Understanding the Cebu earthquake and its surface rupture
The September 30, 2025 magnitude 6.9 earthquake in northern Cebu left more than shaking ground in its wake. A shallow focus of about 5 kilometers produced a surface rupture first observed by geologists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) in Nailon, near Bogo City. This rupture, now named the Bogo Bay Fault, illuminates how energy release, focal depth, and ground conditions combine to reveal the fault’s footprint on the surface.
What creates a surface rupture?
A surface rupture occurs when the energy released at the earthquake source exceeds the strength of the rocks along a fault, causing visible ground displacement at the surface. The likelihood of a surface rupture increases with larger earthquakes and shallower focal depths. However, not every large quake produces a surface rupture—the local geology and fault geometry also play decisive roles.
The discovery: from Nailon to the Bogo Bay Fault
PHIVOLCS announced the identification of the Bogo Bay Fault three days after the quake, recognizing a newly active structure distinct from the pre-existing Bogo Fault. The offshore-to-onshore trace is a key reminder that many faults stretch beneath the sea, where offshore geophysical surveys and seismic profiling help map hidden segments that contribute to future seismic risk estimates.
Fault finders and knowledge sharers
Structural geologists, often labeled fault finders, pursue the responsible fault responsible for an earthquake. In addition to initial data like epicenter, magnitude, depth, and fault mechanism, they use satellite interferometry, aftershock patterns, and offshore seismic profiles to outline rupture extents. When a quake affects remote or underwater regions, the search expands beyond visible ground features to subsurface imaging and ongoing aftershock mapping.
Techniques in fault tracing
Key methods include: analyzing epicenter data, aftershock distributions to delineate rupture zones, and interferometric synthetic aperture radar (InSAR) for surface deformation. Offshore geophysical surveys then visualize subsurface structures beneath the seabed, revealing hidden fault segments that land surveys might miss.
Aftershocks and the rupture zone
Aftershocks follow the main event as the fault continues to adjust. These smaller quakes help scientists infer the rupture length and the geometry of the fault plane. For the Cebu event, aftershocks track a northeasterly trend from the offshore epicenter toward San Remigio, passing through Nailon and Bogo City, and extending toward Tabogon and Tabuelan. The pattern informs estimates of the rupture’s land and offshore extents.
Implications for resilience and public safety
Detecting and mapping faults like the Bogo Bay Fault strengthens resilience by informing building codes, land-use planning, and early-warning strategies. As fault finders become knowledge sharers, their transparent data sharing helps communities understand risk, apply mitigation measures, and respond more effectively to future earthquakes.
A call for ongoing collaboration
The Cebu event underscores the need for ongoing cooperation among universities, government agencies, and local communities. Field investigations, offshore surveys, and rapid data dissemination are essential to building a robust knowledge base that supports disaster preparedness and recovery planning in Cebu and nearby regions.
Note: The author led the geology group of the UP Quick Response Team during investigations in Leyte and Cebu, with support from UP NIGS, UP Resilience Institute, and regional offices of Mines and Geosciences Bureau. Ongoing work continues to define the full extent of the Bogo Bay Fault and its implications for seismic hazard in the area.
