Phivolcs warns of unstable ground in 38% of General Santos barangays

CAGAYAN DE ORO, Philippines – The Philippine Institute of Volcanology and Seismology (Phivolcs) said liquefaction threatens at least 38% of barangays in General Santos City after the magnitude 7.8 earthquake, as scientists mapped zones where water-saturated ground may lose stability under strong shaking.

The June 8 earthquake left more than 60 people dead, dozens missing, and over 1,000 injured, while damaging and destroying infrastructure across parts of Mindanao.

In General Santos alone, 22 people were killed and more than 500 others injured in the powerful earthquake, showed city government data as of Monday, June 15.

The quake also adversely affected at least 180,519 people, destroyed 3,004 houses and damaged 22,469 others in the city, the latest data showed.

Phivolcs said on Sunday, June 14, that at least 10 of the city’s 26 barangays were classified as high-risk zones, a designation that signals a localized but severe failure of soil under stress.

Liquefaction happens when strong shaking makes wet, water-filled soil lose its strength, so solid ground can behave more like liquid. It can cause the ground to crack, buildings to sink, and sand and water to bubble up to the surface, especially in low-lying areas near rivers or the coast.

The high-risk barangays identified by Phivolcs include Buayan, Baluan, Lagao, Bula, Katangawan, Tinagacan, Batomelong, and parts of City Heights, San Isidro and Conel.

Kathleen Papiona, a senior science research specialist at Phivolcs, said residents and developers in these areas should have structural engineers assess building foundations and ensure compliance with seismic design standards, including for future construction.

The remaining General Santos barangays were placed under moderate and low-risk categories, though Phivolcs said the classifications remain subject to refinement as new data and field validation become available.

Papiona said the presence of liquefaction hazards does not prohibit development but stressed that compliance with the National Building Code and structural standards remains essential, particularly for critical infrastructure.

Phivolcs said General Santos remains prone to the process in which water-saturated soil temporarily loses strength during strong shaking, leading to ground cracks, subsidence and surface deformation.

During a briefing at the General Santos City Disaster Risk Reduction and Management Office (CDRRMO), Phivolcs clarified that cracks and ground fissures reported after the magnitude 7.8 earthquake were not caused by any active on-land fault.

Papiona said the earthquake originated from the offshore Cotabato Trench, with no recorded movement of active faults on land in the affected areas.

She said the offshore trench generated strong ground shaking that, based on Phivolcs data, reached intensity 8 in some areas, accounting for the widespread impact across parts of Mindanao.

More than 5,000 aftershocks had been recorded as of Sunday morning, including over 1,300 plotted on seismic maps and 76 felt by residents. – Rappler.com