7.8 magnitude Mindanao earthquake, not yet “The Big One” – PHIVOLCS

The 7.8-magnitude tremor generated by the Cotabato Trench that rattled Mindanao cannot yet be called “The Big One,” as it is not the maximum possible earthquake the trench can produce, according to the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

“The Big One” refers to the strongest possible earthquake that may occur in a specific area, triggered by a trench or fault, Bhenz Rodriguez, senior science research specialist at PHIVOLCS said.

The strongest possible earthquake is usually determined based on the length of the generating fault or trench and historical seismicity data.

In the case of the recent 7.8-magnitude earthquake that occurred offshore of Sarangani last Monday, it is not regarded as “The Big One” because an 8.1-magnitude earthquake was already recorded in 1976, known as the Moro Gulf earthquake, which serves as a historical benchmark for the Cotabato Trench.

“Based on seismicity and historical data, what happened was not ‘The Big One.’ What we recorded was around magnitude 7.8, and that is not the maximum for the Cotabato Trench,” Mr. Rodriguez said in an online interview.

“However, since the Cotabato Trench has already generated a 7.8-magnitude earthquake in this area, the likelihood of another 8.1-magnitude event in the same segment may be lower, although it remains possible,” Mr. Rodriguez added.

Several areas in the country lie near active fault systems capable of producing major earthquakes, commonly referred to as “The Big One,” which can cause widespread damage.

Metro Manila’s version of “The Big One” is projected to be a 7.2-magnitude earthquake triggered by movement along the West Valley Fault System.

In such an event, fatalities could reach up to 48,000, with economic losses estimated at $48 billion, according to the Greater Metro Manila Area Risk Assessment Project.

As “The Big One” continues to pose a major threat to the capital, Mr. Rodriguez urged continued preparedness at the community level, including regular earthquake drills and ensuring the structural integrity of public and private infrastructure and residences.

More importantly, he emphasized that Filipinos must also be prepared at the personal and family level.

Meanwhile, aftershocks following the 7.8-magnitude Mindanao earthquake are expected to last between two weeks and two months, Mr. Rodriguez said.

“We can expect aftershocks to continue for two weeks to two months. Over time, the aftershocks will gradually become smaller and less frequent,” Mr. Rodriguez said.

As of 6:00 p.m., PHIVOLCS had recorded 3,254 aftershocks related to the earthquake, which occurred across 852 locations.

Of the total aftershocks, 64 were felt, ranging in magnitude from 1.2 to 6.4.

Amid continued tremors, Mr. Rodriguez urged affected residents to prepare for aftershocks by ensuring that if their homes are still standing, these should be inspected by structural engineers to assess possible damage or risk of collapse.

The same precaution should also be applied to residents living in landslide-prone areas. — Edg Adrian A. Eva