The 1,250-km-long, NNW-trending, arc-parallel Philippine fault, one of the world’s most active tectonic structures, traverses the Philippine archipelago and has been the source of surface-rupturing earthquakes during the last four centuries. In this paper, we will discuss Philippine fault distribution and segmentation in Mindanao Island by integrating detailed fault mapping together with new geological and paleoseismic data and the analysis of historical surface-rupturing earthquakes. Using geometric segmentation criteria, we have identified nine geometric segments separated by discontinuities such as en echelon steps, bends, changes in strike, gaps, steps and bifurcation in the surface trace. Fault segments ranges from 20 to 100 km in length and are capable of generating earthquakes ofMw6.6 toMw7.4. The results of our study have important implications for earthquake generation potential and seismic hazard assessment of the Philippine fault in Mindanao Island.

At 00:43 UTC on 27 July 2022, a 15-km deep major earthquake with magnitude (Mw) 7.0 struck Northwestern Luzon, Philippines. 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. More than a thousand landslides, rockfalls and tension cracks were mapped, near the epicentral region, in the northwestern part of the Central Cordillera. Most of the landslides were shallow-seated, many of which were situated along road cuts. 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. Sea level disturbance was also observed in some coastal areas of Ilocos Sur and La Union. Damages to buildings and infrastructures were documented in areas that experienced PEIS VI (very strong), equivalent to MMI VI and PEIS VII (destructive). 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. There is no clear indication of a surface rupture based on InSAR analysis and field investigation. 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. Peak ground acceleration (PGA) records show a unidirectional rupture propagation and are congruent with the spatial distribution of earthquake impacts. 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. The need to understand these earthquake sources in the country is needed for a better seismic hazard and risk assessment.

The Philippine archipelago is tectonically complex and seismically hazardous, yet few seismic hazard assessments have provided national coverage. This article presents an updated probabilistic seismic hazard analysis for the nation. Active shallow crustal seismicity is modeled by faults and gridded point sources accounting for spatially variable occurrence rates. Subduction interfaces are modeled with faults of complex geometry. Intraslab seismicity is modeled by ruptures confined to the slab volume. Source geometries and earthquake rates are derived from seismicity catalogs, geophysical data sets, and historic-to-paleoseismic constraints on fault slip rates. The ground motion characterization includes models designed for global use, with partial constraint by residual analysis. Shallow crustal faulting near metropolitan Manila, Davao, and Cebu dominates shaking hazard. In a few places, peak ground acceleration with 10% probability of exceedance in 50 years on rock reaches 1.0 g. The results of this study may have utility for defining the design base shear in the National Structural Code of the Philippines.

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The 1973 Ragay Gulf earthquake produced an onshore surface rupture approximately 30 km in length along the Guinayangan segment of the Philippine fault in southern Luzon Island. Through geologic mapping and paleoseismic trenching, we have characterized the amount of coseismic offsets, the average recurrence interval, and the slip rate of the segment. The coseismic offsets we identified in the field were fairly constant along the fault, ranging from 1 to 2 m. Paleoseismic trenching at the Capuluan Tulon site exposed stratigraphic evidence for three or possibly four surfacerupturing events after the deposition of strata dated at AD 410–535. The average recurrence interval was calculated to be 360–780 years, which is close to that for the Digdig fault, the source fault of the 1990 central Luzon earthquake. The slip rate, based on the calculated recurrence interval and offsets during the 1973 earthquake, has been calculated to be 2.1–4.4 mm/yr. This rate is significantly smaller than the geodetic slip and creep rates of 20–25 mm/yr estimated for the Philippine fault on the islands of Masbate and Leyte. The slip rate deficit may be explained by the possibilities of underestimation of the recurrence interval due to possible missing paleoseismic events within the stratigraphic records, the occurrence of larger earthquakes in the past, and the aseismic fault creep between the surface-rupturing earthquakes.