Cebu was rocked by an M5.7 earthquake: an aftershock preceding the M6.9 earthquake

To read this article in Filipino (Google automatically translates), click here.

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Please refer to the Philippine Institute of Volcanology and Seismology (PHIVOLCS) for the latest information on this earthquake.

A 5.7 magnitude earthquake struck Cebu today: a large aftershock of the deadly M6.9 earthquake that occurred on September 30. This is the largest aftershock of the sequence so far. The earthquake occurred at 1:06 a.m. local time on October 13 (10/12/2025 UTC).

Figure 1: Earthquakes since September 15, 2025 in northern Cebu, Philippines. Earthquakes are colored over time. The mainshock (M6.9) and largest aftershock (M5.7) are marked. Earthquakes are also displayed on the right on a timeline and appear below the map in a time series.

Since the main shock occurred, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) has been busy. Through a quick field investigation, they discovered the fault they believe was responsible for the earthquake: the newly named Bogo Bay Fault. From what we can tell, this fault is aligned with a straight extension of the coast in the Gulf (and is presumably responsible for this straightening).

Figure 2: A screenshot from Google Earth showing the straight section of shoreline that appears identical to the newly mapped Bogo Bay fault.

We also have some preliminary satellite images that help visualize the rupture and its effects, also from PHIVOLCS. The images below are from the Japanese ALOS-2 satellite, which is particularly adept at measuring large deformations of the Earth. The upper image shows the displacement in the satellite's direction of view (northeast). The area southeast of the fault (red) has moved about 2 meters to the southwest, a reasonable value for an earthquake of this magnitude. The area northwest of the fault (blue) has moved northeast. Less movement is observed in the second image, which shows displacement in the northwest direction. This makes sense: this is a strike-slip fault, so we wouldn't expect to see much movement toward or away from the fault itself.

Guided by satellite imaging and field mapping, PHIVOLCS mapped the bend of the fault toward the southwest and then south. This is interesting, because it means that slip on the fault must cause stress around this curved region, as the lateral slip crust becomes increasingly restricted. This is called compressional bending of the strike-slip fault.

In fact, the recent M5.7 aftershock is a thrust event, corresponding to northwest-southeast pressure.

Figure 4: Earthquake sequence map, with the fault traced from the PHIVOLCS satellite image and extending offshore toward the main epicenter. At the southwestern end of the fault path, movements should cause stress, as seen in the M5.7 aftershock.

Although the magnitude of this earthquake was much smaller than the main shock, it may have caused similar shaking intensity in some areas close to the epicenter. USGS PAGER estimates that a maximum shaking intensity of VII (very strong) may have been felt by more than 100,000 people. PHIVOLCS indicates lower intensity (maximum V: moderate) – we are not sure whether this reflects a difference in methodology, or a more precise assessment by PHIVOLCS based on reports.

Large, rare aftershocks are a normal part of the seismic sequence. The basic rule for aftershock sequences is called Bath's Law: In a typical sequence, the largest aftershock will be 1.2 units smaller than the mainshock. So, in this case, M5.7 is exactly what we should expect.

In general, we also expect the rate of aftershocks to decrease by 1/time. In other words, if there are N/10 aftershocks on the first day after the earthquake, this rate should decrease to N/10 on day 10, and N/100 on day 100; This is the famous Omori-Otsu law of aftershock decay.

Unfortunately, these “laws” are not really laws at all, but rather observations that represent typical behavior. Not many earthquake sequences have M-1.2 aftershocks – likewise, many earthquakes will have larger aftershocks, or even trigger triggering events larger than the original mainshock. So, while the most likely scenario is that aftershocks will continue to reduce in frequency over time, there is also less likelihood of larger events occurring. People in the affected area should remain aware of this potential for further events, and follow guidance provided by PHIVOLCS and regional authorities.

Hubbard, J. and Bradley, K. (2025). Devastating M7.4 subduction earthquake shakes Mindanao, Philippines. Earthquake visions. https://earthquakeinsights.substack.com/p/damaging-m74-subduction-earthquake

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