The 30 September 2025 Mw 6.9 Cebu earthquake occurred on a previously unmapped offshore fault near the coast of Bogo City and produced severe impacts, including reported casualties, widespread infrastructure damage, and localized ground effects such as liquefaction in Medellin and San Remegio. Initial focal mechanism interpretations indicate strike-slip faulting, with candidate nodal planes corresponding to either a right-lateral, NE-trending fault or a left-lateral, NW-trending fault. However, the observed pattern of aftershock migration toward the southwest, together with reported ground rupture indicating right-lateral motion, suggests that the NE-trending right-lateral plane is the more plausible seismogenic source. Farther to the southwest, aftershock mechanisms appear to transition from dextral strike-slip to thrusting, implying spatial variability in fault behavior and stress regime that warrants a coherent structural explanation. This study aims to characterize the fault rupture and associated ground deformation of the event by integrating earthquake sequence analysis, field- and satellite-based deformation measurements, and subsurface structural constraints from seismic profile interpretation.
The project will synthesize three complementary datasets to build an event-scale fault model. (1) Earthquake data analysis: hypocenter and focal mechanism information will be compiled from local and international catalogs (e.g., PHIVOLCS, USGS, GCMT and other network products) to evaluate aftershock propagation, spatiotemporal clustering, and fault geometry/kinematics; processing and visualization will be implemented primarily using the Generic Mapping Tools (GMT). (2) Ground deformation analysis: coseismic and postseismic surface deformation will be assessed using both field observations of rupture and satellite Synthetic Aperture Radar (SAR) data; SAR products from repositories such as the Alaska Satellite Facility will be processed through open-source InSAR workflows (e.g., ISCE) to generate deformation fields and to constrain the distribution of slip and strain beyond the immediate rupture trace. (3) Seismic profile interpretation: available seismic reflection/refraction profiles will be requested from relevant public and private sources (including government and petroleum-sector datasets, as available) and interpreted using institutional-licensed platforms (e.g., IHS Kingdom Suite and TNavigator). The resulting subsurface structural interpretation will be reconciled with earthquake and InSAR constraints. The combined outputs (aftershock geometry/kinematics, measured deformation gradients, and subsurface structure) will be integrated to produce a consistent fault and deformation model that explains the occurrence and spatially varying mechanisms of the Mw 6.9 Cebu earthquake.
This study addresses a critical gap highlighted by the event itself: a damaging Mw 6.9 earthquake sourced by a previously unmapped fault demands rapid, defensible characterization of rupture geometry and deformation footprint to improve tectonic understanding and inform risk reduction. By explicitly integrating earthquake sequence behavior, direct and satellite-derived deformation observations, and independent subsurface structural constraints, the project is positioned to (i) identify the most likely seismogenic fault plane and its kinematics, (ii) quantify the spatial distribution of coseismic deformation (including areas susceptible to secondary hazards such as liquefaction), and (iii) clarify why faulting style appears to vary along strike from strike-slip to thrusting toward the southwest. The resulting fault model and deformation interpretation can strengthen scientific understanding of active deformation in northern Cebu and adjacent offshore areas, support improved interpretations of regional fault system connectivity, and provide a robust evidence base that can be translated into hazard-relevant products for agencies and local stakeholders (e.g., prioritized zones for reconnaissance, monitoring, and future mapping).
This research addresses the UN Sustainable Development Goals for Sustainable Cities and Communities (SDG 11) and Industry, Innovation, and Infrastructure (SDG 9).
Cover photo from: https://www.rappler.com/science/earth-space/cebu-not-known-earthquake-prone-until-september-30-2025/
