slip faults within the accretionary wedge.
On 27 July 2022 (UTC), the M w 7.0 Abra earthquake struck the northwestern region of Luzon, the Philippines. This earthquake is the largest crustal seismic event in northern Luzon since the 1990 M w 7.7 earthquake (Silcock & Beavan, 2001;Yoshida & Abe, 1992). The earthquake occurred ∼20 km east of Vigan city, beneath the western flank of the Cordillera Central, with a hypocenter depth of 15 km (PHIVOLCS, 2022) (Figure 1). The earthquake is characterized by an oblique faulting and the focal mechanism of the mainshock exhibits two nodal planes dipping ∼70° to the north-west and ∼30° to the east, respectively (Bonita et al., 2015;Ekström et al., 2012;Punongbayan et al., 2015; USGS, 2022a) (Table S1 in Supporting Information S1). Strong ground shaking generated by the mainshock was perceived throughout Luzon island, including Manila, one of the most populous metropolises in the world, ∼300 km away from the epicenter (PHIVOLCS, 2022). The emergency field investigation after the mainshock reported massive damages of local infrastructures, induced landslides in mountainous areas, and liquefaction in western Luzon (Perez et al., 2023). The distribution of aftershocks shows a north-south trend, roughly parallel to the strike of geological units in this region (Figure 1). However, no coseismic surface ruptures were found, leaving the seismogenic fault of the mainshock concealed. According to the focal mechanism (Figure 1), this event is likely associated with an oblique blind fault.
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