Location:

Period:

16 Mar 2009 14:15:56 - 16 Mar 2009 17:44:53 (3 hours 28 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.2; 196 km SSE of Sarangani, Philippines; (11 Feb 2009) (43km from the swarm center)

10 swarms found nearby.

2000

PS20000418.1(183.7km)

17 Apr

1 day 6 hours

7 earthquakes

2009

PS20090211.1(37.0km)

11 Feb

2 days 11 hours

67 earthquakes

S20090212.1(15.4km)

11 Feb

2 days 13 hours

50 earthquakes

PS20090614.1(198.9km)

14 Jun

15 hours

5 earthquakes

2014

PS20141221.1(174.8km)

21 Dec

2 hours

5 earthquakes

2019

PS20190106.1(139.4km)

6 Jan

20 hours

8 earthquakes

2022

PS20220508.1(23.9km)

8 May

14 hours

5 earthquakes

2023

S20230118.1(99.4km)

18 Jan

3 days 19 hours

63 earthquakes

PS20230124.1(74.5km)

23 Jan

1 day 3 hours

7 earthquakes

2026

PS20260608.1(190.0km)

7 Jun

3 days 12 hours

38 earthquakes

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Seismic Swarm PS20090316.1 Southeast of Sarangani, Philippines

Seismic swarm PS20090316.1 occurred on 16 March 2009, approximately 242 km southeast of Sarangani in Mindanao, Philippines. The sequence began at 14:15 UTC and concluded at 17:44 UTC, spanning 3 hours and 28 minutes. During this period, five earthquakes were recorded, with magnitudes ranging from 5.0 to 6.3 and focal depths between 18 km and 45 km.

The events unfolded in rapid succession. The initial shock at 14:15:56 registered magnitude 6.3 at 35 km depth. Two magnitude 5.0 events followed within minutes at 14:24:36 (45 km depth) and 14:27:39 (35 km depth). Activity resumed later with a magnitude 5.7 event at 17:42:03 (18 km depth) and a final magnitude 5.2 shock at 17:44:53 (35 km depth). This tight temporal clustering without a dominant mainshock-aftershock pattern characterizes swarm behavior, often linked to fluid migration or aseismic slip along fault segments.

The swarm location lies within the tectonically complex Philippine Mobile Belt, where the Philippine Sea Plate converges with the Sunda Plate. Subduction along the Philippine Trench to the east and the Cotabato Trench to the southwest drives regional deformation. The Philippine Fault system, a major left-lateral strike-slip structure, traverses Mindanao and accommodates significant shear. Shallow to intermediate-depth seismicity in this sector reflects both crustal faulting and slab-related processes.

Historical records since 2000 indicate three prior swarms in the broader area, with one in 2000 and two in 2009. A notable magnitude 7.2 earthquake struck on 11 February 2009, centered 196 km south-southeast of Sarangani and only 43 km from the swarm epicentral zone. This event preceded the March swarm by roughly one month, suggesting possible stress triggering or shared fault involvement within the regional network.

Such swarms contribute to understanding transient seismic hazards in subduction-influenced arcs. Depths spanning the upper crust to around 45 km align with the transition from brittle to ductile regimes in this setting. Continued monitoring aids in distinguishing swarm sequences from foreshock activity preceding larger events.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for regional tectonics and historical events.
Philippine Institute of Volcanology and Seismology (PHIVOLCS) tectonic framework reports.