7.8 magnitude quake in Mindanao triggered by Cotabato trench — PHIVOLCS

The magnitude 7.8 earthquake that shook Mindanao on Monday morning, with the highest Intensity VII reported in General Santos City, was likely triggered by movement along the Cotabato Trench, according to the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

“Based on the location of the earthquake, focal mechanism, and aftershock distribution, the earthquake may be attributed to subduction along the Cotabato Trench,” PHIVOLCS said in its primer report on the earthquake.

The Cotabato Trench was also responsible for the deadly August 17, 1976 Moro Gulf earthquake, in which a magnitude 7.9 earthquake triggered a massive tsunami that devastated 700 kilometers of coastline bordering Moro Gulf, PHIVOLCS said in its damaging earthquake compilation report.

The tsunami waves reached a maximum height of 9 meters, or 30 feet, causing the majority of the 7,079 reported deaths and missing persons in Regions IX and XII following the quake.

In total, at least seven significant earthquakes were recorded in Sarangani and nearby provinces between 1917 and 2023, with magnitudes ranging from 5.7 to 8.0, PHIVOLCS said in its primer report.

The bureau said these areas are particularly vulnerable to earthquakes because they face the Cotabato Trench, a major earthquake-generating structure, and several nearby active faults, as well as several nearby active faults, some of which may be hidden beneath soil and sediment.

“The Cotabato Trench and these active faults are capable of generating minor to great earthquakes,” it said.

Meanwhile, PHIVOLCS had already lifted the tsunami warning in relation to the magnitude 7.8 earthquake as of 3:19 pm.

Earlier, tsunami waves of 1.4 meters in height were recorded in Kiamba, Sarangani, while waves of less than one meter were reported in Kalamansig, Sultan Kudarat; Maasim, Sarangani; Zamboanga City; Mati City; and Tandag City.

Aftershocks, though, remain expected in areas near the epicenter and may persist for several days to weeks.

PHIVOLCS cautioned residents to continue practicing “Drop, Cover, and Hold” during aftershocks.

It also advised residents to report structures that may have been weakened or damaged by the earthquake to municipal or city engineering offices for assessment.

Damaged buildings should not be reoccupied until they are certified safe by structural engineers. — Edg Adrian A. Eva