Major earthquake strikes the Philippines, followed by unusually large aftershocks

A magnitude 7.6 megathrust earthquake occurred near Mindanao Island. Strong shaking, a small tsunami and many very large aftershocks have followed.
 

By Ross S. Stein, Ph.D., Temblor, Inc. and Shinji Toda, Ph.D., Tohoku University
 

Citation: Stein, R. S. and Toda, S., 2023, Major earthquake strikes the Philippines, followed by unusually large aftershocks, Temblor, http://doi.org/10.32858/temblor.330
 

This article is also available in Tagalog.
 

On Dec. 2, 2023, at 10:37 p.m. local time, a shallow magnitude 7.6 shock occurred just off Mindanao Island of the Philippines, launching a small tsunami and causing strong shaking. The event struck at one of the two Philippines islands with the highest earthquake potential, according to Temblor’s earthquake occurrence model that’s derived from GEAR1 (Bird et al., 2015). Two magnitude 6.9 aftershocks followed at sites where our stress calculations suggest stress had been raised by the mainshock, with more large aftershocks likely to follow.
 

 

Why is the Philippines so seismically active?

The Philippine Islands are part of a volcanic arc, and are the product of two opposing subduction zones — the Philippine and East Luzon Trenches to the east, and the Manila-Negros-Cotabato Trenches to the west. To the east, the Philippine Sea oceanic tectonic plate plunges beneath the arc. To the west, various microplates either collide with or subduct beneath the islands. An additional complexity is the Philippine Fault, a San Andreas-like transform fault sandwiched between the trenches. All three systems have high slip rates.

As a result, on average, the Philippines experiences at least one earthquake with a magnitude greater than or equal to 6.8 every year — the same rate as the main Japanese island of Honshu, which covers a similar area. These are among the half-dozen most seismically active places on Earth (Aurelio et al., 2022).
 

How often should we expect quakes of this size in Mindanao?

The magnitude 7.6 shock struck on part of the Philippine Trench that records a convergence rate of about 40 millimeters/year (1.6 inches/year; Holt et al., 2018). This is about as fast as the Cascadia Subduction Zone in the Pacific Northwest off Washington, Oregon and British Columbia. The preliminary U.S. Geological Survey (USGS) finite fault model for the magnitude 7.6 shock shows the peak slip is about 1.5 meters (or 1,500 millimeters, or nearly 5 feet). Taken together, the convergence rate and slip suggest a rough time between similar-sized events of about 40 years. In other words, it takes about 40 years of convergence at a rate of 40 mm/yr to produce 1.5 meters of slip on the fault. This is consistent with the last major earthquake to strike the region, which occurred 34 years ago. That event was also a magnitude 7.6, and it ruptured part of the fault 30 kilometers (about 19 miles) to the southeast of this recent sequence.

Temblor’s earthquake occurrence model, which derives from GEAR1 (Bird et al., 2015), gives the annual frequency of all earthquakes everywhere on Earth. The model is a blend of strain rate measured by GPS instruments and past earthquakes. These components capture the steady stressing of fault systems, and the sudden release of those stresses in earthquakes.

GEAR1 as portrayed in the Temblor app gives the magnitude of an earthquake that has a 1% per year chance of occurring within 100 kilometers (about 60 miles) of any location. So, if a resident at that location (as noted in Figure 1) lived to be 85 years old, they would have a greater than 50% chance of experiencing a quake of the listed magnitude, in this case magnitude 7.4. In the model, one of the two sites throughout all of the Philippines with the highest potential for large shocks lies at the 2023 epicenter. The other site is 400 kilometers (about 250 miles) to the north, on eastern Samar Island, where three shocks with magnitudes greater than 7.0 struck in 1995-1996.
 

Should we expect more large aftershocks, and if so, where?

All shallow earthquakes trigger abundant aftershocks, but this one has been usually productive thus far. Whether that will continue is difficult to forecast. In general, aftershocks become less frequent with time, but not smaller with time. So, the number of aftershocks that will happen in the first 10 days is about the same as the number of aftershocks that will occur over the next 100 days, but the largest shock in the first 10 days will be about the same as the largest in the next 100 days. So, vigilance and precautions are key.

We can compare the aftershock rate and the time decay of aftershocks for the first three days of the 2023 sequence to two other magnitude 7.6 Philippine shocks, which struck in 1989 and 2012. The 2023 aftershock rate is two to three times higher than its predecessors, and, thus far, the aftershock rate is decaying more slowly than the 1989 and 2012 aftershocks did. This suggests that large, damaging aftershocks have a higher chance of occurring in this earthquake sequence than is typical for earthquakes of this size in this area.

We can also try to forecast where aftershocks are more likely to occur. We do this by calculating the Coulomb stress change, which derives from the hypothesis that faults that are unclamped and sheared will be promoted toward failure — they’re more likely to rupture. Those stressed in the opposite sense will be inhibited from failure, which means they’re less likely to break.
 

 

In these calculations (Figure 2), the most likely sites for additional aftershocks are farther offshore (good news), or to the north or south along the coast (not as good news). The three calculation methods do not fully agree, and this reflects scientists’ inadequate understanding of the conditions that nucleate earthquakes. Nevertheless, calculations like these have performed much better than chance in explaining the distribution of aftershocks and progressive mainshocks in numerous settings, and so we believe they have some utility for the public and for science (Stein, 1999).
 

Science editor: Dr. Alka Tripathy-Lang, Ph.D.
Reviewer: Dr. Wendy Bohon, Ph.D.
 

References

Aurelio, M., Catugas, S.D., Ramirez, A.B., Aurelio, S.C. and Lagmay, A.M.F. (2022), Two large quakes hit Abra, Philippines, in three months. What does this mean?, Temblor, http://doi.org/10.32858/temblor.288

Bird, P., D.D. Jackson, Y.Y. Kagan, C. Kreemer, and R.S. Stein (2015), GEAR1: A Global Earthquake Activity Rate Model Constructed from Geodetic Strain Rates and Smoothed Seismicity, Bull. Seismol. Soc. Amer., 105, 2538–2554, doi: 10.1785/0120150058.

Holt, A. F., L. H. Royden, T. W. Becker, C. Faccenna (2018), Slab interactions in 3-D subduction settings: The Philippine Sea Plate region, Earth Planet. Sci. Letts., 489, 72-83, https://doi.org/10.1016/j.epsl.2018.02.024.

Kagan, Y.Y. and D.D. Jackson (2014), Statistical earthquake focal mechanism forecasts, Geophys. J. Int., 197, 620–629, doi: 10.1093/gji/ggu015

Stein, R.S. (1999), The role of stress transfer in earthquake occurrence, Nature 402, 605–609, https://doi.org/10.1038/45144
 

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