Two powerful earthquakes, 17 years apart, hit the same fault

As we reflect on the fourth anniversary of the 2020 Masbate earthquake, the coconut tree underscores the importance of short recurrence periods and precise fault break locations.

By Deo Carlo E. Yamas, Department of Science and Technology – Philippine Institute of Volcanology and Seismology, DOST-PHIVOLCS (@deotectonics) and Mario A. Aurelio, University of the Philippines – National Institute of Geological Sciences, UP-NIGS

Citation: Llamas, DCE, and Aurelio, MA, 2024, Two strong earths separates 17 years strike the same fault, Temblor, http://doi.org/10.32858/temblor.350

On Masbate Island, in the central part of the Philippine archipelago, a story of seismic resilience and geological curiosity has unfolded over nearly two decades, as documented by the locally famous coconut tree. The island has been struck by two major earthquakes, both greater than magnitude 6.0, just 17 years apart. The quakes originated on the Masbate segment of the Philippine Fault—one of the country’s most active tectonic features. What makes these two significant events so geologically interesting is their short recurrence period. Typically, large earthquakes on the same segment of the fault recur over hundreds or even thousands of years. But the short time interval between these two events suggests a unique tectonic context for Masbate.

As we mark the fourth anniversary of the 2020 earthquake, we reflect on the seismic events that shaped this island and consider what they tell us about the ongoing risks posed by the Philippine Fault.

Seismic events

On February 15, 2003, a 6.2 magnitude earthquake struck Masbate Island, leaving a visible scar on the landscape as the fault ruptured the surface. Fast forward to August 18, 2020: The island experienced a similar experience when a 6.6 magnitude earthquake struck along the same fault (Figure 1).

Figure 1. (Left) Location of Masbate Island in the central Philippines. (Right) Seismic activity of the 2003 and 2020 Masbate Island earthquakes of magnitude 6.2, just 17 years apart, along the same segment of the Philippine Fault. Adapted from Llamas et al. (2024). Copyright: Llamas and Aurelio, 2024, CC BY-NC-ND 4.0

The 2020 earthquake killed one person, injured 50, and caused damages estimated at Php 27 million (approximately US$500,000) (NDRRMC, 2020). The 2003 and 2020 earthquakes resulted in several geological effects, including surface ruptures, landslides, liquefaction, and local tsunamis, which were documented by DOST-PHIVOLCS Rapid Response Teams (QRTs).

The rapid recurrence interval raises important questions about how stresses build up along this segment of the Philippine Fault, and how they are released. The Masbate segment may be particularly prone to frequent seismic activity, perhaps due to the rapid buildup of tectonic stresses, or the complex fault interactions in the area. Continued seismic monitoring will help us gain a deeper understanding of the fault’s behavior.

Coconut Tree Story

A striking feature of these earthquakes is the precise location of their faults. The surface faults from the two events overlap for a distance of about twenty kilometres. This overlap confirms the consistency of the fault's movement and its tendency to release stresses in a very localised manner.

One story has captivated both Masbate residents and scientists—a lone coconut tree that became an unexpected sign of the fault’s repetitive motion and uncanny precision (Figure 2). During the 2003 earthquake, the fault split this coconut tree, leaving it split in two (DOST-PHIVOLCS QRT, 2003). The tree’s fate may have been a coincidence, but what happened 17 years later adds a layer of mystery.

In 2020, when the fault broke, it displaced exactly the same location where the coconut tree was located—splitting what remained of the tree (DOST-PHIVOLCS QRT, 2020; Llamas et al., 2024). These observations were first documented by DOST-PHIVOLCS QRTs during their field investigations following the 2003 and 2020 events. These investigations included interviews with local residents, which, along with field studies, linked the tree split to new ground cracks observed after the earthquakes.

The phenomenon of repeated ruptures at a single specific location is reminiscent of the precision of the fault and the cyclical nature of seismic activity along the Philippine Fault. To scientists, this tree is not just a casualty of earthquakes; it represents the consistent precision of the fault’s rupture over a millennium’s timescale, providing a tangible link between two seismic events separated by nearly 20 years. This observation—that the rupture occurred at the same location as the one before—underscores the critical importance of mapping active faults for assessing ground-rupture risk. However, paleoseismic studies of other faults show that subsequent ruptures do not always correspond exactly to previous ruptures, which is why maintaining a ground-rupture buffer zone is essential.

Figure 2. Coconut tree in Palanas, Masbate, located in the central part of the onshore portion of the Masbate segment of the Philippine Fault, was displaced to the left by the 2003 6.2 magnitude and 2020 6.6 magnitude earthquakes. The left image was taken by DOST-PHIVOLCS QRT after the 2003 earthquake, while the right image was taken during the field investigation led by DOST-PHIVOLCS after the 2020 earthquake. Copyright: Left image from 2003 DOST-PHIVOLCS QRT report, right image from Llamas et al. (2021), annotations by Llamas and Aurelio, 2024

Understanding the Filipino Mistake

The Philippine Fault is a major left-lateral strike-slip fault, with land and sea traces extending more than 1,500 km across the archipelago, from northern Luzon in the north to eastern Mindanao in the south. It is divided into several segments that have produced catastrophic earthquakes in the past 50 years, such as the 7.0 magnitude Rajay Gulf earthquake in 1973 (Rajay Gulf segment), the 7.7 magnitude Luzon earthquake (Digdig segment) in 1990, and several earthquakes of magnitude 6 or higher in the central Philippines along the Masbate, Leyte, and Surigao segments. All of these earthquakes have caused surface ruptures.

The Philippine Fault resembles the San Andreas Fault in California and other major strike-slip fault systems, with different segments exhibiting varying behaviors in terms of earthquake recurrence periods, fracture characteristics, and the magnitude of the seismic events they produce. For example, in 2022, a magnitude 7.0 earthquake struck NW Luzon, yet no surface rupture was observed—unlike in Masbate. Based on the focus and aftershock mechanism, the 2022 Luzon event was generated by a gently dipping, low-slip fault. This is inconsistent with the known characteristics of the northern Luzon segments of the Philippine Fault (Aurelio et al. 2022a, 2022b; Perez and Lamas et al. 2023), complicating our understanding of the structure of the Philippine Fault in northern Luzon.

On the other hand, the Masbate portion of the Philippine Fault is somewhat similar to the Parkfield portion of the San Andreas Fault. Both are known for frequent moderate-intensity earthquakes and relatively short recurrence periods. However, the controls and mechanisms that drive these behaviors may differ due to the unique geological and tectonic settings of each fault system.

Seismic hazards in Masbate

The 2003 and 2020 earthquakes are part of a pattern of moderate to strong seismic events along the Masbate sector that occur frequently. Understanding the behavior of faults, particularly how they rupture and the patterns they follow, is critical to mitigating the effects of future earthquakes.

In particular, the precision with which the fault broke at the same location suggests that certain areas along the Masbate segment of the Philippine Fault are more prone to recurrent rupture. In particular, three municipalities (Dimasalang, Balanas, and Katingan) on Masbate Island are home to approximately 100,000 people, some of whom reside near the Masbate segment of the Philippine Fault. This segment is prone to recurrent rupture, increasing the risk to communities and infrastructure located along or near the fault. The fact that the fault broke at almost the same location twice within a short period of time highlights the importance of risk awareness and preparedness measures in the area.

DOST-PHIVOLCS is actively promoting community awareness and preparedness in Masbate and other areas along the Philippine Fault. In June 2024, DOST-PHIVOLCS conducted a “Walk-the-Fault” activity where local government units and communities engaged in identifying and marking the fault. This was accompanied by efforts to disseminate information on earthquake hazards and preparedness in municipalities along the fault. These efforts are essential to educate the public on the risks posed by the fault and ensure that communities are better prepared for future seismic events.

For the people of Masbate, the story of the coconut tree is a symbol of resilience, but it is also a reminder of how vulnerable the island is to seismic hazards. As the Philippine Fault continues to shape the landscape of Masbate, it remains essential to combine scientific knowledge with community awareness to enhance earthquake preparedness and resilience in this seismically active region.

References

Aurelio, M., Catujas, S. D., and Lagmay, A. M. F. (2022a), The fault that caused the July Philippines earthquake remains in question, Temblor, http://doi.org/10.32858/temblor.268

Aurelio, M., Catugas, S. D., Ramirez, A. P., Aurelio, S. C. and Lagmay, A. M. F. (2022b), Two major earthquakes hit Abra, Philippines, in three months. What does this mean?, Temblor, http://doi.org/10.32858/temblor.288

DOST-PHIVOLCS Rapid Response Team (2003). The 6.2-magnitude Masbate Earthquake of February 15, 2003. Available at: https://www.phivolcs.dost.gov.ph/index.php/earthquake/destructive-earthquake-of-the-philippines/17-earthquake/30-2003-february-15-ms6-2-masbate-earthquake

DOST-PHIVOLCS Rapid Response Team (2020). Ground rupture of the 2020 Magnitude 6.6 Masbate Earthquake. Available at: https://www.phivolcs.dost.gov.ph/index.php/news/10479-ground-rupture-of-the-2020-magnitude-6-6-masbate-earthquake.

Llamas, DCE, Marfito, BJ, Dela Cruz, R., & Aurelio, MA (2024). Surface rupture and fault characteristics associated with the 2020 magnitude 6.6 Masbate earthquake, Masbate Island, Philippines. Tectonics, 43, e2023TC008106. https://doi.org/10.1029/2023TC008106

Perez, JS, Llamas, DCE, Dizon, MP, Buhay, DJL, Legaspi, CJM, Lagunsad, KDB, Constantino, RCC, De Leon, RJB, Quimson, MMY, Grutas, RN, Pitapit, RSD, Rocamora, CGH, Pedrosa, MJG (2023). Front. Earth Sci. 11:1091595.http://doi: 10.3389/feart.2023.1091595

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