Researchers have discovered four new fault zones beneath the sea west of Luzon, branching out from the main Philippine Fault Zone

The Philippine Fault Zone (PFZ) is a major fault system linked to earthquakes and crustal movement across the Philippines. Understanding how the PFZ works is essential for earthquake risk reduction. However, its full extent—especially where it ends in northwest Luzon is still unclear.

In this study, we used advanced imaging tools, including high-resolution seafloor maps and seismic profiles, to get a clearer picture of what lies offshore. We discovered four new fault zones beneath the sea west of Luzon that appear to branch off from the main PFZ. One of these branches seems to extend the main fault to the Manila Trench—a major undersea feature where earthquakes and tsunamis can also originate. This fault may even be splitting the trench into two parts.

What makes this more intriguing is that the Manila Trench bends sharply as it moves southward—by about 35 degrees—and this matches with the direction of the PFZ. Below this area, seismic data show that the Earth’s crust is torn at deeper levels. In the north, one part of the tectonic plate is dipping gently downward, while in the south it dips much more steeply. This tearing may be caused by two major geologic collisions happening at the same time—one from the Palawan Microcontinental Block pushing in from the southwest, and another from Benham Rise pushing from the northeast.

Why does this matter? These newly mapped faults and the tearing of the Earth’s crust could help explain how and where large earthquakes might happen in the future. By identifying these geologic structures, we can better understand the complex forces shaping Luzon and, more importantly, improve hazard assessments and earthquake preparedness for communities living near these potentially hazardous fault zones.

Authors: Shu-Kun Hsu (Department of Earth Sciences, National Central University, Taiwan | Institute of Earth Sciences, Academia Sinica, Taipei), Wen-Nan Wu (Department of Earth Sciences, National Central University, Taiwan), Lien-Kai Lin (Department of Earth Sciences, National Central University, Taiwan), Shiou-Ya Wang (Center for Environmental Studies, National Central University, Taiwan), Yi-Ching Yeh (Department of Earth Sciences, National Central University, Taiwan), Leo T. Armada (National Institute of Geological Sciences, University of the Philippines Diliman), Carla B. Dimalanta (National Institute of Geological Sciences, University of the Philippines Diliman), Kuan-Ting Chen (Department of Earth Sciences, National Central University, Taiwan), Yun-Jie Tsai (Department of Earth Sciences, National Central University, Taiwan), and Ching-Hui Tsai (Center for Environmental Studies, National Central University, Taiwan)

Read the full paper: https://doi.org/10.1016/j.jseaes.2025.106720

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