The 7.7-magnitude earthquake that struck the Mindanao Sea in the Philippines on Monday (Jun. 8) serves as a reminder that Southeast Asia lies within a highly active tectonic region. The earthquake, which had the potential to trigger a tsunami affecting several parts of Indonesia, was caused by the subduction of one plate beneath another via a thrust fault mechanism.
A geology lecturer from the Faculty of Engineering, Universitas Gadjah Mada (FT UGM), Gayatri Indah Marliyani, Ph.D., explained that earthquakes are common in tectonic plate convergence zones, such as those around the Philippines and Indonesia. In the Mindanao Sea, there is a subduction zone, an area where one tectonic plate descends beneath another.
“This subduction process occurs very slowly, only a few centimeters per year, but energy continues to accumulate over tens to hundreds of years,” said Gayatri on Thursday (Jun. 11).
According to her, the relationship between tectonic stress and earthquakes is straightforward. When accumulated stress exceeds the strength of the surrounding rock, the fault plane ruptures and shifts suddenly, generating an earthquake. In a thrust fault mechanism, she explained, one block of rock is pushed upward relative to another due to compressional forces.
Gayatri noted that special attention is needed when such movement occurs beneath the sea. If the movement causes sudden uplift or subsidence of the seafloor, the overlying water column is displaced, potentially generating tsunami waves.
“The combination of a large magnitude, offshore source, relatively shallow depth, and thrust fault mechanism creates conditions that warrant concern regarding tsunami potential,” she said.
Gayatri further explained that the main cause of the earthquake was tectonic activity within the Mindanao Sea subduction zone. She emphasized that the earthquake source did not originate from a single onshore fault extending toward Sulawesi, but rather from active tectonic plate movement in the region. She added that tremors were felt as far as Sulawesi because seismic wave energy from major earthquakes can travel hundreds to thousands of kilometers from the source.
“The larger the earthquake’s magnitude, the wider the area that can feel the shaking,” she said.
She added that the earthquake’s epicenter and depth play critical roles in determining tsunami potential. Earthquakes occurring beneath the sea generally have a greater chance of generating tsunamis than those on land. However, magnitude alone is insufficient to determine whether a tsunami will occur, as fault mechanism and the scale of seafloor displacement are also key factors.
Gayatri said the Mindanao earthquake should serve as a reminder for Indonesia, which is also located within the Pacific Ring of Fire. She stressed that Indonesia has numerous major earthquake sources, including subduction zones along Sumatra, Java, Nusa Tenggara, Maluku, and Sulawesi, as well as active onshore faults. Nevertheless, she emphasized that the Mindanao earthquake does not mean a major earthquake will immediately occur in Indonesia, as each seismic source has its own characteristics and cycle.
“What matters most is using this event as momentum to improve preparedness,” she said.
According to Gayatri, Indonesia’s long history of earthquakes and tsunamis underscores that these hazards are real risks that must be addressed collectively. She therefore recommended continued disaster education, evacuation drills, earthquake-resistant infrastructure development, and stronger early warning systems.
Regarding disaster mitigation, she reminded coastal communities to evacuate immediately when tsunami warnings are issued by authorities. She stressed that people should not wait for natural signs such as receding seawater or incoming waves, as evacuation time is often extremely limited. In such situations, a quick response is essential for survival.
“For people who experience strong and prolonged shaking in coastal areas, the principle to remember is: ‘strong earthquake, move away from the coast immediately,’” she said.
In addition, Gayatri urged the public to pay attention to designated evacuation routes and, if available, vertical evacuation shelters, and proceed to them without delay. She also emphasized the importance of relying on official information sources, including the Meteorology, Climatology, and Geophysics Agency, the National Disaster Management Agency, regional disaster management agencies, local governments, and other relevant institutions. The spread of unverified information can cause panic and disrupt evacuation efforts.
In closing, Gayatri emphasized that earthquakes and tsunamis are part of natural dynamics and geological processes that cannot be prevented. However, their impacts can be minimized through strong preparedness.
“The most important thing is to build a culture of disaster awareness. Vigilance does not mean living in fear. With good knowledge and proper response, risks can be reduced, and more lives can be saved when disasters occur,” she concluded.
Author: Hanifah
Editor: Gusti Grehenson
Post-editor: Rajendra Arya
Photo: BMKG