EXPLAINER: What is an earthquake swarm?

The magnitude 7.8 earthquake that struck Mindanao on June 8, 2026, originated in the Cotabato Trench, according to the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Before the earthquake, experts recorded a series of weak tremors, or an earthquake swarm, in the Cotabato Trench. PHIVOLCS clarified that this does not mean the swarm triggered the mainshock.

An earthquake swarm is a series of consecutive moderate-sized earthquakes with no distinct mainshock. These tremors are typically around magnitude 4 or 5 and can continue for days, months, or even more than a year.

PHIVOLCS said an earthquake swarm differs from the typical earthquake sequence, which begins with a strong mainshock followed by smaller aftershocks.

In an earthquake swarm, there is no single dominant earthquake. Instead, it consists of a cluster of similarly sized earthquakes, with the strongest tremor occurring at any point in the sequence, including the middle or later stages.

From May 26 to June 26, PHIVOLCS recorded 287 earthquakes near the Manila Trench, which is capable of generating earthquakes greater than magnitude 8.

According to PHIVOLCS, the occurrence of an earthquake swarm does not automatically indicate that a major earthquake will follow. In many cases, the tremors gradually weaken and eventually stop without a major mainshock.

Although earthquake swarms may or may not precede stronger earthquakes, experts say they should not be ignored.

Unlike weather systems, earthquakes cannot be accurately predicted, making preparedness essential.

Before an earthquake, prepare for a possible evacuation by keeping an emergency go-bag or disaster supply kit ready. It is also important to familiarize yourself with the safest and quickest routes to designated evacuation areas.

During an earthquake, remember the safety protocol: Duck, Cover, and Hold.

People living in coastal communities should also watch for signs of a tsunami, including prolonged or strong ground shaking, the sudden retreat or unusual behavior of the sea, and a loud roaring sound coming from the ocean. — Lyjah Tiffany Bonzo, with research from GMA Digilab/VBL, GMA News