Seismic Swarm S20110712.2 near Maricalom, Philippines: Geological Context and Event Analysis

The Philippine archipelago lies within the Pacific Ring of Fire, where complex tectonic interactions drive frequent seismicity. The region around Maricalom, situated on Negros Island in the central Visayas, experiences earthquakes primarily due to the subduction of the Philippine Sea Plate beneath the Eurasian Plate along the Philippine Trench to the east and the Sulu Trench to the west. Additional deformation occurs along the Philippine Fault system and subsidiary structures such as the Negros Fault. These settings produce both shallow crustal events and deeper subduction-related activity.

Seismic swarm S20110712.2 was recorded 19 km west-southwest of Maricalom. The sequence began at 20:52 on 11 July 2011 and concluded at 14:16 on 12 July 2011, encompassing 28 earthquakes over 17 hours and 24 minutes. All events were of moderate magnitude, ranging between 4.2 and 4.9, with the majority occurring at depths of 10 km. Two events registered at greater depths of 18 km and 20 km. The swarm lacked a single dominant mainshock, a characteristic feature of swarm sequences often linked to fluid migration or localized stress adjustments along fault networks.

The temporal distribution showed peak activity during the first several hours, with multiple events exceeding magnitude 4.7 between 20:52 and 23:41 on 11 July. Activity continued at a lower rate through the early morning and midday of 12 July before terminating. Depths remained consistently shallow except for the two deeper shocks, consistent with activity within the upper crust in this tectonically active margin.

Historical records indicate that this was the sole swarm documented in the area since 1 January 2000. No prior swarms have been identified in the same locale during the preceding eleven years, underscoring the episodic nature of clustered seismicity in the Visayas region.

Such swarms provide valuable data on fault behavior and stress transfer in subduction-influenced settings. Continued monitoring by the Philippine Institute of Volcanology and Seismology supports improved hazard assessment for nearby communities on Negros and adjacent islands.

References

Philippine Institute of Volcanology and Seismology (PHIVOLCS) seismic bulletins
USGS Earthquake Catalog
Global CMT Project focal mechanism database