Search

Unknown seismogenic structures lurking beneath convergent margins introduce substantial uncertainty in seismic hazard assessments. In northwestern Luzon, the Mw7.0 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 Mw7.0–7.7, given the thick seismogenic layer of ∼30 km in northern Luzon. On 27 July 2022, the Mw7.0 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. 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 Global Navigation Satellite System and Sentinel‐1 Aperture Radar reveal the Abra rupture on an oblique blind fault beneath northern Luzon The coseismic rupture brought the shallow parts (5 km depth) of adjacent faults up to 2 bars closer to failure Inferred slip and depth distribution of seismicity reveal the thick seismogenic layer of 30 km on the northern Philippine Fault Zone

Since 2001, we have set up a dense geodetic network with 52 campaign-mode GPS sites and seven continuously recording GPS stations as well as six leveling routes in the Taitung area, Taiwan. Our aim was to better characterize near-fault crustal deformation of active faults at the plate suture of the Philippine Sea plate and Eurasia in southeastern Taiwan. On 1 April 2006, a moderate shallow earthquake (Mw6.1, depth 10.8 km) occurred within this network. This earthquake resulted from rupturing of a geologically unknown or suspected fault (called the Y fault) located underneath the eastern margin of the Central Range. After removing the impacts of secular motions and postseismic slip, we estimated the coseismic displacements of the Peinan earthquake from the GPS and leveling measurements before and after the main shock. Three deformation types with distinct slip behaviors were revealed in three different regions: (1) near the epicenter—around 45 mm movement in the S-SSW direction with +20 to -20 mm vertical motion, in the northern part of the Y fault; (2) south of the epicenter across the southern part of the Y fault—approximately 35 mm in a westward movement with -60 mm subsidence (footwall side) and 40 mm in a SSW movement with at least 50 mm uplift (hanging-wall side), in the southern part of the Y fault; (3) northeast away from the epicenter—about 10 mm in a northward displacement with +15 to -10 mm vertical motions, in the Longitudinal Valley and on the western flank of the Coastal Range. This unique coseismic deformation pattern sheds new light on the characteristics of the suture zone between the Eurasian and Philippine Sea plates at the southernmost Longitudinal Valley. We used GPS and leveling measurements to invert for the fault geometry and the coseismic slip distribution. The optimal modeled fault is an 80° west-dipping fault at a depth of 0.5–20 km. The highest slip of about 0.33 m is located to the south of the hypocenter at a depth of 9–16 km. The total geodetic moment in our optimal model is 2.3 × 1018Nt-m, which is equivalent to an earthquake of Mw6.2. The surface coseismic displacements as well as the inferred coseismic slip distribution indicate a drastic change of slip behaviors in the middle of the Y fault. The left-lateral slippage near the hypocenter turned dramatically to reverse faulting with left-lateral component as rupturing propagated to the southern portion of the fault, suggesting that a possible right-lateral faulting occurred that coseismically cross cut the northern middle Peinanshan massif in the Longitudinal Valley.

The MW 6.5 Chengkung earthquake occurred in eastern Taiwan at 04:38 UTC on 10 December 2003. The GPS data from eighteen continuously recording stations (CORS) and 86 campaign-surveyed stations (CSS) collected 18 days to 9 months before and 6 days to 4 months after the main shock are utilized to analyze the coseismic and postseismic deformation associated with the Chengkung earthquake. The earthquake resulted from rupturing of the Chihshang fault, a 25-km-long segment of the Longitudinal Valley Fault (LVF). The coseismic horizontal displacements in the hanging wall showed a fan-shape distribution with vectors towards the west. On the other hand, the movements of the revealed a mirror fan-shape with relatively lesser amounts of displacement. The largest coseismic displacement, which reached 126 mm and 263 mm in the horizontal and vertical components, occurred near the epicenter area in the hanging wall. The largest postseismic displacements in 109 days, which approached 59 mm and 68 mm in the horizontal and vertical components, occurred near the surface trace of the Chihshang fault (TAPO) and near the epicenter area (CHEN), respectively. The stations near the Chihshang fault indicated a more significant postseismic displacement than coseismic one.