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Unknown seismogenic structures lurking beneath convergent margins introduce substantial uncertainty in seismic hazard assessments. In northwestern Luzon, the Mw 7.0 Abra earthquake on 27 July 2022 highlights the seismic activity along an unmapped blind fault underneath the Cordillera Central. By integrating coseismic displacements constrained by radar satellite imagery and Global Navigation Satellite System, we image oblique coseismic slip at 11–22 km depth with peak slip of ∼1 m beneath the Philippine Fault Zone in northern Luzon. The southward propagation of coseismic slip and aftershocks terminated at a distance of 50 km from the northern end of the 1990 Luzon earthquake rupture, leaving a seismicity gap in between. Coulomb stress changes of reaching 2 bars are imparted at the shallow portions of the Vigan‐Aggao and Abra River faults, where the updated 100‐year seismic potential is increased to Mw 7.0–7.7, given the thick seismogenic layer of ∼30 km in northern Luzon. Plain Language Summary: On 27 July 2022, the Mw 7.0 Abra earthquake hit northwestern Luzon, the Philippines, killing 11 people, damaging more than 30,000 buildings, and causing US$34 million worth of damage. The earthquake occurred on a previously unrecognized deep fault with no observable surface ruptures. In this study, we collected coseismic displacements from Global Navigation Satellite System and Interferometric Synthetic Aperture Radar to investigate the source fault and coseismic slip distribution. Our model shows that the coseismic slip occurred beneath the Philippine Fault Zone in northern Luzon at a depth of 11–22 km. The coseismic slip and aftershocks propagated southward and terminated at a distance of 50 km from the northern tip of the 1990 Luzon coseismic surface rupture. This leaves a seismicity gap between latitudes 16.5°N–17°N along the Philippine Fault Zone. The mainshock‐induced stress perturbation on the nearby active faults, together with the unusually thick seismogenic layer on the northern Philippine Fault Zone, implies a high seismic hazard in Luzon. Successive crustal deformation and seismicity monitoring will provide valuable constraints for the energy budget of earthquakes across northern Luzon, where local communities are threatened by inland crustal earthquakes and megathrust events in the future. Key Points: Global Navigation Satellite System and Sentinel‐1 Aperture Radar reveal the Abra rupture on an oblique blind fault beneath northern LuzonThe coseismic rupture brought the shallow parts (5 km depth) of adjacent faults up to 2 bars closer to failureInferred slip and depth distribution of seismicity reveal the thick seismogenic layer of 30 km on the northern Philippine Fault Zone [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The southern Ryukyu margin near Taiwan accommodates the relative plate motion and backarc rifting. This region has suffered from tsunamis and large earthquakes with magnitudes of about eight in the history, yet insufficient offshore data still hamper our ability to fully characterize fault slip behaviors. We demonstrate how a seafloor geodetic technique, with the combination of the Global Navigation Satellite System and acoustic ranging, can help us constraining fault slip behaviors and assessing seismic risks. We infer a fraction of oblique plate motion is accommodated by the subduction interface capable of hosting earthquakes with moment magnitudes ranging from 7.5 to 8.4. Incorporating seismological observations, we find different trends of the compressive axes of earthquakes, seafloor movement, and the forearc convergence, which may indicate some degree of strain partitioning. The trench‐parallel motion is partially taken up by seismic slip near Taiwan, subducting plate deformation, and strike‐slip faults within the forearc. More near‐trench observations are crucial to assessing seismic hazard and evaluating strain partitioning along the southern Ryukyu Trench. Key Points: Rapid convergence across the southern Ryukyu margin increases eastward from 92 mm/yr offshore Taiwan to 123 mm/yr near the Gagua RidgeOblique convergence is accommodated in part by the plate interface fault capable of hosting earthquakes with Mw 7.5–8.4Different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip partitioning [ABSTRACT FROM AUTHOR]

Abstract: We measured seafloor movement using a Global Navigation Satellite Systems (GNSS)/Acoustic technique at the south of the rifting valley in the western end of the Okinawa Trough back‐arc basin, 60 km east of northeastern corner of Taiwan. The horizontal position of the seafloor benchmark, measured eight times between July 2012 and May 2016, showed a southeastward movement suggesting a back‐arc opening of the Okinawa Trough. The average velocity of the seafloor benchmark shows a block motion together with Yonaguni Island. The westernmost part of the Ryukyu Arc rotates clockwise and is pulled apart from the Taiwan Island, which should cause the expansion of the Yilan Plain, Taiwan. Comparing the motion of the seafloor benchmark with adjacent seismicity, we suggest a gentle episodic opening of the rifting valley accompanying a moderate seismic activation, which differs from the case in the segment north off‐Yonaguni Island where a rapid dyke intrusion occurs with a significant seismic activity. [ABSTRACT FROM AUTHOR]