What triggered the powerful magnitude 7.8 earthquake in Mindanao?

Just a few minutes after half past seven in the morning of Monday, June 8, on the first day of the new school calendar instituted by the Department of Education (DepEd) for school year 2026-2027, a powerful magnitude 7.8 earthquake struck offshore General Santos City in Mindanao. Early reports of massive damage to infrastructure in the city have started coming in.

The likely culprit

Mindanao Island is surrounded by three active subduction trenches. The Sulu Trench runs offshore on the west coast of the Zamboanga Peninsula, the Philippine Trench skirts the eastern coast of Mindanao from Siargao Island in the north to Mati in Davao Oriental in the south, and in the middle, the Cotabato Trench hugs the western coast of the main island.

The Cotabato Trench takes an arcuate geometry which is concave to the east trending generally northwesterly, following the curving coastline from Sindangan in the north to Sarangani in the south. Until there, the trench then extends southwards and notably bends more northerly towards the Indonesian islands of Sangihe, and adopts the name Sangihe Trench.

Based on preliminary reports from local and global seismological observatories, it is likely the northern section of the Sangihe Trench and/or the southern segment of the Cotabato Trench that ruptured and generated the earthquake.

Magnitude estimation

The Philippine Institute of Volcanology and Seismology (Phivolcs) earlier reported that the magnitude of the earthquake was magnitude 7.0, while global networks registered magnitudes between 8.0 and 8.2, which are more than 30 times more powerful. But as more earthquake data were collected, Philvolcs upgraded the magnitude to 7.8 while global networks downgraded to the same magnitude. Both local and global networks also revised their focal depths from an earlier shallow 10 kilometers to around 40 kilometers.

It is important to be reminded that when earthquakes strike, magnitude and focal depth estimates should be treated as very preliminary. More earthquake information, including unfortunately more earthquakes in the form of aftershocks need to happen and be collected by the seismological observatories to improve their earthquake parameter calculations.

The energy released by a magnitude 8.0 earthquake is about 33 times more powerful than that released by a magnitude 7.0. This relationship is very important in evaluating (and therefore preparing for) the capability of the earthquake to inflict damage to infrastructure that also often accompanies death.

With reference to the current magnitude estimate of 7.8, the earlier Phivolcs magnitude of 7.0 was underestimated by about 23 times in energy release, while the global magnitude of 8.1 was overestimated by about 5 times. – Rappler.com

Mario A. Aurelio, PhD is a professor at the National Institute of Geological Sciences – University of the Philippines (UP NIGS). His fields of interest include earthquake studies.