A magnitude-5.8 earthquake that occurred off the coast of Batangas in southern Luzon, Philippines, only three weeks after a magnitude-6.6 earthquake struck the same region, is a reminder of the local vulnerability to earthquake hazards.
By Mario Aurelio, Director of the University of the Philippines National Institute of Geological Sciences, Mahar Lagmay, Executive Director, University of the Philippines Resilience Institute-Nationwide Operational Assessment of Hazards Center (@nababaha), John Agustin Escudero, Structural Geology and Tectonics Laboratory at the University of Philippines National Institute of Geological Sciences, and Sandra Catugas, Structural Geology and Tectonics Laboratory at the University of Philippines National Institute of Geological Sciences
Citation: Aurelio, M., Lagmay, M., Escudero, J. A., and Catugas, S., 2021, Philippines fault jolts Batangas again, with magnitude-5.8 quake, Temblor, http://doi.org/10.32858/temblor.198
At 11:08 p.m. (local time) on Friday, the 13th of August 2021, a magnitude-5.8 earthquake was generated by the subduction of the South China Sea underneath southern Luzon along the Manila Trench. This earthquake occurred just three weeks after a magnitude-6.6 earthquake struck offshore Batangas in Luzon, Philippines, on July 24, 2021, along the same subduction-related fault (Aurelio et al., 2021). This most recent earthquake was felt by residents in Metropolitan Manila some 120 kilometers away, especially those living in high-rise residential buildings.
The focal mechanism solution (also known as beachball diagrams), location, and depth of the two earthquakes are almost identical and close to each other (Figure 1). Only their magnitudes differ by about one order of magnitude: i.e., the energy released by the August 13 event is about 30 times weaker than that released by the July 24 event. This leads some to consider that the August 13 event was an aftershock of the July 24 event.
The July 24 mainshock produced one magnitude-5.8 aftershock less than 10 minutes after the mainshock. And preliminary Coulomb stress transfer modeling indicates that the two magnitude-5.8 earthquakes of August 13 and July 24 (the magnitude-5.8 aftershock) fall within the increased stress lobe of the magnitude-6.6 event of July 24 (Figure 1). This indeed suggests that the two magnitude-5.8 earthquakes were promoted, or triggered, by the magnitude-6.6 mainshock. In that sense, they can be considered aftershocks. Nevertheless, it is unusual to have two aftershocks less than one magnitude unit smaller than the mainshock. So alternatively, the August 13 event could be interpreted as an independent earthquake that would have occurred even if the magnitude-6.6 shock had not struck.
Regardless of the origin of the earthquakes, their closely spaced relationship in space, time and focal mechanism (beachball) strongly suggests that this segment of the Manila Trench is highly likely to generate more earthquakes again. Records indicate that within a 50-kilometer radius around the epicenter of the July 24 magnitude-6.6 event, six earthquakes of magnitude 5.0 or greater have occurred in the past year, including a magnitude 6.3 on Christmas Day 2020 (Aurelio, 2021). An additional 25 have struck in the last decade, with 11 in the last five years, for an average of 2.5 earthquakes per year in the last 10 years (Table 1). This attests to the very high frequency of earthquakes capable of inflicting damage to the region.
The Friday the 13th event serves once more as an important reminder to populations of the provinces of Batangas, Cavite, Laguna and Mindoro in southern Luzon, and nearby Metro Manila to strengthen preparedness efforts against the hazards of seismic events originating from the southern segment of the Manila Trench.
References
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Aurelio, M., Lagmay, M., Escudero, J. A., and Catugas, S. (2021). Latest Philippine earthquake reveals tectonic complexity, Temblor, doi.org/10.32858/temblor.191
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