At 10:37 on the night of December 2, 2023, the strongest earthquake to hit this part of eastern Mindanao since 1992 struck off the coast of Surigao del Sur. Nearly three years later, the official record of what it did to Bislig City is worth knowing, because the fault that caused it has not gone anywhere.

How strong it was

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) measured the quake at magnitude 7.4 and placed it offshore near Hinatuan, the town just north of Bislig. The United States Geological Survey rated it slightly higher, at magnitude 7.6, at a depth of about 40 kilometers. PHIVOLCS reported Intensity VII, which it classifies as "destructive," in Tandag City.

The quake came from the Philippine Trench, the deep undersea boundary east of Mindanao where the Philippine Sea plate slides beneath the islands. The USGS described it as shallow oblique-thrust faulting along that subduction zone. It was the first major earthquake in the area since a magnitude 7.1 and 7.5 doublet in 1992, which also produced a tsunami.

A tsunami reached Bislig

PHIVOLCS issued tsunami warnings for Surigao del Sur and Davao Oriental that night. Waves did arrive, though they were small. A tsunami of 18 centimeters was recorded at Lawigan in Bislig, according to PHIVOLCS data reported by The Philippine Star. The highest wave recorded in the country, 64 centimeters, was observed on Mawes Island. In Davao City, the tsunami measured 8 centimeters.

What it did to Bislig

The earthquake killed three people across Mindanao, and one of them was in Bislig. The deaths were attributed to collapsing concrete walls.

The damage to homes in the city was significant. The Department of Social Welfare and Development's final DROMIC report on the earthquake, issued on February 5, 2025, counted 66 houses destroyed and 1,507 damaged in Bislig alone. Across all affected areas, more than 10,000 houses were damaged, 444 of them destroyed.

The Civil Aviation Authority of the Philippines also reported minor damage at Bislig Airport, along with the airports in Butuan, Surigao, Sayak and Tandag.

Surigao del Sur Governor Alexander Pimentel suspended classes in affected areas until December 6 and cancelled Christmas parties in government offices that year to encourage donations to quake victims instead.

It kept shaking

PHIVOLCS recorded 8,403 aftershocks in the weeks that followed. The strongest, a magnitude 6.9 quake on December 4, 2023, struck near Cagwait, although PHIVOLCS treated it as a separate earthquake. On August 2, 2024, two more quakes of magnitude 6.8 and 6.3 struck within the same area.

Why it matters now

The Philippine Trench sits off Bislig's coast. Large earthquakes linked to it or to this stretch of coast were recorded in 1897, 1921, 1924 and 1992 before the 2023 quake, and the 1921 and 1992 events also produced tsunamis. The 2023 quake showed that waves can reach Bislig's shoreline and that falling concrete walls are a real danger when the ground shakes.

Families living near Bislig Bay should know their nearest evacuation route and higher ground. PHIVOLCS treats strong ground shaking near the coast as a natural tsunami warning sign, so residents should move away from the shore right away rather than wait for an official announcement.

Sources

  • Wikipedia, "December 2023 Mindanao earthquake," citing PHIVOLCS, USGS, NDRRMC and DSWD DROMIC reports
  • DSWD DROMIC Terminal Report on the Mw 7.4 Earthquake Incident in Hinatuan, Surigao del Sur, February 5, 2025
  • The Philippine Star, "Tsunami warning lifted after Magnitude 7.4 quake strikes off Surigao del Sur," December 3, 2023