EXPLAINER: Why did the magnitude 7.8 Sarangani quake happen?

CEBU CITY, Philippines — The magnitude 7.8 earthquake that struck offshore Sarangani on Monday, June 8, 2026, may be linked to movement along the Cotabato Trench, according to the Philippine Institute of Volcanology and Seismology (Phivolcs).

The earthquake occurred at 7:37 a.m. Philippine Standard Time.

Phivolcs traced the epicenter 32 kilometers west of Maasim, Sarangani. The quake had an estimated depth of 33 kilometers.

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What caused the earthquake?

Phivolcs said the quake’s location, focal mechanism, and aftershock pattern point to subduction along the Cotabato Trench.

But what does this mean?

The Cotabato Trench is a major earthquake-generating structure off the coast of Mindanao.

In simple terms, a trench is an area where one part of the Earth’s crust moves beneath another. Scientists call this process subduction.

READ: Mindanao earthquake: Mandaue, Lapu-Lapu ready to send help

When pressure builds up underground, rocks can suddenly move. That sudden movement causes an earthquake.

Why Sarangani is quake-prone

Sarangani and nearby provinces sit near a seismically active area.

Phivolcs said Sarangani faces the Cotabato Trench. The area also has nearby local faults. Some of these faults may be hidden under recent deposits.

These structures can generate minor to great earthquakes.

Aftershocks may continue

As of 11 a.m. on June 8, Phivolcs recorded 138 aftershocks from the magnitude 7.8 earthquake.

The aftershocks ranged from magnitude 1.3 to magnitude 6.7.

Phivolcs said aftershocks may continue for several days to weeks. Some may still be felt in nearby provinces.

Tsunami waves recorded

The earthquake also triggered tsunami waves in some coastal areas.

Phivolcs recorded waves of about one meter along the coasts of Kiamba and Maasim in Sarangani, and Kalamansig in Sultan Kudarat.

The agency also recorded smaller tsunami waves in Mati City and Zamboanga City.

Phivolcs reminded coastal communities to watch for natural signs of a local tsunami. These include strong shaking, a sudden rise or fall of seawater, and a roaring sound from incoming waves.

Residents should move to higher ground immediately if they observe these signs.

Not linked to volcanic activity

Phivolcs said the earthquake was tectonic in origin.

This means movement in the Earth’s crust caused the quake. It was not caused by volcanic activity.

Matutum and Parker volcanoes are the nearest active volcanoes to Sarangani. However, Phivolcs said the earthquake does not indicate volcanic activity.

The agency urged the public to prepare for aftershocks, avoid damaged structures, and follow updates only from DOST-Phivolcs and other authorities.

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