Seismic Swarm PS20110319.2 Off Miyako, Japan: Geological Context and Event Analysis

Seismic swarm PS20110319.2 occurred in a tectonically active offshore region 82 km east of Miyako, Japan, within the Japan Trench subduction zone. This area marks the boundary where the Pacific Plate subducts beneath the Okhotsk Plate, generating frequent earthquakes through thrust faulting and crustal stress accumulation. The swarm initiated at 18:32 on 18 March 2011 and concluded at 12:03 on 20 March 2011, spanning 41 hours and 30 minutes with a total of eight recorded events.

The sequence began with a magnitude 5.1 earthquake at 49 km depth on 18 March at 18:32:49 UTC. Subsequent activity included a magnitude 5.5 event at 41 km depth later that evening. On 19 March, the swarm intensified with four events: a magnitude 5.9 at 15 km depth, followed closely by magnitudes 5.4 and 5.5 at depths of 31 km and 28 km, respectively. An additional magnitude 5.4 occurred at 39 km depth that evening. Activity on 20 March featured a magnitude 5.1 at 28 km depth and concluded with a magnitude 5.8 at 42 km depth.

These events displayed variable focal depths between 15 km and 49 km, consistent with the complex stress regime near the plate interface. Magnitudes remained moderate, ranging from 5.1 to 5.9, indicating a cluster of aftershock-like activity rather than a single mainshock sequence. Depths suggest involvement of both upper crustal and deeper subduction-related faulting.

The Miyako region lies along the Pacific Ring of Fire, where historical seismicity reflects ongoing subduction. Since 2000, eight swarms have been documented in the vicinity, with two occurring in 2008 and six in 2011. This pattern underscores episodic clustering tied to plate motion rates averaging 8–9 cm per year. Notable strong earthquakes nearby include the magnitude 7.0 event 30 km east of Ishinomaki on 20 March 2021, located 99 km from the swarm center, and the magnitude 7.0 quake 27 km south-southwest of Ōfunato on 26 May 2003, 67 km from the center. Both highlight the persistent seismic hazard in the Tohoku offshore domain.

Insights from the swarm reveal temporal clustering within a short window, with peak activity on 19 March. Depth variations point to distributed fault slip across multiple planes, potentially influenced by post-subduction stress transfer. Such swarms contribute to understanding precursory or triggered seismicity in subduction settings.

References

United States Geological Survey Earthquake Catalog
Japan Meteorological Agency Seismic Data
Global CMT Project Database