A magnitude 6.8 earthquake struck offshore eastern Mindanao in the southern Philippines, resulting from renewed movement along a subducting fracture zone. The tremor, which occurred at 6:23 a.m. local time, was centered 48 kilometers east-northeast of Lingig, Surigao del Sur, at a depth of 25 kilometers. This event follows a magnitude 7.4 earthquake that hit the region eight months earlier, indicating a possible connection between the two seismic occurrences.
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that the earthquake did not cause significant damage or casualties, although it was followed by six aftershocks ranging in magnitude from 4.8 to 6.2. These aftershocks occurred at shallower depths, further unsettling the region.
Seismotectonic Background
The earthquake’s origin is tied to the tectonic activity of the Philippine Fault and the Philippine Trench, both of which play a significant role in generating earthquakes in the region. The Philippine Fault is a left-lateral fault stretching over 1,200 kilometers, while the Philippine Trench marks the westward subduction of the Philippine Sea Plate beneath the Philippine Mobile Belt. The interaction between these two tectonic features has been responsible for numerous devastating earthquakes in the past.
The subducting Philippine Sea Plate enters the trench at a slight angle and steepens as it descends, with the slab detectable at depths of over 150 kilometers. The August 3 earthquake occurred at a depth consistent with the gently dipping section of the slab, similar to the December 2023 magnitude 7.4 event.
CST Modeling and Earthquake Triggers
Using Coulomb Stress Transfer (CST) modeling, researchers examined the relationship between the December 2023 and August 2024 earthquakes. The model indicated that the August 3 earthquake occurred in a region where stress had increased due to the December 2023 event. This suggests that the earlier earthquake may have triggered the later one by altering the stress distribution along the subduction zone.
However, the CST model did not indicate that the December 2023 earthquake influenced the aftershocks of the August 3 event. A subsequent CST model, using the August 3 mainshock as the source, showed that all aftershocks occurred in regions of increased stress, supporting the idea that these were primarily caused by the release of energy along the upper segments of the subduction thrust.
Implications and Future Risks
The CST models underscore the importance of understanding stress distribution in complex tectonic settings like those in eastern Mindanao. The recent earthquakes highlight the ongoing risks in the region, particularly in areas where extinct fracture zones are being subducted. Similar tectonic settings in the Philippines, such as the Manila Trench, pose significant risks to densely populated areas like Metro Manila.
As the region continues to experience seismic activity, experts emphasize the need for ongoing vigilance and preparedness for future earthquakes, particularly in high-risk areas with large populations.