The need to understand these earthquake sources in the country is needed for a better seismic hazard and risk assessment.įor the past 10 years, damaging earthquakes with magnitude (M w) greater than 6 have occurred in the Philippines. Although earthquake parameters, deformation analysis and field data suggest that the Abra River Fault is the probable causative fault, the derived geometry and kinematics from the seismotectonic analysis challenge our existing understanding of the nature of the Abra River Fault, as well as the other segments of the Philippine Fault. Peak ground acceleration (PGA) records show a unidirectional rupture propagation and are congruent with the spatial distribution of earthquake impacts. The spatial distribution of geologic impacts, such as earthquake-induced landslides and liquefaction, is strongly controlled by the causative fault, the direction of rupture propagation and geology. There is no clear indication of a surface rupture based on InSAR analysis and field investigation. Earthquake data, including hypocentral location, aftershock distribution, focal mechanism solutions and strong motion data, and InSAR observation indicate that the earthquake was generated by an almost north-south striking reverse left-lateral oblique fault that is gently dipping to the east. Damages to buildings and infrastructures were documented in areas that experienced PEIS VI (very strong), equivalent to MMI VI and PEIS VII (destructive). Sea level disturbance was also observed in some coastal areas of Ilocos Sur and La Union. Liquefaction manifested as lateral spreads, sand boils, fissures, ground subsidence, and localized swelling was documented along the coastal areas of Ilocos Sur and river channels in Abra and Ilocos Sur. Most of the landslides were shallow-seated, many of which were situated along road cuts. More than a thousand landslides, rockfalls and tension cracks were mapped, near the epicentral region, in the northwestern part of the Central Cordillera. The strongest ground shaking felt was at PHIVOLCS Earthquake Intensity Scale (PEIS) VII (destructive), equivalent to Modified Mercalli Intensity (MMI) VII, in Abra and along the coastal areas of Ilocos Sur.
Department of Science and Technology-Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS), Quezon City, PhilippinesĪt 00:43 UTC on 27 July 2022, a 15-km deep major earthquake with magnitude (M w) 7.0 struck Northwestern Luzon, Philippines.
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