Two strong earthquakes separated by 17 years strike the same fault

As we reflect on the fourth anniversary of the 2020 Masbate earthquake, a coconut tree underscores the significance of short recurrence intervals and precise locations of fault ruptures.
 

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

Citation: Llamas, D. C. E., and Aurelio, M. A., 2024, Two strong earthquakes separated by 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 a locally famous coconut tree. This island has been struck by two significant earthquakes, both stronger than magnitude 6.0, just 17 years apart. The earthquakes were generated by the Masbate segment of the Philippine Fault — one of the country’s most active tectonic features. What makes these two significant events geologically intriguing is this short recurrence interval. Typically, large earthquakes on the same fault segment recur over hundreds or even thousands of years. But, the brief interval between these events suggests a unique tectonic context in Masbate.

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

The seismic events

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

 

The 2020 earthquake killed one person, injured 50 people, and caused about PHP 27 million (approximately $500,000 USD) in damage (NDRRMC, 2020). Both the 2003 and 2020 earthquakes led to several geological impacts, including surface ruptures, landslides, liquefaction and local tsunamis, which were documented by the DOST-PHIVOLCS quick response teams (QRTs).

The rapid recurrence interval raises important questions about how stress accumulates along this segment of the Philippine Fault, and how it is released. The Masbate segment may be particularly prone to frequent seismic activity, potentially due to a rapid build-up of tectonic stress, or complex fault interactions in the region. Ongoing seismic monitoring will help us gain a deeper understanding of the fault’s behavior.
 

The tale of the coconut tree

Another striking feature of these earthquakes is the precise location of their ruptures. The surface ruptures from both events overlap for a 20-kilometer stretch. This overlap underscores the consistency of the fault’s movement and its tendency to release stress in a highly localized manner.

One story has captivated both Masbate locals and scientists — a lone coconut tree that became an unexpected marker of the fault’s frequent movement and curious precision (Figure 2). During the 2003 earthquake, the fault rupture sliced through this coconut tree, leaving it split in half (DOST-PHIVOLCS QRT, 2003). The tree’s fate could have been seen as a mere coincidence, but what happened 17 years later adds a layer of intrigue.

In 2020, as the fault ruptured, it displaced precisely the same location that the coconut tree straddles — further splitting what remained of the tree (DOST-PHIVOLCS QRT, 2020; Llamas et al., 2024). These observations were first documented by the DOST-PHIVOLCS QRTs during their field investigations following both the 2003 and 2020 events. These investigations included interviews with locals, which, along with field studies, linked the tree’s split to the fresh ground ruptures observed after the earthquakes.

This phenomenon of repeated rupture at one exact location is a stark reminder of the fault’s precision and the cyclic nature of earthquake activity along the Philippine Fault. For scientists, this tree is more than just a casualty of the earthquakes; it represents the unwavering accuracy at the millennial timescale of fault rupture, offering a tangible link between two seismic events separated by nearly 20 years. This observation — that the rupture occurred at the same location as before — underscores the critical importance of active fault mapping for assessing ground rupture hazards. However, paleoseismic studies of other faults show that subsequent ruptures don’t always align perfectly with previous ones, which is why maintaining a buffer zone for ground rupture hazard is essential.
 

 

Understanding the Philippine Fault

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

The Philippine Fault is similar to the San Andreas Fault in California and other major strike-slip fault systems, with different segments exhibiting varying behaviors in terms of earthquake recurrence intervals, rupture characteristics, and the magnitude of seismic events they produce. For example, in 2022, a magnitude 7.0 earthquake hit northwestern Luzon, yet no surface rupture was observed — unlike in Masbate. Based on focal mechanism and aftershocks, the 2022 Luzon event was generated by a gently dipping oblique fault. This is not consistent with the known characteristics of the northern Luzon segments of the Philippine Fault (Aurelio et al. 2022a, 2022b; Perez, Llamas, et al., 2023), which complicates our understanding of the structure of the Philippine Fault in northern Luzon.

On the other hand, the Masbate segment of the Philippine Fault is somewhat analogous to the Parkfield segment of the San Andreas Fault. Both are known for frequent moderate-magnitude earthquakes and relatively short recurrence intervals. However, the controls and mechanisms driving these behaviors may differ due to the unique geological and tectonic settings of each fault system.
 

Masbate’s seismic hazards

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

In particular, the precision with which the fault has ruptured in the same location suggests that certain areas along the Masbate segment of the Philippine Fault are more susceptible to repeated rupture. In particular, three municipalities (Dimasalang, Palanas and Cataingan) on Masbate Island house approximately 100,000 people, some of whom reside near the Masbate segment of the Philippine Fault. That segment is susceptible to repeated rupture, which increases the risk for communities and infrastructure located along or near the fault. The fact that the fault ruptured in nearly the same location twice within such a short time period highlights the importance of hazard awareness and preparedness measures in the region.

DOST-PHIVOLCS has been actively working to enhance community awareness and preparedness in Masbate and other areas along the Philippine Fault. In June 2024, DOST-PHIVOLCS conducted a “Walk-the-Fault” activity during which local government units and communities participated in locating and placing markers along the fault. This was accompanied by efforts to disseminate information on earthquake hazards and preparedness in municipalities transected by the fault. These efforts are crucial for educating the public about the risks posed by the fault and ensuring 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 the island’s vulnerability to seismic hazards. As the Philippine Fault continues to shape the landscape of Masbate, it remains essential to integrate scientific knowledge with community awareness to enhance earthquake preparedness and resilience in this seismically active region.
 

References

Aurelio, M., Catugas, S.D., and Lagmay, A.M.F. (2022a), Fault that caused a July quake in the Philippines still in question, Temblor, http://doi.org/10.32858/temblor.268

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

DOST-PHIVOLCS Quick Response Team (2003). 2003 February 15 Ms 6.2 Masbate Earthquake. 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 Quick 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, D. C. E., Marfito, B. J., Dela Cruz, R., & Aurelio, M. A. (2024). Surface rupture and fault characteristics associated with the 2020 magnitude (MW) 6.6 Masbate earthquake, Masbate Island, Philippines. Tectonics, 43, e2023TC008106. https://doi.org/10.1029/2023TC008106

Perez, J.S., Llamas, D.C.E., Dizon, M.P., Buhay, D.J.L., Legaspi, C.J.M., Lagunsad, K.D.B., Constantino, R.C.C., De Leon, R.J.B., Quimson, M.M.Y., Grutas, R.N., Pitapit, R.S.D., Rocamora, C.G.H., Pedrosa, M.G.G. (2023). Front. Earth Sci. 11:1091595.
http://doi: 10.3389/feart.2023.1091595
 

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