Earthquake model successfully forecasts location of August 3 magnitude 6.8 event
The earthquake that struck offshore eastern Mindanao in the southern Philippines on August 3, 2024 resulted from renewed movement of an extinct fracture zone subducting into Earth's mantle. That movement along the subduction zone was responsible for a magnitude 7.4 event that occurred eight months earlier.
On August 3, 2024 at 6:23 a.m. local time, a magnitude 6.8 earthquake struck offshore, east of central Mindanao in the southern Philippines. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) located the event about 48 kilometers east-northeast of the town of Lingig (population approximately 35,000) in the province of Surigao del Sur.
The quake ruptured at a depth of 25 kilometers (16 miles). Within four days, this mainshock was succeeded by at least six aftershocks with magnitudes between 6.2 and 4.8, all at shallower depths. Fortunately, no major infrastructure damage or casualties were reported.
Seismotectonic setting
The left-lateral Philippine Fault, which stretches more than 1,200 kilometers in distance, has several southern segments that traverse Eastern Mindanao (Aurelio, 1992; Quebral, 1994) (Figure 1a). Offshore and to the east, the Philippine Trench marks the westward subduction of the Philippine Sea Plate underneath the Philippine Mobile Belt (Cardwell et al., 1980; Aurelio, 2000a; Aurelio and Peña, 2010). The Philippine Fault-Philippine Trench pair operates under a shear partitioning mechanism, where the oblique convergence between the Philippine Sea Plate and the Philippine Mobile Belt (Rangin et al., 1999) can be divided into westward frontal subduction along the Philippine Trench and left-lateral slip along the Philippine Fault (Aurelio, 2000b). Both tectonic features are responsible for generating numerous earthquakes in the region, many of which have been devastating.
The subducting Philippine Sea Plate enters the Philippine Trench at a slight westerly angle (between 15 and 30 degrees). The plate begins to steepen between depths of 10 and 20 kilometers, eventually reaching its steepest angle of about 45 degrees at around 50 kilometers depth. The slab can be detected to a depth of more than 150 kilometers in some places (Cardwell et al., 1980; Aurelio, 2000) (Figure 1b). The 25-kilometer depth of the magnitude 6.8 earthquake places this event at the end of the gently dipping section of the subducting slab at almost the same depth as the magnitude 7.4 earthquake of December 3, 2023 (Aurelio et al., 2023).
Triggered earthquakes
We explored the relationship between the December 2023 magnitude 7.4 earthquake and the August 2024 magnitude 6.8 event using a Coulomb Stress Transfer (CST) model. All CST models require a source fault and a receiver fault. In our model, the fault that generated the magnitude 7.4 event served as the source fault. The fault that generated the magnitude 6.8 earthquake served as the receiver fault. Red areas represent zones of increased stress, whereas blue areas represent zones of decreased stress (Figures 1a and 1b). The focus of the August 3 event is located in a red zone, suggesting that this event was triggered by the magnitude 7.4 event.
However, in the same CST model (Figures 1a and 1b), none of the aftershocks of the magnitude 6.8 earthquake plot in the stress increase (red) zones. This may suggest that the magnitude 7.4 earthquake of December 3, 2023 did not influence the generation and distribution of the August 2024 aftershocks.
In search of a more coherent explanation of the distribution of the August 2024 aftershocks, we created another CST model. In this case, the fault that generated the magnitude 6.8 event served as the source fault. The fault that generated the largest aftershock (magnitude 6.2) served as the receiver fault (Figures 2a and 2b).
In this model, we observe that all aftershocks have focal depths that are shallower than the mainshock; all aftershocks plot within the stress increase (red) zones. Five of the aftershocks, including the largest (magnitude 6.2) plot east and above the mainshock, whereas only one plots to the west. This strongly suggests that the aftershocks were generated by the release of energy mostly along the upper, gently dipping segments of the subduction thrust, whereas one (magnitude 5.4) aftershock may have been associated with an east-dipping backthrust on the overriding plate.
Predictive power of CST modeling
The CST models presented above demonstrate the predictive power of this modeling technique, especially when properly taken within the context of complex tectonic settings such as those found in eastern Mindanao. Based on our modeling, the recent magnitude 6.8 mainshock occurred in a region in which stress increased as a result of the December 2023 magnitude 7.4 event. The December 2023 event likely resulted from subduction of an extinct fracture zone (Aurelio et al., 2023). We posit that the recent mainshock resulted from renewed movement along the subduction zone where the extinct fracture zone continues to be consumed at the trench.
In our earlier work, we wrote, "the population in eastern Mindanao should be reminded to be alert against any strong earthquakes in the future that may be triggered by this magnitude 7.4 event." Indeed, the "future" event occurred eight months later (Aurelio et al., 2023).
At this point, it is important to reiterate that similarly complicated tectonic settings around the Philippines are capable of generating similarly large magnitude earthquakes. Special attention should be given to areas with large populations, such as in metro Manila and environs (approximate population of 25 million); the offshore Manila Trench is another location where an extinct spreading ridge is currently being subducted (Pautot et al., 1986; Aurelio, 2000a).