Cotabato Trench linked to 3 major Mindanao quakes in 50 years

MANILA, Philippines — The magnitude 7.8 earthquake that struck off the coast of General Santos City on Monday, June 8, originated from movement along the Cotabato Trench—the same active offshore trench linked to two other major earthquakes in Mindanao over the last five decades, according to Phivolcs and earthquake records.

"Aktibo 'yung Cotabato Trench. Marami tayong nare-record dito na mga earthquakes na maliliit," Winchelle Sevilla, chief science research specialist at the Seismology Division of Phivolcs, said Monday. (The Cotabato Trench is active. We have recorded several small earthquakes from that trench.)

Sevilla said earthquake swarms, or clusters of small-to-moderate earthquakes, were also recorded in parts of Sultan Kudarat from January to May due to movements associated with the Cotabato Trench.

A swarm is different from an aftershock sequence. Aftershocks follow a main earthquake, while a swarm refers to many earthquakes occurring in the same area without one clearly dominant main shock.

The Cotabato Trench is an oceanic trench southwest of Mindanao, where tectonic movement can generate powerful undersea earthquakes. A quake’s magnitude measures the energy released at the source, while its intensity refers to how strongly the shaking is felt or recorded in a specific area.

The June 8 earthquake was recorded at magnitude 7.8, but shaking intensity varied across affected areas depending on distance from the epicenter, depth, ground conditions and structures.

Deadly history

The same trench system was linked to the August 17, 1976 Moro Gulf earthquake, one of the deadliest disasters in Philippine history.

Based on Phivolcs records, the offshore earthquake was generated by movement along the Cotabato Trench. It struck at 12:11 a.m. near the Moro Gulf and triggered destructive tsunami waves that devastated coastal communities in Mindanao.

The earthquake and tsunamis killed thousands, with government and historical accounts placing the toll at at least 4,000 dead, around 3,000 missing and more than 9,000 injured.

Affected areas included Pagadian City, Zamboanga del Sur, Zamboanga City, Basilan and Sulu, as well as Sultan Kudarat, Maguindanao, Cotabato City, Lanao del Sur and Lanao del Norte.

2002 Mindanao quake

Another major earthquake linked to the Cotabato Trench struck southern Mindanao on March 6, 2002, at 5:16 a.m. Philippine time, according to the U.S. Geological Survey.

USGS recorded the quake at magnitude 7.5, with its epicenter southwest of Sultan Kudarat. It affected parts of Sultan Kudarat, Sarangani, North Cotabato and South Cotabato.

Disaster records cited in past reports placed the toll from the 2002 quake at eight deaths and 41 injuries, with damage estimated at P4.175 million. More than 7,000 families were also affected.

As of 2 p.m. Monday, at least 15 people had been killed in the June 8 earthquake that struck off Sarangani province, according to the Office of Civil Defense.