EXPLAINER: What is a coastal uplift?

MANILA, Philippines — The coastlines in Sarangani and Davao Occidental are now the focus of scientific investigations after the magnitude 7.8 earthquake that struck Mindanao on June 8 exposed coral reefs that were once submerged.

As the Philippine Institute of Volcanology and Seismology (Phivolcs) explained, the phenomenon is called coastal uplift, a vertical deformation of the coast in response to the upward and downward movement of the crust along a fault or subduction zone.

Phivolcs said coastal uplift or subsidence may happen suddenly during an earthquake, or coseismic, or gradually during slow progressive deformation of the crust, or interseismic.

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As a result, areas that were previously submerged have been exposed.

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READ: Coastal uplift seen in Sarangani, Davao Occ. after Mindanao earthquake

Last week’s earthquake, which killed 65 people based on government data as of Monday, June 15, had an offshore epicenter located 32 kilometers west of Maasim, Sarangani, and an estimated depth of 33 kilometers.

Based on the location of the earthquake, its focal mechanism and aftershock distribution, the earthquake may be attributed to subduction along the Cotabato Trench, one of the six active trenches in the Philippines.

READ: Magnitude 8.2 quakes possible along PH trenches, says Phivolcs

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A shift in the Cotabato Trench “pushed upward part of the coastlines of Sarangani and Davao Occidental […] exposing the bottom of the sea that was originally submerged,” Phivolcs said in a statement.

The trench, which lies as close as 50 kilometers off the coast of southern Mindanao, is the site of frequent seismic activity, including a “swarm” of thousands of mostly small earthquakes recorded in the first month of the year.

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Phivolcs said the movement resulted in almost 2 meters of coastal uplift, with the seabed raised above its former level, exposing wide stretches of coral reefs and seagrass beds.

Because of the uplift, the coastal land expanded by as much as 200 meters in some areas, while tidal levels and shoreline positions changed.

The phenomenon was recorded in Barangay Balangonan in Jose Abad Santos, Davao Occidental, and Barangay Panglan in Glan, Sarangani.

While Phivolcs pointed out that such a landform created after a strong earthquake does not indicate that a tsunami is imminent, exposed corals and seagrass beds had started “dying off alongside their resident organisms such as reef fishes, eels, clams, and shells.”

Because of this, residents said they were concerned that the stench of decaying marine life could poison them.

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Last year, a 1-meter coastal rise was also observed in Barangay Nailon in Bogo City, Cebu, as a result of a magnitude 6.9 earthquake that rocked the Visayas on Sept. 30. /dm