Location:

Period:

15 Oct 2013 00:12:32 - 16 Oct 2013 23:37:28 (1 day 23 hours 24 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.1; 4 km SE of Sagbayan, Philippines; (15 Oct 2013) (46km from the swarm center)

3 swarms found nearby.

2011

PS20110711.2(158.0km)

11 Jul

12 hours

9 earthquakes

2012

PS20120206.1(54.4km)

6 Feb

1 day 0 hours

8 earthquakes

2025

PS20250930.1(149.4km)

30 Sep

1 day 0 hours

7 earthquakes

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Seismic Swarm PS20131015.1: Analysis of the October 2013 Events Near Sagbayan, Philippines

The seismic swarm designated PS20131015.1 occurred in the vicinity of Sagbayan on Bohol Island, Philippines, beginning at 00:12 on 15 October 2013 and concluding at 23:37 on 16 October 2013. Over this 47-hour period, 17 earthquakes were recorded, highlighting a concentrated episode of seismic activity in a tectonically active region.

The sequence initiated with a magnitude 7.1 mainshock at a depth of 19 km, located 4 km southeast of Sagbayan. This event was followed by numerous aftershocks, predominantly clustered at shallow depths between 6 km and 20 km. Notable subsequent events included magnitudes of 5.7, 5.6, and several in the 5.0–5.3 range, with the final recorded tremor registering 5.3 at 23:37 on 16 October. Depths remained generally consistent within the upper crust, suggesting activation along local fault structures.

Bohol Island lies within the Philippine Mobile Belt, a complex zone of deformation influenced by the convergence of the Philippine Sea Plate and the Sunda Plate. The region features a network of active faults, including segments of the Philippine Fault System and subsidiary structures such as the North Bohol Fault. These features accommodate regional strain through strike-slip and thrust mechanisms, resulting in recurrent seismicity. Historical records indicate that Bohol has experienced multiple significant earthquakes, with the 2013 sequence representing one of the stronger events since 2000.

Prior to 2013, two earlier swarms had been documented in the area since 2000, occurring in 2011 and 2012. The 2013 swarm, centered near Sagbayan, aligns with the broader pattern of clustered activity along known fault lines. The magnitude 7.1 event of 15 October stands as the strongest recorded in the immediate vicinity during this timeframe, underscoring the potential for moderate-to-large events in this setting.

Such swarms provide valuable insight into fault behavior, often indicating stress redistribution following a principal shock. The shallow focal depths observed throughout the sequence are characteristic of crustal faulting in the Philippine archipelago, where tectonic interactions drive frequent seismic release. Monitoring efforts by national agencies continue to track these patterns to refine hazard assessments for the region.

References

USGS Earthquake Catalog
PHIVOLCS Seismic Bulletins
Philippine Institute of Volcanology and Seismology Regional Tectonic Summaries