Shaken again: Unraveling the sources of earthquakes in southwestern Luzon, Philippines

In late January, a moderate earthquake struck offshore southwestern Luzon, Philippines, and was felt across Metro Manila. DOST-PHIVOLCS scientists explain what’s going on.
 

By Deo Carlo E. Llamas (@deotectonics), Jeffrey S. Perez, Jonard Jhon S. Acid, John Patrick S. Naing, Crystel Jade M. Legaspi-Delos Santos and Kathleen L. Papiona, Department of Science and Technology – Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS)
 

Citation: Llamas, D.E.C., Perez, J.S., Acid, J.J.S., Naing, J.P.S., Legaspi-Delos Santos, C.J.M., and Papiona, K.L., 2025, In late January, a moderate earthquake struck offshore southwestern Luzon, Philippines, and was felt across Metro Manila, Temblor, http://doi.org/10.32858/temblor.356
 

On January 20, 2025, at 06:42 PM local time, a magnitude 5.5 earthquake struck offshore, between the islands of Mindoro and Luzon, at a depth of 90 kilometers (56 miles; red star in Figure 1). The tremor was widely felt. In Lubang, an island that’s located between Luzon and Mindoro islands and is part of the province of Occidental Mindoro (Figure 1), shaking reached an intensity of IV on the PHIVOLCS Earthquake Intensity Scale (PEIS) (moderately strong; equivalent to Modified Mercalli Intensity or MMI IV). In various parts of Metro Manila, the Philippine capital with a population of about 15 million, the shaking intensity was PEIS III (weak; equivalent to MMI III), particularly for people living in Manila’s high-rise condominiums.
 

 

Though no significant damage has been reported, this event is part of a larger pattern of seismic activity in the region (Figure 2). The onshore and offshore areas of southwestern Luzon exhibit high seismic activity due to its complex tectonic setting, which includes subduction along the Manila Trench and the presence of numerous active faults. Since 2020, the region has experienced several felt earthquakes with magnitudes ranging from 5.0 to 6.6 at intermediate depths (ranging from 70 to 300 kilometers depth, or about 40 to 190 miles; Aurelio et al., 2021; DOST-PHIVOLCS, 2025), underscoring the tectonic processes linked to subduction along the Manila Trench. In addition to these deeper earthquakes, the region is also prone to shallow seismic events with depths of less than 70 kilometers (about 40 miles), making it a key area for understanding varied earthquake sources and their differing risks.
 

 

Intermediate-depth earthquakes tied to subducting slab

The recent magnitude 5.5 earthquake and similar events in the past few years are associated with the subducting slab (sinking section of Earth’s crust) along the Manila Trench, based on their locations and depths (Figure 3).

The Manila Trench is a major subduction zone where the Sunda Plate dives beneath Luzon Island (Figure 2). In this region, the subducting slab dips steeply, approaching a near-vertical orientation as it descends into the mantle, in contrast to the gentler dip observed in northern Luzon (Bautista et al., 2001; Chen et al., 2015). This nearly vertical slab of old, cold oceanic crust is illustrated in the earthquake cross section (a “slice” or “side view” of the Earth) shown in Figure 3.
 

 

The January 20 earthquake’s focal mechanism, based on data from PHIVOLCS SWIFT-CMT (Nakano et al., 2008; Bonita et al., 2015; Punongbayan et al., 2015), reveals reverse faulting, but the tensional axis — a critical indicator of the direction of least compressional stress and maximum tensile (or pulling) stress — is oriented in the down-dip direction relative to the slab. This suggests the earthquake was likely caused by the forces pulling the slab into Earth’s mantle, a process called slab pull. This happens when the weight of the descending plate causes it to stretch, creating tension in the deeper parts of the subducting slab. Think of a stretched rubber band; the rubber band experiences tensile stress in the direction that we are pulling and eventually snaps when pulled too far. Similarly, the subducting slab undergoes increasing tension as it sinks, leading to stress accumulation and eventual rupture.

Interestingly, most of the recent intermediate-depth earthquakes in the region have also exhibited reverse faulting mechanisms with tensional axes (red arrows in Figure 3) oriented in the down-dip direction of the subducted slab (Chen et al., 2015). This pattern highlights the consistent influence of slab dynamics on the seismicity in the area.

Intermediate-depth earthquakes like the January 20 event are less likely to cause severe ground shaking at the surface due to their depth. However, they can be felt over large distances because seismic waves from deep sources travel efficiently through Earth’s interior. These events serve as reminders of the dynamic and ongoing subduction processes beneath the Philippines.
 

Shallow earthquakes show active faults

Though intermediate-depth earthquakes are common, the southwestern Luzon region is also prone to shallow earthquakes originating from faults that cut through Earth’s crust. One notable recent example is the April 2017 earthquake sequence, which involved several magnitude 4 and 5 events occurring in the offshore Batangas region (Figures 1 and 2) at depths of between 5 to 10 kilometers (3 to 6 miles). These earthquakes caused significant damage to structures as well as liquefaction and landslides. The 2017 earthquake sequence was linked to movement along the Batangas Bay Fault (Figure 1), part of a complex network of strike-slip faults recently mapped in the offshore area (Sarmiento et al., 2022; Llamas et al., 2022).

Another significant event in the region was the 1994 magnitude 7.1 Mindoro earthquake (Figure 2), which originated from the Aglubang River Fault that extends offshore near the Verde Island Passage (Figure 1). This earthquake caused widespread destruction, including ground ruptures, landslides, and a destructive tsunami that impacted the coastal areas of Mindoro and Batangas (DOST-PHIVOLCS QRT, 1994).

Shallow earthquakes pose a unique threat because of their proximity to the surface. Ground shaking from these events can be severe, causing widespread damage to buildings, infrastructure, and livelihoods.
 

Recent fault mapping efforts

Because the region frequently experiences earthquakes and little was previously known about its faults, DOST-PHIVOLCS recently conducted a detailed mapping of active faults, both onshore and offshore, in southwestern Luzon. This effort utilized high-resolution topographic and bathymetric data, improving upon the previously approximate fault locations.

Analysis of landforms in this area has revealed a complex system of mainly strike-slip faults. These faults are part of a conjugate system where faults running east-west, like the Lubang Fault and the Verde Fault System, are interconnected with faults running northwest, such as the Balayan Fault, Marikaban Fault, Calatagan Fault, Batangas Bay Fault, Aglubang River Fault. Additionally, there are faults that run northeast, like the North and South Golo Faults, which are classified as normal faults (Figure 1). On land, new fault systems have also been identified in Batangas province, including the Lobo Fault System and the Calubcub Fault (Llamas et al., 2022). Table 1 provides a summary of basic characteristics of the faults mapped in the offshore southwestern Luzon area.
 

 

These mapping initiatives are critical for understanding the seismic hazards of the region. By identifying active faults and assessing their potential for generating earthquakes, researchers can provide valuable insights into the risks faced by coastal and inland communities.
 

Potential for larger earthquakes and tsunamis

The various earthquake sources in the southwestern Luzon region have the potential to generate larger, more destructive events. The extent of the mapped active faults indicates that earthquakes in this area could reach magnitudes greater than 7, posing a significant threat to coastal communities in Batangas, Mindoro, and surrounding provinces (Figure 1).

Though strike-slip faults are not typically associated with large tsunamis, the possibility of tsunami generation cannot be ruled out. A notable example is the 1994 Mindoro earthquake that caused a devastating tsunami, reaching a maximum runup height of 8.5 meters (nearly 28 feet; Bautista et al., 2012). A recent study (Ramirez et al., 2022) suggests that the tsunami was more likely the result of a submarine landslide during the earthquake rather than fault movement. This underscores that, under certain conditions, strike-slip earthquakes can still generate tsunamis, despite the primary fault motion being horizontal.

The Manila Trench is another source of tsunamigenic earthquakes in the region. Recent studies (Salcedo, 2011; Qiu et al., 2019) revealed that different segments of this trench may generate earthquakes with magnitudes greater than 8. Such events could produce tsunamis as high as 10 meters (nearly 32 feet), which would greatly affect more than 800 kilometers (about 500 miles) of coastline in western Luzon Island including Metro Manila. Such a tsunami may also affect countries around the West Philippine Sea, including Taiwan, China, Vietnam, and Malaysia.
 

Balancing awareness, preparedness, and mitigation

Earthquakes are not surprising in this region, given its proximity to the Manila Trench and the presence of numerous active faults. As has been happening recently, even a deep and moderately-sized earthquake would shake a wide area and be felt in the highly populated metropolis of Manila.

We anticipate significantly stronger and more destructive ground shaking if we consider the maximum magnitude that individual faults are capable of generating. Specifically, an earthquake from surrounding active faults in the southwestern Luzon area might generate ground shaking ranging from PEIS VII (destructive ground shaking) to PEIS VIII (very destructive) based on available hazards maps. Ground shaking hazards can be mitigated by following the provisions of the National Building Code and the Structural Code of the Philippines.

Recently, DOST-PHIVOLCS released the Seismic Hazard Atlas for the Design Earthquake of the Philippines (SHADE PH), which contains regional and national ground motion maps derived from both probabilistic (PSHA) and deterministic seismic hazard analysis (DSHA). PSHA estimates the likelihood of ground motion occurring over a specified period, while DSHA depicts the worst-case ground motion from specific earthquake scenarios (Anderson, 1997). These maps are essential tools for engineers, urban planners, and decision-makers, helping them design safer buildings and communities that can better withstand future earthquakes.

The potential threat of tsunamis also remains a concern. In response, DOST-PHIVOLCS has led different activities to increase earthquake and tsunami awareness and preparedness. In 2006, the first Tsunami Early Warning System (TeWS) in the Philippines was established on Lubang Island, north of Mindoro Island. This cost-effective set-up provides tsunami warnings in high-risk coastal communities.

A near real-time tsunami detection system was established in five pilot communities from 2011 to 2013. At present more than 30 tsunami detection instruments comprise the Philippine Tsunami Network, which aims to provide tsunami advisories. Timely issuance of tsunami warnings may lessen the impact and casualties, but awareness and preparedness of communities living in this area should be a priority (Perez and Martinez-Villegas, 2023).
 

References

Anderson, J. G. (1997). Benefits of scenario ground motion maps. Engineering Geology, 48(1-2), 43–57. doi:10.1016/s0013-7952(97)81913-8

Aurelio, M., Catugas, S.D., Escudero, J. A., Lagmay, A.F.M. and Tapang, G., (2021), Luzon, Philippines, sees sixth strong earthquake in five months, Temblor, http://doi.org/10.32858/temblor.225

Bautista, M.L.P., Bautista, B.C., Salcedo, J.C., Narag, I.C., (2012). Philippine Tsunamis and Seiches 1589–2012. Philippine Institute of Volcanology and Seismology (PHIVOLCS), Diliman, Quezon City.

Bautista, B. C., Bautista, M. L. P., Oike, K., Wu, F. T., & Punongbayan, R. S. (2001). A new insight on the geometry of subducting slabs in northern Luzon, Philippines. Tectonophysics, 339(3-4), 279-310. doi:10.1016/S0040-1951(01)00120-2

Bonita JD, Kumagai H, Nakano M (2015) Regional moment tensor analysis in the Philippines: CMT solutions in 2012–2013. J Disaster Res 10:18-24

Chen, P. F., Olavere, E. A., Wang, C. W., Bautista, B. C., Solidum Jr, R. U., & Liang, W. T. (2015). Seismotectonics of Mindoro, Philippines. Tectonophysics, 640, 70-79.

Department of Science and Technology – Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS) (2025). PHIVOLCS earthquake catalog available at: https://earthquake.phivolcs.dost.gov.ph/ (Accessed January 22, 2025).

Llamas, D.C. E., Constantino, R.C., & Papiona, K. L. P. (2022). Tectonic structures in Southwestern Luzon, Philippines [poster presentation]. Geological Society of the Philippines Geological Convention 2022, Quezon City, Philippines. doi: 10.13140/RG.2.2.35849.97128.

Nakano M., Kumagai H., Inoue H. (2008) Waveform inversion in the frequency domain for the simultaneous determination of earthquake source mechanism and moment function. Geophys J Int 173:1000-1011. doi:10.1111/j.1365-246X.2008.03783.x

NAMRIA. (2013). Interferometric Synthetic Aperture Radar Imagery – Digital Elevation Model of the Philippines. 1:10,000. National Mapping And Resource Information Authority (NAMRIA). Fort Andres Bonifacio, Taguig City, Philippines.

Perez, J. S. and Martinez-Villegas, M. M. (2023), Two offshore earthquakes in the Philippines: What do coastal communities need to know?, Temblor, http://doi.org/10.32858/temblor.329

Philippine Statistics Authority. (2016). Philippines – Subnational Administrative Boundaries [Map]. Quezon City, Philippines. 

PHIVOLCS Quick Response Team (1994). 15 November 1994 Mindoro earthquake: Preliminary report of investigation (Special Report No. 2). Philippine Institute of Volcanology and Seismology (PHIVOLCS). Diliman, Quezon City.

Punongbayan J, Kumagai H, Pulido N, Bonita JD, Nakano M, Yamashina T, et al. (2015) Development and operation of a regional moment tensor analysis system in the Philippines: Contributions to the understanding of recent damaging earthquakes. J Disaster Res 10:25-34

Qiu, Q., Li, L., Hsu, Y.-J., Wang, Y., Chan, C.-H., and Switzer, A. D. (2019). Revised earthquake sources along Manila trench for tsunami hazard assessment in the South China Sea, Nat. Hazards Earth Syst. Sci., 19, 1565–1583, https://doi.org/10.5194/nhess-19-1565-2019.

Ramirez, A. B. G., Ramos, N. T., Nawanao Jr, L. P., Mangahas-Flores, R. Z., Narag, I. C., Baba, T., Chikasada, N. & Satake, K. (2022). An earthquake-triggered submarine mass failure mechanism for the 1994 Mindoro tsunami in the Philippines: Constraints from numerical modeling and submarine geomorphology. Frontiers in Earth Science, 10, 1067002.

Salcedo, J. C. (2011). Earthquake source parameters for subduction zone events causing tsunamis in and around the Philippines. Bull. Int. Inst. Seismol. Earthquake Eng., 45, 49.

Sarmiento, K. J. S., Aurelio, M. A., Flores, P. C. M., Carrillo, A. D. V., Marfito, B. J., Abigania, M. I. T., Daag, A.S., & Siringan, F. P. (2022). Seafloor structures and static stress changes associated with two recent earthquakes in offshore southern Batangas, Philippines. Frontiers in Earth Science, 9, 801670.

Warren, L. M., Hughes, A. N., & Silver, P. G. (2007). Earthquake mechanics and deformation in the Tonga‐Kermadec subduction zone from fault plane orientations of intermediate‐and deep‐focus earthquakes. Journal of Geophysical Research: Solid Earth, 112(B5).

Wells, D. L., & Coppersmith, K. J. (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the Seismological Society of America, 84(4), 974-1002. https://doi.org/10.1785/BSSA0840040974

Wessel, P., Luis, J. F., Uieda, L., Scharroo, R., Wobbe, F., Smith, W. H. F., & Tian, D. (2019). The Generic Mapping Tools Version 6. Geochemistry, Geophysics, Geosystems, 20(11), 5556–5564. https://doi.org/10.1029/2019GC008515
 

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