Earthquakes generated at subduction zones are often the most destructive because they come from a huge contact surface where one tectonic plate dives beneath another, allowing stress to build up over large areas before suddenly releasing. In southern Mindanao, the Cotabato subduction zone has produced major earthquakes in the past, including the 1976 Moro Gulf event that triggered a deadly tsunami. Despite this history, there is still no clear, consolidated picture of which partsof the Cotabato subduction zone have ruptured in large earthquakes, how big those ruptures were, and whether there are “quiet” sections that may be overdue. This ongoing project is reconstructing the rupture history of the Cotabato subduction zone to identify possible seismic gaps and translate those findings into clearer implications for earthquake hazard in southern Mindanao.
Work is underway to build a database of large Cotabato-subduction earthquakes (focusing on about magnitude 7 and above) using multiple earthquake catalogs and historical sources. Instrumental records from agencies and global repositories (e.g., PHIVOLCS, USGS, and the International Seismological Centre) are being compiled, cleaned, and checked for consistency, and older accounts prior to modern instrumentation are being incorporated where reliable. For each major event, the project estimates the likely rupture area using available information such as aftershock patterns (the spatial “cloud” of smaller quakes that often outlines the main rupture) and earthquake source mechanisms from global moment tensor catalogs (useful because subduction earthquakes can vary in how they slip depending on where they occur along the plate boundary). These event-by-event rupture footprints are then assembled into space–time maps showing which sections of the subduction interface have broken in the past and which have remained relatively quiet. Finally, the project estimates earthquake recurrence by combining: (1) the rate at which plates converge along the trench (a proxy for how quickly strain is “loaded”), and (2) a seismic-moment accumulation approach (after Molnar-type formulations) that relates slip deficit over time to the size and frequency of characteristic large earthquakes.
The project aims to deliver a rupture-history map for the Cotabato subduction zone, similar to those used in other well-studied subduction systems, but tailored to southern Mindanao. In plain terms: it will show which stretches have already “spent” their accumulated strain in recent history and which stretches may still be storing it. This matters because hazard planning is not just about knowing that a subduction zone exists, but about understanding where the next large rupture is more plausible and how long it may take for strain to rebuild to damaging levels. By integrating scattered earthquake records into one coherent timeline and set of rupture footprints, the project strengthens the scientific basis for identifying potential seismic gaps, improves expectations for recurrence behavior, and supports more informed earthquake and tsunami hazard discussions for coastal and inland communities affected by Cotabato-subduction earthquakes.
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 Ferrario, M.F., Perez, J.S., Dizon, M. et al. Environmental effects following a seismic sequence: the 2019 Cotabato—Davao del Sur (Philippines) earthquakes. Nat Hazards 120, 6125–6147 (2024). https://doi.org/10.1007/s11069-024-06467-7
