Stress releases and seismic gaps: Earthquake sequences strike Eastern Mindanao, Philippines
In late 2023, a series of earthquakes struck off the coast of eastern Mindanao, an island in the southern part of the Philippines. A tsunamigenic magnitude 7.4 earthquake occurred on Dec. 2, which was followed by numerous aftershocks, including some greater than magnitude 6. Just two days later, another earthquake with a magnitude of 6.8 struck about 70 kilometers northeast of the powerful magnitude 7.4 earthquake.
Then, on Feb. 10, 2024, two shallow moderate earthquakes struck onshore. The first, a magnitude 5.8, was quickly followed by a magnitude 5.2 event and subsequent aftershocks. Both earthquakes were widely felt in the region (Figure 1). These seismic events have prompted a closer examination of their origins and potential connection.
Here, we analyze the earthquake sequences in Eastern Mindanao in detail. We found that the magnitude 7.4 earthquake potentially triggered the magnitude 6.8 earthquake, whereas the Feb. 2024 earthquakes occurred in a surprising location, highlighting the complexity of earthquake interactions. We also explore the geological context, historical seismicity, and implications of these seismic activities and suggest future scenarios along the Philippine Fault in eastern Mindanao.
Tectonics and seismicity of Mindanao
Mindanao, the second-largest island in the Philippines, is known for its high seismic activity, having experienced numerous moderate to major earthquakes throughout its history (Table 1; Bautista and Oike, 2000; Perez and Tsutsumi, 2017).
East of the island lies the Philippine Trench. There, the Philippine Sea Plate converges northwestward toward Mindanao at a rate of approximately 10 centimeters per year (Cardwell et al., 1980; Argus et al., 2011). The trench-parallel component of this oblique convergence is accommodated by the Philippine Fault (Fitch, 1972). In other words, because the oceanic Philippine Sea Plate subducts beneath the Philippine Trench at a non-perpendicular angle relative to the trench, the motion that’s parallel to the trench must be accommodated elsewhere.
The Philippine Fault stretches for about 1,500 kilometers, north-northwest to south-southeast, across the Philippine archipelago. Running approximately 500 kilometers in eastern Mindanao, this fault is a significant geological feature in the region (Tsutsumi and Perez, 2013; Perez et al., 2015) (Figure 1). Here, the Philippine Fault consists of several segments, including the Surigao, Esperanza, Agusan Marsh, West Compostela Valley (WCV), Central Compostela Valley (CCV), Nabunturan, East Compostela Valley (ECV), Caraga River, Mati, and Lianga segments. These segments have been differentiated according to various criteria. For example, in some cases, geometric or structural features distinguish the fault segments. Elsewhere, the segments’ past rupture histories, like temporal characteristics such as coseismic behavior observed in historical surface rupture or paleoseismic studies, help delineate them from one another (Perez et al., 2015; Wesnousky, 2006; McCalpin and Nelson, 2009).
Eastern Mindanao has a history of significant seismic activity (Figure 2a) along the Philippine Fault. Records dating back to the 19th century unveil several significant seismic events. For instance, an 1879 earthquake along the Surigao segment had an estimated moment magnitude of 7.4 (Perez and Tsutsumi, 2017). An 1891 quake along the WCV segment had an estimated magnitude of 7.2. In 1893, another shock along the CCV segment had an estimated magnitude of 7.3 (Bautista and Oike, 2000; Perez et al., 2015). More recently, in 2023, eastern Mindanao, particularly the province of Davao de Oro (formerly known as Compostela Valley), was affected by several moderate to strong earthquakes generated by segments of the Philippine Fault (Figure 2b).
After the Dec. 2023 magnitude 7.4 event: Investigating more recent quakes along the Philippine Fault
The Dec. 2023 magnitude 7.4 earthquake that struck part of the Philippine Trench produced surprisingly large aftershocks. Days later, a magnitude 6.8 earthquake occurred at a shallower depth, which redirected seismic activity northward and triggered a separate cluster of tremors. Llamas and others (2024) suggest that the two events ruptured separate sections of the Philippine Trench.
Roughly 150 kilometers to the west of the Philippine Trench lies the Philippine Fault, which, as mentioned above, experienced its own sequence of earthquakes in February 2024. The first event in the seismic sequence, a magnitude 5.8 earthquake, occurred on Feb. 10 at 11:22 a.m. local time, and was located 5 kilometers southeast of Esperanza, Agusan Del Sur (population approximately 60,000). Its focal mechanism suggests strike-slip faulting, indicating horizontal movement along the fault. This earthquake could be linked with the Esperanza segment of the Philippine Fault or other associated local faults that traverse this region.
Shortly after, at 1:21 p.m. local time, a magnitude 5.2 earthquake struck 11 kilometers southwest of Esperanza, Agusan Del Sur. The focal mechanism of this event indicates northwest-striking normal faulting. This earthquake is probably linked to the Lianga Fault, which branches east-southeast from the Philippine Fault. Following these events, several aftershocks were recorded ranging in magnitude from 2.0 to 3.1.
A question arising from these earthquakes is whether they are interconnected and influenced by stress changes from the earlier magnitude 7.4 mainshock. To understand the seismic dynamics ensuing in eastern Mindanao, we generated Coulomb stress change models using actual focal mechanisms and optimally oriented strike-slip faults as receiver faults (receiver faults do not participate directly in the earthquake rupture, but instead "receive" the transfer of stress).
Coulomb stress change considers both shear and normal stress changes resulting from an earthquake. This method is based on the principle that faults experiencing stress increases (depicted as red in Figure 3) are prone to failure, making them more likely to experience earthquakes. Conversely, faults with stress decrease (depicted in blue in Figure 3) are less likely to see failure and consequently are less likely to generate earthquakes (Stein, 1999).
Notably, as per our stress change model using actual focal mechanisms (Figure 3), the magnitude 7.4 earthquake resulted in a stress buildup directed toward the location of the magnitude 6.8 earthquake, likely setting off a triggering effect (Figure 3a). On the other hand, the faults that generated the February 2024 magnitude 5.8 and 5.2 earthquakes experienced stress decreases from the previous events (Figures 3b and 3c). This is contrary to our expectations, which highlights the complexity of how earthquakes interact and the need for further analysis.
However, by looking only at the normal stress change, it is worth noting that the faults that generated the 5.8 and 5.2 earthquakes experienced unclamping or positive normal stress change (Figure 4) induced by their preceding events, a factor that can be considered for possible earthquake triggering.
Our Coulomb stress change model using optimally-oriented strike-slip faults indicates areas of stress increase along segments of the Philippine Fault, including the Agusan Marsh, ECV, CCV, and Nabunturan segments (Figure 1). This suggests that stress has been transferred to these segments, and they were brought closer to failure. As a result, we observe a rise in seismic activity along the Agusan Marsh segment, characterized by an observed increase in stress following the Dec. 2023 earthquake sequence. Specifically, the number of earthquakes occurring in the two months following the Dec. 2023 sequence surpassed the total number of earthquakes recorded in the entire year preceding that sequence.
Historical context and previous seismicity
Interestingly, the area of Feb. 2024 seismic activity falls within a seismic gap positioned between the 1879 event to the north and the 1891 and 1893 earthquakes to the south. Historical records (as shown in Table 1) indicate that the Esperanza and Agusan Marsh segments of the Philippine Fault have not produced any major earthquakes - those greater than magnitude 7.0 - in the historic past. However, it's worth noting that instrumental records also document previous moderate earthquakes in the region of the Feb. 2024 earthquakes. For instance, those records include a magnitude 5.0 event and a magnitude 5.1 event in 1999 (Figure 2a).
Earlier earthquake sequence: The 2023 Davao de Oro earthquakes
On Feb. 1, 2023, a magnitude 6.0 earthquake jolted the province of Davao de Oro (formerly known as Compostela Valley) and nearby areas. Based on the epicenter and focal mechanism, this earthquake could be associated with the CCV segment of the Philippine Fault.
About a month later, on Mar. 6, 2023, a sequence of earthquakes greater than magnitude 5.0 struck again, approximately 25 kilometers south. This earthquake sequence produced events ranging from magnitudes 5.3 to 5.9. The focal mechanisms of these earthquakes suggest movement along a strike-slip fault that could be associated with the Philippine Fault or other nearby, local strike-slip faults.
The February-March 2023 earthquake sequence occurred within transtensional basins that form as the Philippine Fault steps to the left. The Compostela Valley segment is an example of a left-stepping segment that results in such a basin. Moreover, the sequence exhibited a southward propagation of seismic activity toward an area where documented stress release has been minimal in the past. The significant stress releases during the 1891 and 1893 earthquakes, which occurred to the north of the 2023 events, could have inhibited recent northward propagation of seismic activity. South of these transtensional basins --- and of the 2023 sequence --- lies the Mati segment of the Philippine Fault, which has no documented occurrences of large earthquakes in paleoseismic and historical records (Perez et al., 2015).
To investigate the stress changes caused by the 2023 sequence of earthquakes in Davao de Oro, we also employed Coulomb stress change modeling using optimally oriented strike-slip faults as receiver faults. The model shows that the 2023 sequence caused an increased stress along the WCV, ECV, Nabunturan, Caraga River, and Mati segments by 0.1 to 0.2 bars, implying that these segments of the Philippine Fault have been brought closer to failure (Figure 2b). A commonly suggested threshold for potential earthquake triggering is a Coulomb stress increase of 0.1 bar, as proposed by several studies (e.g., Harris, 1998; King et al., 1994; Stein, 1999).
Implications for future earthquakes
A seismic gap refers to a segment of a fault that has not experienced significant seismic activity in recent history, implying that stress may be accumulating. The presence of historical seismic gaps, particularly in the Esperanza, Agusan Marsh, and Mati segments of the Philippine Fault, raises concerns about the potential for earthquakes along these fault segments. Additionally, our modeling suggests that these fault segments have experienced notable increases in stress due to recent earthquakes, which increases the likelihood of fault failure and earthquake occurrence.
Considering the potential for earthquakes exceeding magnitude 7 along these fault segments based on their length (as indicated in Table 1), one might wonder: could there be further seismic activity in areas affected by the 2023 Dec. and 2024 Feb. earthquakes due to incomplete stress release?
Predicting the exact timing and magnitude of future earthquakes is not possible. Instead, we address these concerns with caution and preparedness rather than alarm. A significant earthquake along the Philippine Fault in eastern Mindanao could lead to widespread damage to both infrastructure and communities due to their proximity to the fault. Thus, increased earthquake awareness and preparedness should occur at all levels of society. Measures to strengthen infrastructure and community resilience should be prioritized.
As a guide, DOST-PHIVOLCS has developed different materials for earthquake preparedness and has conducted several Information, Education, and Communication campaigns throughout the country. Meanwhile, the Philippine government has institutionalized a nationwide simultaneous earthquake drill that takes place every quarter of the year (Figure 5). New constructions and/or retrofitting of structures must adhere to the National Building Code of the Philippines.
Additionally, it's crucial to account for other hazards that often accompany earthquakes and exacerbate damage, like landslides and liquefaction. Earthquake-related hazard maps and other informational materials can be accessed for free from the official website of DOST-PHIVOLCS and hazardhunter.georisk.gov.ph.