The Philippine island of Palawan — considered a stable part of the seismically active country — was recently shaken by two earthquakes in two days, highlighting the need to reassess seismic hazards.
By Deo Carlo E. Llamas (@deotectonics), Jonard Jhon S. Acid, Jeffrey S. Perez, Ishmael C.Narag, John Patrick S. Naing and Crystel Jade M. Legaspi, Department of Science and Technology – Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS)
Citation: Llamas, D. C. E., Acid, J. J. S., Perez, J. S., Narag, I. C., Naing, J. P. S. and Legaspi, C. J. M., 2024, Earthquakes rattle Palawan, Philippines: Is the region as stable as we thought?, Temblor, http://doi.org/10.32858/temblor.345
The Philippines, known for its intense seismic activity, forms a collection of islands of varying shapes and sizes in the western Pacific Ocean. The islands and their many earthquakes result from a complex tectonic setting. To the west, the Eurasian Plate subducts beneath the Philippine archipelago along several trenches, including the Manila, Negros, and Sulu Trenches.
Palawan, a long, thin island that trends northeast-southwest, appears to point from the main Philippine archipelago to the island of Borneo. Palawan sits atop the Eurasian plate and has traditionally been considered a seismically stable region. In contrast, the rest of the Philippines is located within the Philippine Mobile Belt, a highly dynamic area marked by active tectonic movements, studded with volcanoes, and replete with earthquake activity.
However, on 11 June 2024 at 2:58 p.m local time, an offshore, shallow earthquake with magnitude 5.1 shook Palawan (Figure 1). The highest felt intensity was reported in the municipality of Roxas, Palawan at PHIVOLCS Earthquake Intensity Scale (PEIS) III (Weak; equivalent to Modified Mercalli Intensity (MMI) III). Two days after this event, on 13 June 2024 at 3:00 p.m. local time, a magnitude 4.2 earthquake struck in the offshore Coron, Palawan region (Figure 1), 250 kilometers to the north. For this event, the highest reported felt intensity was PEIS II (Slightly Felt; equivalent to MMI II).
These events not only startled the residents of the island, but also caught some scientists in the Philippines off guard. To that end, we present a rapid reassessment of Palawan’s seismic potential. We accomplished this by using historic earthquakes, the PHIVOLCS earthquake catalog, and detailed, high-resolution bathymetric maps. We found that despite its apparent stability, Palawan does, indeed, have active faults nearby and can experience shaking from large earthquakes that strike elsewhere in the Philippines.
Historical seismicity: Rare events in a tranquil setting
Although Palawan is often portrayed as a seismically stable region, instrumental records provide evidence of occasional earthquakes, some of which were felt across the island. Such events are rare and generally of low magnitude (Figure 1).
The largest Palawan earthquake on record was the 19 December 1940 magnitude 5.9 earthquake. Between 2013 and 2015, Palawan experienced a few moderate-sized earthquakes, ranging from magnitude 3.9 to 4.7, primarily occurring offshore. Most of these events were not felt by residents. An exception was the 12 October 2015 magnitude 4.7 earthquake, which was felt in Bataraza, Palawan. During this event, residents experienced ground shaking lasting three to five seconds, rated at PEIS IV (Moderately Strong).
Though located relatively far from the active plate boundaries in and around the Philippines, Palawan experiences earthquakes because elastic strain energy can still build up in Earth’s crust far from active plate boundaries. In other words, tectonic stress from surrounding active regions can extend into Palawan, causing strain to accumulate. This strain eventually needs to be released through movement along faults. Given Palawan’s long history of tectonic evolution alongside the surrounding marginal basins, numerous old structures exist that can accommodate the build-up of strain. However, Palawan is located in a relatively stable tectonic region, so the process of strain accumulation and release occurs at a much slower rate than in regions directly situated on active plate boundaries.
Investigating the source: Where is the fault?
Following the 11 June 2024 earthquake, we at PHIVOLCS quickly mobilized to understand the underlying cause. The epicenter was located within the northwest Sulu Sea basin, about 90 kilometers southeast of Palawan (Figure 2). This basin, oriented northeast-southwest, was previously a back-arc basin that resulted from the initial subduction of the Celebes Sea basin beneath the proto South China Sea several tens of millions of years ago (Lai et al., 2021). Since that time, the Sulu Sea and Celebes Sea have evolved into marginal basins separated by active trenches.
The focal mechanism of the June 11 earthquake, calculated from the PHIVOLCS SWIFT-CMT solution, indicates that the tremor resulted from movement along a reverse fault. (A focal mechanism tells seismologists the orientation of fault planes and which way they moved.) This focal mechanism suggests movement on a northeast-southwest trending, gently dipping fault plane 10 kilometers below the surface. This is consistent with the orientation of the basin and with previous estimates of maximum horizontal stress in the region (Heidbach et al., 2018).
However, small and moderate-sized earthquakes, like the 11 June tremor, rarely offset faults at the surface. Moreover, the current map of PHIVOLCS does not indicate any active faults in the area, which made initial identification of the source fault difficult.
Our preliminary analyses focused on the region’s active tectonics using high-resolution bathymetric data available from the National Mapping and Resource Information Authority (NAMRIA) of the Philippine government. This analysis reveals potential earthquake-generating faults that had previously been overlooked due to availability of only low-resolution data in past years. We explored recently acquired high-resolution bathymetric data. Using SimpleDEM software, we generated a 3D anaglyph of the bathymetry, which helped us better explore structures on the seafloor. The bathymetry shows significant deformation, suggesting the presence of reverse faults in the region. For instance, we see southeast-facing fault scarps, tectonic warp associated with blind thrusts, and evidence of backthrusts (Figure 2).
Further supporting the presence of these faults, a recent study (Cadenas and Ranero, 2024) identified reverse faults in the northwest Sulu Sea via seismic reflection profiles (Figure 3). These profiles provide cross-sectional images of the subsurface, revealing structures that might be sources of seismic activity. However, the existing seismic profiles are limited in coverage. So, determining the precise orientation and extent of these faults is challenging.
The relative activity of these offshore structures has not been fully assessed. Yet, based on geomorphic features, the seafloor deformation observed, and the occurrence of the June 2024 earthquake, it is plausible that these offshore faults are active and could be potential sources of future earthquakes.
The occurrence of the 13 June 2024 magnitude 4.2 earthquake presents a different scenario and appears more typical due to its proximity to an active tectonic boundary, the Manila Trench (Figure 2). Because it is near an active tectonic margin, the epicentral area of the magnitude 4.2 earthquake is actively deforming, causing this area to experience frequent earthquakes (Figure 1). The focal mechanism of this earthquake indicates strike-slip movement, consistent with previous seismic events in the region, suggesting that deformation is primarily accommodated by strike-slip faults. Based on bathymetry data, a prominent linear feature (indicated by red arrows in Figure 2) aligns with the northwest-striking nodal plane of the focal mechanism, suggesting it could potentially be the source of the earthquake.
Seismic hazards and vulnerability in Palawan
Given the presence of potential earthquake sources in the vicinity of Palawan, future seismic events are a foreseeable occurrence. Beyond earthquakes originating nearby, Palawan faces the threat of ground shaking from more distant seismic sources. Historical records show instances of ground shaking from distant earthquake sources, such as the 1940 magnitude 6.8 earthquake in southwestern Luzon Island, which was felt at MMI V (equivalent to PEIS V (Strong)), in Cullion Island, Palawan (SEASEE, 1985). Additionally, Palawan may have been affected by other significant nearby earthquakes, such as the 1948 magnitude 8.2 Lady Caycay event and 1990 magnitude 7.1 Panay earthquake (Figure 1). Although these distant events did not cause destruction in Palawan, they highlight the island’s exposure to ground shaking from both near and far sources.
Another major concern is the potential for tsunamis, given Palawan’s extensive coastlines and proximity to offshore tectonic structures. Historical records indicate that both northern and southern Palawan were affected by a tsunami during the 1897 magnitude 7.5 Sulu Sea Earthquake. Reports from that event noted wave heights of 2 meters in Cuyo, Palawan, as well as in parts of southern Palawan and northern Borneo (Bautista et al., 2012).
The occurrence of these earthquakes underscores the importance of robust disaster preparedness measures, particularly in anticipating other potential hazards like tsunamis. Relying on isolated events for seismic hazard assessment is inadequate; a comprehensive geological investigation is essential to accurately identify seismic sources. Such an investigation would need to thoroughly reevaluate Palawan’s seismic potential to ensure appropriate seismic safety measures are implemented, especially for critical installations such as proposed nuclear power plants and associated nuclear waste facilities.
Nevertheless, the National Building Code of the Philippines mandates that engineers use a non-zero seismic zone factor when calculating the seismic base shear for standard vertical structures in the Palawan area. In other words, when buildings in Palawan are constructed, engineers must assume that the structures will experience at least some shaking resulting from an earthquake.
Despite its reputation for geological stability, earthquake records and the presence of potential active faults indicate that the Palawan region is vulnerable to seismic events. This highlights the necessity for continuous monitoring and detailed geological investigations to effectively understand and mitigate the risks in a relatively stable region. Although the earthquakes in Palawan may not be surprising, these events highlight the importance of enhancing our understanding and preparedness to better protect the residents and infrastructure of Palawan from the impacts of future earthquakes.
Science editor: Dr. Alka Tripathy-Lang, Ph.D.
Reviewer: Dr. Ross Stein, Ph.D.
References
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