Hidden Philippine Megathrust Could Fire Tsunamis Within Two Minutes of Shaking

Beneath the Sulu Sea, one of the most poorly understood subduction zones in Southeast Asia may be capable of unleashing tsunami waves up to eight meters high onto Philippine coastlines with as little as two minutes of warning, according to a new modeling study published in Natural Hazards and Earth System Sciences. The research, led by Lyndon P. Nawanao Jr. and Noelynna T. Ramos of the University of the Philippines Diliman, together with Toshitaka Baba, Naotaka Chikasada, and Kenji Satake of Japanese institutions, represents the first systematic attempt to map the seismic and tsunami hazard potential of the Negros–Sulu megathrust, a fault system so sparsely studied that it is missing entirely from the most widely used global compilation of subduction zone geometries.

The Philippines sits at the restless junction of the Pacific, Philippine Sea, Eurasian, and Indo–Australian plates, and its archipelago is stitched together by oppositely dipping subduction zones and the sweeping Philippine Fault Zone. The country’s historical record, collated from eyewitness accounts dating back to 1589, documents 41 tsunamigenic earthquakes, twenty of which can be directly tied to subduction zones. Three of these reached magnitude 8.0 or greater, including the devastating 1976 Moro Gulf earthquake along the Cotabato Trench, a shallow rupture that killed an estimated 3,000 to 8,000 people and remains the deadliest tsunami disaster in Philippine history. Yet while the Manila, East Luzon, and Philippine trenches have all been constrained by dense GPS networks and coupling models, the Negros–Sulu subduction zone has languished in a data vacuum, with only a handful of instrumentally recorded earthquakes and a GPS coverage too sparse to identify which parts of the plate boundary may be locked and accumulating strain.

To confront this uncertainty, the team built their rupture scenarios from the ground up using a structural approach rather than relying on geodesy. Analyzing the orientation variations of the frontal wedge, the deformed sediments at the leading edge of the overriding plate, they divided the megathrust into four main segments along the Negros and Sulu trenches, then added two alternative configurations to bracket uncertainties in how far the northernmost segment extends and whether the two southern segments could rupture together. Seismic tomography profiles, which show the shallow slab dipping at angles between 9 and 30 degrees, prompted the researchers to model three dip scenarios of 10, 20, and 30 degrees for every segment, yielding 18 distinct rupture scenarios in total.

Feeding these fault geometries into empirical rupture-scaling relations derived from global subduction earthquakes, the team estimated that a full rupture of each segment could produce moment magnitudes ranging from 8.0 to 8.9, corresponding to average slips of 1.68 to 6.23 meters. The longest scenario, a combined rupture of the Sulu Trench segments stretching 379 kilometers, carries the largest potential magnitude of 8.9. The researchers deliberately chose the linear area-magnitude scaling relation over a bilinear alternative because it yields slightly larger magnitudes, a conservative choice given how little is known about this margin. The fault models assume a shallow blind rupture with its top buried five kilometers beneath the seafloor and pure reverse slip, with the down-dip extent of the seismogenic zone capped at 50 kilometers based on the depth distribution of 207 reverse-oblique earthquakes recorded between 1977 and 2023.

The tsunami simulations themselves were run in the JAGURS numerical code, a state-of-the-art solver that goes beyond classical tsunami modeling by incorporating nonlinear shallow-water equations, both vertical and horizontal seafloor displacements, and Boussinesq dispersion effects that describe how short-period waves scatter as they travel. The team merged 15-arcsecond GEBCO bathymetry with resampled 5-meter interferometric radar terrain data from the Philippine mapping agency to build the seafloor and land surface, positioned 34 synthetic tide gauges around the Sulu Sea, and placed 6,879 sampling points at 10-kilometer intervals along the coastlines of 16 defined coastal regions to extract maximum wave heights, flow velocities, and minimum arrival times over a four-hour simulation window.

The results reveal a stark pattern governed by wave directivity. Coastal communities directly facing the rupture segments, including Sipalay–Bayawan on Negros, Sindangan–Labason and Siocon–Sibuco on western Mindanao, Basilan–Sulu, and the Moro Gulf, face the highest exposure, with modeled wave heights of 5 to 8 meters and flow velocities of 5 to 7 meters per second. More alarming still is the timing: because these coasts lie so close to the trench, the first wave arrives in under two minutes, leaving no time for official warnings and demanding immediate self-evacuation. The Negros Trench scenarios generate waves up to 6 meters with flow velocities reaching 7 meters per second, while Sulu Trench ruptures produce up to 8 meters of wave height. Waves from the Sulu segments can even thread through straits into the Celebes Sea, reaching the Moro Gulf in roughly ten minutes.

Not every coastline faces equal danger, and the study uncovers some elegant physical controls on where waves grow and where they fade. Coastlines protected by wide shallow shelves fare dramatically better: the analysis shows that areas hit by waves of at least 2 meters typically have a nearshore width, measured between the 50-meter depth contour and the shoreline, of just 2 to 4.5 kilometers, whereas coasts with shelves wider than 20 kilometers experience dissipation that suppresses waves below 2 meters, and every location with a shelf exceeding 30 kilometers saw minimal wave heights. Front-line islands also act as natural breakwaters, diffracting and attenuating waves before they reach rearward coasts. Conversely, concave embayments and narrow channels in the interior Philippines can trap and amplify tsunami energy through constructive resonance, occasionally pushing waves up to 2 meters even deep inside the archipelago where direct exposure would otherwise be low.

The team validated their models against historical accounts, comparing modeled maxima with eyewitness reports of past events in the Sulu and Celebes seas, including the 1897 magnitude 7.5 earthquake that reportedly produced waves up to 6 meters along the Sulu archipelago. In most regions the modeled waves slightly exceed the historical accounts, but the comparison also revealed a sobering finding for the Moro Gulf: the highest modeled wave from the Sulu Trench reaches 6 meters, demonstrating that this disaster-prone coastline is vulnerable not only to the nearby Cotabato subduction zone, source of the catastrophic 1976 tsunami, but also to ruptures on the Sulu side of the sea. Prehistoric washover deposits previously discovered along Zamboanga del Sur may likewise record extreme waves from either system.

The authors are careful to emphasize that their estimates should be treated as floors, not ceilings. Because the models assume uniform average slip across each fault plane rather than the heterogeneous slip patches that characterize real megathrust earthquakes, and because trench-breaching ruptures and splay faulting, which can dramatically amplify seafloor uplift, were not included, the maximum wave heights and flow velocities are likely underestimates. Real earthquakes that concentrate slip near the trench, as the 2011 Tohoku-Oki earthquake famously did, can generate far larger tsunamis than average-slip models predict. The study also cannot resolve local inundation without higher-resolution nearshore bathymetry, which the authors identify as an urgent data gap given the demonstrated sensitivity of wave heights to shelf width.

Despite these caveats, the work delivers immediately actionable outputs for a region where scientific information has long lagged behind the hazard. The maps of minimum arrival times, maximum wave heights, and flow velocities offer regional planners a baseline for siting evacuation routes, designing coastal defenses such as seawalls, green embankments, and offshore breakwaters, and identifying locations where vertical evacuation into sturdy elevated structures may be the only viable option given the extraordinarily short warning times. The highest-exposure coastlines also double as the most promising search areas for paleotsunami deposits, sedimentary fingerprints of prehistoric waves that could extend the region’s earthquake record far beyond four centuries of written history. As the authors conclude, in a margin where every seismic record spans fewer than 300 earthquakes and recurrence intervals stretch across hundreds to thousands of years, scenario-based modeling is not merely useful; it is the essential bridge between a dangerous knowledge gap and the coastal communities who live directly above it.

Subject of Research: Seismic and tsunami hazard modeling of the Negros–Sulu megathrust subduction zone in the Philippines

Article Title: Seismic and tsunami hazard potential of the Negros–Sulu megathrust, Philippines

Article References: Nawanao Jr., L. P., Ramos, N. T., Baba, T., Chikasada, N., & Satake, K. (2026). Seismic and tsunami hazard potential of the Negros–Sulu megathrust, Philippines. Natural Hazards and Earth System Sciences, 26(10), 5035-5055. https://doi.org/10.5194/nhess-26-5035-2026

Image Credits: AI Generated

DOI: 10.5194/nhess-26-5035-2026

Keywords: megathrust, tsunami, Philippines, Sulu Sea, subduction zone, earthquake hazard, JAGURS, wave directivity, rupture scaling, paleotsunami, disaster mitigation, Negros Trench