MANILA, Philippines – The magnitude 6.8 earthquake that rocked a large portion of Mindanao on Friday afternoon, November 17, was likely generated by the Cotabato Trench, according to state volcanologists.

The Philippine Institute of Volcanology and Seismology (Phivolcs) said in a primer released on Saturday, November 18, that the earthquake’s epicenter was located 28 kilometers southwest of Sarangani, Davao Occidental, with an estimated depth of 63 kilometers.

It struck at 4:14 pm, causing at least five deaths and triggering panic in parts of Mindanao.

Phivolcs said the highest reported intensity was VII (Destructive), felt in Glan, Sarangani.

Under Intensity VII, many people run outdoors; heavy objects and furniture are toppled; old or poorly built structures suffer considerable damage; even some well-built structures are slightly damaged; cracks may appear on roads, dikes, and concrete walls; and trees shake strongly.

Landslides and liquefaction – when sediments behave like liquid – are also observed at Intensity VII.

‘Seismically active’ region

Phivolcs explained on Saturday that Davao Occidental and nearby provinces “are situated in one of the seismically active regions in the country.”

There are “offshore active faults and trenches, including an unnamed offshore fault east of Davao Occidental and the Cotabato Trench.”

“Additionally, there are local faults in proximity, some potentially concealed by recent deposits capable of generating earthquakes ranging from minor to strong magnitudes,” Phivolcs said.

The agency’s records show at least six major earthquakes, ranging from magnitude 5.7 to 8, affected Sarangani, Davao Occidental, and nearby areas between 1917 and 2017. The 2017 earthquake, which occurred on April 29 that year, was magnitude 7.2.

Phivolcs also said the magnitude 6.8 earthquake on Friday was not linked to volcanic activity, since it was found to be “tectonic in origin.” The nearest active volcano to the town of Sarangani is Parker Volcano, located in the province of South Cotabato.

Aftershocks

As of 8 am on Saturday, or nearly 16 hours since the earthquake, Phivolcs had recorded 58 aftershocks ranging from magnitude 1.4 to 3.9.

Aftershocks are expected to last for several days to weeks in the epicentral area, said Phivolcs, and could also be felt in nearby provinces.

“Communities should prepare for aftershocks. In case of another strongly felt earthquake, it is recommended that people protect themselves by doing the ‘drop, cover, and hold,’” the agency said.

Aftershocks or succeeding earthquakes may also cause further damage to structures already weakened by Friday’s quake.

“Civil engineers from the local government and other organizations are strongly enjoined to inspect buildings and infrastructure to determine their structural integrity and recommend appropriate actions,” Phivolcs said.

“Structurally compromised buildings should not be reoccupied unless certified safe by structural engineers.” – Rappler.com