Two earthquakes of magnitude 5.1 and 5.2 struck the same corner of southern Mindanao 88 minutes apart overnight on October 27 and 28, with U.S. Geological Survey origin times of 23:43 and 01:11 UTC. Both epicentres sit within 7 kilometers of Balangonan, and neither raised a tsunami flag. On Philippine clocks the shaking came at 7:43 and 9:11 on Wednesday morning.
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The pair arrived nearly five months after a far larger earthquake on the Cotabato Trench shook the same coast, and the agencies measuring them do not fully agree on how deep either one was.
Two origin times, one gap
The USGS catalog dates the first shock to 23:43:51 UTC on October 27, a magnitude 5.1 placed 7 kilometers north of Balangonan at a depth of 57.8 kilometers. The second, a magnitude 5.2, followed at 01:11:55 UTC on October 28, 6 kilometers northwest of the same locality and 45.9 kilometers down. Subtracting one origin time from the other gives 88 minutes, and the catalog records no other event of magnitude 5 or greater between them in the same area.
Felt reports were few: 7 for the first shock and 5 for the second, which the agency converts to community intensities of 4.1 and 4.3. Those are numbers from a handful of volunteers and carry wide error bars, but they sit in the range where people feel a clear jolt and unsecured objects move, not the range where buildings are damaged. Both events carry a tsunami flag of zero in the same USGS data, which fits their depth and size: moderate shocks tens of kilometers down rarely displace enough seafloor to matter.
Eighteen kilometers against fifty-odd
The Philippine Institute of Volcanology and Seismology lists the same pair differently. Its earthquake list shows a magnitude 5.1 at 7:43 a.m. and a magnitude 5.0 at 9:11 a.m., each at a depth of 18 kilometers, located relative to Balut Island in Davao Occidental and to Glan in Sarangani rather than to Balangonan. The agency’s second shock is a tenth of a magnitude unit weaker than the USGS figure, and its depths are about a third of the USGS values.
Early solutions from a local network and from a global one can diverge by tens of kilometers in depth, because depth is the hardest coordinate to pin down when the stations that constrain it are distant. Neither set of numbers is final, and the difference shapes the interpretation: an event at 18 kilometers would sit in the upper crust, while one near 50 kilometers could lie on or below the subduction interface, depending on how the slab dips under this stretch of coast.
What a doublet does to a fault
Seismologists use the word doublet for two earthquakes of similar size close together in space and time. The best-studied example in the literature that turned up for this article is the March 2007 pair near Padang Panjang in Sumatra, which Nakano and colleagues described in the Geophysical Journal International. A magnitude 6.4 and a magnitude 6.3 struck about two hours apart, with nearly identical strike-slip mechanisms, but they ruptured different segments of the Sumatran fault, one running southeast and the other northwest.
The authors attributed the second event mainly to static Coulomb stress transfer: slip on the first segment increased the stress on its neighbor and made failure there more likely. They also argued that the boundary between two segments accumulates stress without releasing it, which favors rupture there.
Whether the Balangonan pair follows that pattern is not known. No source mechanism or rupture analysis for either event had been published in the material reviewed, and a magnitude 5.1 followed by a 5.2 could just as easily be two unrelated releases in a crust still adjusting to the June earthquake. Distinguishing the two readings takes the fault-plane solutions: matching orientations and adjacent rupture patches would point to stress transfer, while differing mechanisms would point to a sequence driven by something larger.
A coast already moved in June
The background rate for the whole area was reset on June 8, when a magnitude 7.8 earthquake at 33 kilometers depth, 32 kilometers west of Maasim in Sarangani, struck. PHIVOLCS attributed it to subduction along the Cotabato Trench. In its post-event surveys, reported by the Manila Bulletin, the agency found the coast at Barangay Pangyan in Glan had risen about 2 meters and the shoreline had retreated roughly 200 meters. A tsunami followed, with run-up of up to 2.5 meters at Salaman, Lebak, and by 9 a.m. on June 14 the agency had counted 5,200 aftershocks between magnitude 1.2 and 6.4.
The trench had a recent precedent. After the November 2023 offshore Davao Occidental earthquake, PHIVOLCS wrote in its event primer that “the earthquake was likely generated by the Cotabato Trench,” at a depth of 63 kilometers, and warned that aftershocks could continue for “several days to weeks.”
The 63-kilometer figure from 2023 falls close to the 57.8 and 45.9 kilometers in the USGS solutions. If the USGS depths are right, the October pair belongs to a depth range this trench has produced before; if PHIVOLCS’s 18 kilometers is right, they are shallower crustal events. The next revision of either agency’s catalog will decide which.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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