Department of Science and Technology (DOST) Secretary Renato Solidum Jr. on Wednesday explained why the seabed and coastline in Glan, Sarangani, and nearby areas rose by as much as two meters following the magnitude 7.8 earthquake that struck Mindanao on 8 June.
Solidum, a former director of the Philippine Institute of Volcanology and Seismology (PHIVOLCS), said the phenomenon, known as coastal uplift, occurs when a powerful earthquake pushes the ocean floor and coastline upward.
“Coastal uplift happens when the land is being raised during a major earthquake,” Solidum said in an ambush interview on the sidelines of the National Nutrition Summit organized by the DOST-Food and Nutrition Research Institute at the Sheraton Manila Hotel in Pasay City.
He explained that earthquake movements may be horizontal or vertical, citing the 2013 Bohol earthquake as an example.
“For example, in 2013, at the North Bohol Fault, one side of Bohol rose and the other side fell. The side that fell, facing Cebu, experienced subsidence, while another significant part of Bohol Island, including Maribojoc, rose,” he said.
Solidum said the recent earthquake involved the movement of the Celebes Sea plate beneath Southwest Mindanao along the Cotabato Trench.
“When there is a dive, it does not just move. There is friction because the rocks are interacting,” he explained.
He said the sudden release of accumulated pressure produces a rebound that can permanently raise portions of the coastline.
“When it suddenly moves, there is a rebound. That sometimes becomes permanent,” he said.
According to Solidum, the phenomenon also affects seawater displacement, producing tsunami waves while permanently altering the coastline.
“So there are two results. You will see the tsunami and then permanent coastal uplift. And we saw two meters in another place. It depends on the shape of the coastline,” he said.
Solidum said coastal uplift is not expected to significantly affect human settlements over the long term, as it effectively adds land area, although marine ecosystems will gradually adjust.
“The human community is not affected because they will be added to the land area in the long term. But essentially, the biodiversity will adjust,” he said.
He explained that coral reefs may eventually develop farther from the original shoreline and noted that elevated coral formations and limestone deposits are evidence of previous major earthquakes.
“If you see corals or limestone above the shoreline, it means there was a strong tectonic event that caused these features to rise,” he said, adding that stepped coastal formations known as marine terraces are remnants of repeated uplift events over time.
Asked whether a magnitude 7.8 earthquake could also occur in Metro Manila, Solidum pointed to the Metro Manila Earthquake Impact Reduction Study conducted from 2002 to 2004.
The study examined 18 earthquake scenarios involving faults in and around the capital and identified the magnitude 7.2 West Valley Fault event as the worst-case scenario because of its proximity.
He added that the Manila Trench is capable of generating earthquakes of magnitude 8.4 to 8.5, although shaking in Metro Manila would likely be weaker because the source is located offshore.
Solidum also stressed the importance of strict compliance with the National Building Code to minimize earthquake damage.
“Our building code, when followed, would assure us that the buildings will not collapse. If you follow the design, materials used, and actual detailed construction,” he said.
He added that authorities should ensure newly built structures, particularly residential houses, are properly inspected to verify their ability to withstand major earthquakes.
