Preparing Metro Manila for the big event

A study warns that a 7.2-magnitude earthquake in the West Valley could kill 33,500 people, injure 113,600 and destroy 13% of homes in Metro Manila. Buildings, bridges and public facilities will also be damaged and parts of the city will be isolated

Twelve years ago today, the island of Bohol in the Visayas Islands was the victim of the second strongest instrument-recorded Philippine earthquake to hit the Earth since the July 16, 1990, 7.8-magnitude North Luzon earthquake.

At twelve minutes past eight in the morning on October 15, 2013, a Tuesday and the day Filipino Muslims were celebrating Eid al-Adha, Bohol was struck by a strong earthquake measuring 7.2 magnitude. The epicenter of the earthquake was located near the town of Sagbayan, northwest of Bohol, at a shallow focal depth of 12 kilometers.

The earthquake was responsible for killing more than 200 people, injuring nearly a thousand people, and affecting more than 650,000 families. Infrastructure damage was severe and widespread, including broken bridges, displaced road sections, numerous collapsed government and commercial buildings, residential homes, and even the world-famous Chocolate Hills.

Even more devastating is the destruction of several centuries-old churches declared by UNESCO as World Heritage places of worship. Of the nearly a dozen churches damaged, the hardest hit was the Cathedral of Nuestra Señora de la Luz in the town of Lun on the coast in the southwest corner of the island. The church was literally destroyed, and all the precious artwork, architecture, culture and history disappeared with it, except for the image of the Virgin Mary, which miraculously remained intact even after it fell from its place on the facade of the church.

Same size as expectations

The 2013 Bohol earthquake unleashed a powerful magnitude of 7.2. This is the same magnitude as the earthquake that would be expected to occur if the West Valley Fault (WVF) that runs through Metro Manila ruptures.

How is the power of 7.2 estimated? Earthquake magnitude is a function of several fault parameters, including fault rupture length and fault surface displacement (SD). Rupture length is a measure of how much a section of a fault will slide during an earthquake. On the other hand, surface displacement is the amount of movement of known marks on the ground (e.g., road markings, tree alignments, fences, etc.) that are displaced by faulting.

Empirical equations, generated by processing numerous earthquake events around the world and analyzing their effects, are used to estimate magnitude as a function of rupture length and surface displacement. As the length of the rupture increases and the surface displacement increases, the volume increases logarithmically. The rupture length of the 2013 Bohol earthquake based on aftershock distribution was approximately 90 km. This is the same estimated length as the West Valley Fault, which runs from Bulacan in the north to Cavite in the south.

Why is WVF thought to be caused by rupture? The average time taken by one earthquake occurring on a particular segment of the fault to the next segment is called the recurrence period.

Recurrence intervals can be estimated by performing paleoarchaeological studies, which usually involve excavating a trench on a known portion of an active fault.

On trench walls, faults are examined to understand their nature, mechanism and chronological order, among other things. Once this is understood, soil samples containing organic matter are extracted for radiometric dating. For paleoseismic studies, the 14C dating method is usually used due to carbon's short half-life (thus providing accurate age intervals within a few hundred years). A fault is said to be active if it shows signs of movement within the past 10,000 years.

In a trench portion of the West Valley Fault in Rodriguez, Rizal, geologists from the United States Geological Survey (USGS) and PHIVOLCS in a 2000 study found two to four fault events generating large-magnitude earthquakes in the last 1,500 years, giving an average recurrence period of 400 to 600 years.

Six major earthquakes are believed to have struck the Metro Manila area between 1599 and 1863. Taking into account the margin of error in the dating technique and the uncertainty in the date of the last major earthquake to hit Metro Manila, a conservative recurrence interval of 400 years and a penultimate major earthquake in 1599 would mean a repeat of an earthquake that should have already occurred in 1999. But that has not happened yet, perhaps thanks to the margin of error in the carbon dating technique, temporarily benefiting Metro Manila and its environs. This gives the city more time to prepare for what many call “the big city.”

What are the estimated damages?

A 7.2 magnitude earthquake could cause many deaths and massive infrastructure damage in Metro Manila and its surrounding areas. In 2004, the Japan International Cooperation Agency (JICA) funded and conducted a study for the Metropolitan Manila Development Authority (MMDA) and PHIVOLCS entitled: Earthquake Impact Reduction Study in Metro Manila, to assess, among other things, the city's vulnerability to earthquake disasters.

The study conducted an inventory of population and infrastructure and examined their vulnerability in the event of a particular earthquake similar to the metropolitan area. In a model including a magnitude 7.2 earthquake generated by the West Valley Fault, the JICA study estimated the following impacts: about 33,500 people would die, 113,600 would be injured, 168,300 or about 13% of all residential buildings would be severely damaged, and 339,800 or 25.6% would be partially damaged. 11% of medium-rise buildings (10-30 floors) and 2% of high-rise buildings (more than 30 floors) will be severely damaged, while vital structures such as bridges will collapse, and electrical cables, power and water lines will be cut, isolating certain sectors of the city from each other.

Current readiness and readiness plans and strategies are based on the findings of this study. But since it was conducted more than 20 years ago, it needs to be revised keeping in mind that the input parameters have changed significantly including the increase in the population of major cities and the modification of the infrastructure landscape.

Full circle

The magnitude 7.2 Bohol earthquake was triggered by a previously unknown fault that was eventually christened by the PHIVOLCS North Bohol Fault. Among its best manifestations is that discovered in the city of Inabanga, leading other authors to mistakenly call it Inabanga.

The most visible ground profile of the earthquake was initially discovered crossing a road in Anunang Village in Inabanga Township by a journalist covering the aftermath of the earthquake. This discovery was confirmed as a superficial rupture caused by the earthquake days later by Dr. Teresito “Toto” Paculcol, who had been leading the PHIVOLCS rapid response team, and now the director of PHIVOLCS.

Earlier before the discovery of the surface rupture in Inabanga, I met in Lun City with Director Toto, my former structural geology student at the National Institute of Geological Sciences (UP NIGS), while my team was also conducting surveys of earthquake-induced ground deformation. The night after he found the surface trace, Director Toto texted me: “Dr. Marr, I found the bug!” I could only imagine the excitement and satisfaction on his face after he discovered what he called the holy grail of all active fault mappers – surface rupture.

Dr. Bakulkul's successor, as Director of PHIVOLCS, is my faculty colleague Dr. Renato “Rene” Solidum, who is now Secretary of the Department of Science and Technology (DOST). For his part, Secretary Rene was succeeded by the late Dr. Raymundo S. Punongbayan, the first director of PHIVOLCS. Dr. Punongbayan, fondly called RSP by his peers, was my professor and Secretary Rene's Professor in Structural Geology.

We've come full circle, actually! – Rappler.com

Mario A. Aurelio, Ph.D., is a professor at the National Institute of Geological Sciences at the University of the Philippines, and includes earthquake studies among his research interests. He received his PhD in Structural Geology and Tectonics from the Pierre and Marie Curie University – Paris VI University, in Paris, France. He is a recipient of the 2025 Distinguished Alumni Award for Conservation in Earth Sciences Education, presented by the University of the Philippines Alumni Association.

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