Several decades since I defended my doctoral dissertation, my visits to Masbate had been few and far between, until I started preparing for a three-week field work on the island for my incoming class of geology seniors in July.
On June 10, I had the chance to visit an oddly shaped coconut tree in Matugnao (“cool” or “cold” in Masbateno), a hilly village in the eastern town of Palanas. For an earthquake scientist like myself, the tree inside the one-hectare field tended to by a poor farmer surviving on copra production was instantly an object of curiosity and fuel to its further study: it stood like a bent tripod struggling to support its lofty, leafy top.
The adult coconut tree took the brunt of Nature’s forces twice. From its natural underpinning to the lower section of its trunk, the tree sustained a crack after an earthquake rumbled through Masbate and ruptured the ground in 2003. Again, it met a similar fate in 2020, when the same segment of the quake fault moved.
In geological terms, the 17-year interval between two quake events of significant magnitudes occurring along the same fault segment is an extreme rarity. Even more so is the fact that the second temblor sliced the coconut tree in the precise area hit by the first.
The case may be the only documented phenomenon of its kind to date. It seems to defy current understanding of earthquake dynamics so more in-depth studies are needed.
According to traditional earthquake theory, the period between successive ruptures along the same fault, known as “recurrence interval,” would range from hundreds to thousands of years. For instance, studies indicate that the West Valley Fault that traverses eastern Metro Manila has ruptured about four times in the last 1,500 years. This suggests a recurrence interval of about 375 years.
Compared with the 2020 quake in Masbate, a 17-year recurrence interval is excessively short and extremely rare, and the phenomenon equally very rare.
When a fault moves to cause an earthquake of moderate to strong magnitude (5.0 or higher on the Richter scale), stresses are believed to be released from the tearing fault surface, and it takes a considerable length of time to accumulate new stresses. Only when the stress threshold of the fault, or its ability to contain the accumulated stresses, is exceeded will the fault rupture again.
Numerous active faults (showing evidence of “activity” in the past 10,000 years) cut across the Philippines, and dozens are known to be capable of triggering destructive quakes. Among these are the six subduction trenches (one tectonic plate dives underneath another) surrounding the archipelago and a number of strike-slip faults (tectonic blocks slide past each other).
Most of the active faults appear in existing maps, but several erstwhile unknown faults have also generated devastating quakes in recent years, such as the Negros quake (magnitude 6.9) on Feb. 6, 2012, the Bohol quake (magnitude 7.2) on Oct. 15, 2013, and the Zambales quake (magnitude 6.1) on April 22, 2019.
Magnitude refers to the amount of seismic energy released by an earthquake. A magnitude 7.0 quake is around 33 times “stronger” than one with a magnitude 6.0.
The faults in Masbate are part of the Philippine Fault System (PFS), a zone of more than 1,200 kilometers that stretches the entire length of the country, from Cagayan and Ilocos Norte in the north to Davao in the south. Segments of the PFS have generated disastrous quakes in the past 50 years, such as the magnitude 7.3 quake in Ragay Gulf in 1973 (Bicol segment), and those of magnitude 6 or higher along the Masbate, Leyte and Surigao segments.
So far, the strongest quake (magnitude 7.8) measured along any segment of the PFS occurred when its northern Luzon segment ruptured on July 16, 1990. It wreaked havoc over more than a 100-km radius from its epicenter in Nueva Ecija, claiming 1,200 lives and causing P10 billion worth of damage to infrastructure in as far north as Baguio and Dagupan cities, according to government seismologists.
In Masbate, the two recent quakes were much weaker, or at least 50 times lower in energy released than the July 1990 event. But what makes these occurrences interesting is their proximity to each other in both space and time, an extreme rarity in the current understanding of fault dynamics.
On Feb. 15, 2003, Masbate trembled with a magnitude 6.3 quake, whose epicenter was located off Magcaraguit Island near the coastal town of Uson and 28 km southeast of the capital, Masbate City, and at a focal depth of 15 km, according to the Philippine Institute of Volcanology and Seismology (Phivolcs). The quake was felt all over the province and its neighbors.
Except for a chapel wall that collapsed, several houses with cracked walls and floors, and ground fissures, no major damage to structure and loss of lives was reported in the sparsely populated affected communities. Another casualty was the split coconut tree in Matugnao, which would again face further punishment 17 years later during the Aug. 18, 2020 quake.
The magnitude 6.6 quake killed at least one person and injured 50, and affected 1,524 families, according to the National Disaster Risk Reduction and Management Council. It damaged many schools and houses, and several roads and bridges.
In 2017, three undergraduate geology students, Krystel, Pamela and Elisha, from Adamson University predicted in general terms the occurrence of the 2020 quake while on training at the Structural Geology and Tectonics Laboratory of the University of the Philippines’ National Institute of Geological Sciences (UP-NIGS). They used the Coulomb Stress Transfer Theory.
Deo, a graduate student from UP-NIGS, will soon present the results of his study on the subject in his master’s thesis.
The rare natural phenomenon observed in Masbate is another stark reminder that in the science of earthquakes, there are still plenty of unknowns. Lightning can, and often does, strike the same place repeatedly, according to the US National Oceanic and Atmospheric Administration, but scientists cannot yet declare so for earthquakes in close intervals.
Efforts should be strengthened to pursue more in-depth research in this field, especially that the Philippines has hundreds of active faults, many of them still unmapped, and several of these faults pass through or are near population centers and major infrastructure, such as bridges, elevated highways and business districts, and even the mothballed Bataan Nuclear Power Plant.
Mario A. Aurelio, PhD, is a professor at the University of the Philippines and teaches at the National Institute of Geological Sciences in UP Diliman. He is the main author of a Commission on Higher Education-sponsored guide book for senior high school teachers on disaster risk reduction and management. His French dissertation involved structural, kinematic and geodynamic studies of the Philippine Fault. —ED.