In the aftermath of the magnitude 7.8 earthquake that struck offshore south of Sarangani Peninsula in Mindanao on June 8, 2026, numerous social media outfits both from official government institutions involved in the study of earthquakes, as well as from ordinary netizens, have posted discussions on the coastal uplift phenomenon observed on several shores on the peninsula.

The official posts have explained very well the science behind the phenomenon, but some netizen posts and their commentaries have provided rather confusing information sometimes sending alarming messages that may cause undue panic to the public.

On the ground, a common question asked by locals is whether or not the coastal uplift, which is also expressed by the recession of the shoreline, could be a sign that the sea can come back in the form of a tsunami.

It’s a matter of thrust

Plate Tectonics Theory states that the earth’s crust is transected by numerous faults. Faults are cracks that separate blocks of rocks that move against each other. In general, there are three types of faults as a function of the relative motion of the blocks. When one block moves horizontally on a vertical fault plane against the other block, the fault is said to be a “strike-slip fault.” A “normal fault” happens when, in an inclined fault plane, the block resting on top of the other moves downwards.

When the block above the inclined plane moves upward, the fault is called a “thrust fault” or simply, a “thrust” — the overhanging block is thrust upwards. For analogy, imagine a car being loaded from behind a trailer truck along a ramp. As the car is driven onto the trailer, it is “thrust” upwards along the ramp.

The size of faults ranges from microscopic (a microscope is needed to discern the relative movement of blocks) to the scale of tectonic plates. When faults move, the friction between the blocks generate shaking. At the scale of tectonic plates, fault movement generates the shaking of the ground or the earth, hence earthquakes.

The magnitude 7.8 Sarangani earthquake of June 8, 2026, was generated by the movement along a thrust fault. At the scale of tectonic plates, this thrust fault is expressed in the form of a “trench,” a term that refers to an elongated depression usually underneath the ocean. The dynamic processes that occur along a trench at depth define a subduction system/zone. The Philippine Institute of Volcanology and Seismology (Phivolcs) has reported that the Sarangani earthquake was generated by fault movement along the Cotabato subduction zone.

The Cotabato Trench is one among five trenches that surround Mindanao Island, the other four being the Sulu, Philippine, Sangihe, and Halmahera trenches. The Sulu Trench runs offshore on the west coast of the Zamboanga Peninsula while the Philippine Trench skirts the eastern coast of Mindanao from Siargao Island in the north to Mati in Davao Oriental in the south. In central Mindanao, the Cotabato Trench hugs the western coast of the main island. The Sangihe and Halmahera trenches are manifested in the Davao Gulf and Mollucas Sea south of Mindanao Island.

Coastal uplift

When a subduction trench generates an earthquake with sufficient magnitude, the crust above the inclined thrust fault is pushed upwards or uplifted. This uplift can be measured using techniques in geodesy, the science of measuring dimensions of the earth. On land a geodesic technique involves measuring distances and elevations on the surface of the earth before and after an earthquake. The difference in the measurements provides information on the amount of displacement of the ground caused by the earthquake.

Nowadays, satellite technology also provides additional techniques to measure ground deformation resulting from earthquakes. One such technology is called interferometry, where images of a specific area of the earth are gathered by a satellite at different passing times. Since the satellite is always “above” the surface when it passes the same area over the earth at different times, it can detect if the ground has moved closer or farther away from it in between passes. Ground moving closer to a satellite positioned directly above means uplift, while subsidence is manifested when the ground moves farther away from the satellite.

Results from preliminary interferometric analysis of the earthquake by international space observatories indicate uplift in the southern tip of Sarangani Peninsula, with maximum upward motion detected in the towns of Glan, Sarangani province; and Jose Abad Santos, Davao Occidental.

Onsite verification confirms the satellite-observed uplift, particularly in such villages as Pangyan, Burias, Big Margus, Small Margus, and Batulaki in Glan; and Kitayo, Bukid, and Camalian in Jose Abad Santos. When uplift occurs along a coast, what was once seabed submerged underwater may rise and be exposed to the surface and become “new” land. The uplift allows the shoreline to recede seaward, thus pushing back the tide line.

One telltale sign of a tsunami is a receding shoreline. A tsunami is born out in the sea, when the seabed moves vertically due to thrusting which pushes the water column above it upwards that then creates the broad waves that rush towards shore. A tsunami wave has a short wave height in the middle of the ocean where the water column is deep. It grows in height as it reaches shallow waters onshore.

The recession of the shoreline during a tectonic uplift is permanent, or at least until the next earthquake takes place again, while it is short-lived in the case of a tsunami. In Sarangani, the shoreline has receded by as much as 200 meters. Since the recession is uplift related, seawater is not expected to go back and create the tsunami feared by residents of coastal villages in the peninsula. At least, not until the next earthquake strikes again. – Rappler.com

Acknowledgements

The author is leading a team of geologists sent to Ground Zero by the UP Resilience Institute to assist communities affected by the earthquake in post-disaster assessment. He wishes to acknowledge the contributions of his teammates, Abigail Manahan and Micko Palma, geologists and graduate students of UP Diliman.

Mario A. Aurelio, PhD is a professor at the National Institute of Geological Sciences – University of the Philippines (UP NIGS). Among his fields of interest include earthquake studies.