Seismic Swarm PS20110312.2: Earthquake Activity Near Funehikimachi-funehiki, Japan

Seismic swarm PS20110312.2 occurred in a tectonically active region of Fukushima Prefecture, Japan, approximately 11 km east-northeast of Funehikimachi-funehiki. The event began at 09:16 on 11 March 2011 and concluded at 05:45 on 12 March 2011, registering six earthquakes over 20 hours and 28 minutes. This swarm unfolded within the broader context of the Pacific Ring of Fire, where the Pacific Plate subducts beneath the Okhotsk Plate at rates of 8–9 cm per year, generating frequent seismic activity along the Japan Trench.

The sequence included the following events: a magnitude 5.0 earthquake at 09:16:29 on 11 March at 53 km depth; a magnitude 5.1 event at 11:31:13 on 11 March at 58 km depth; a magnitude 5.0 earthquake at 17:30:47 on 11 March at 23 km depth; a magnitude 5.2 event at 19:45:21 on 11 March at 11 km depth; a magnitude 5.1 earthquake at 20:34:41 on 11 March at 36 km depth; and a magnitude 4.3 event at 05:45:23 on 12 March at 35 km depth. Depths ranged from shallow crustal levels to intermediate depths, consistent with the complex faulting in the subduction zone.

Japan’s geological history reflects persistent subduction-driven seismicity. Major historical events include the 869 Jogan earthquake and the 2011 Tohoku earthquake of magnitude 9.0, which occurred on the same day as the onset of this swarm and triggered widespread aftershock sequences. Since 2000, eight swarms have been documented in the area, with one in 2004, three in 2008, and four in 2011, underscoring episodic clustering of moderate-magnitude events.

Notable strong earthquakes since 2000 include a magnitude 7.4 event on 16 March 2022 located 57 km east-northeast of Namie, approximately 69 km from the swarm center, and a magnitude 7.3 event on 13 February 2021 located 73 km east-northeast of Namie, about 84 km from the swarm center. These occurrences highlight the region’s capacity for large-magnitude ruptures along the plate interface and associated crustal faults.

Such swarms provide valuable data for understanding stress transfer and fault interactions in subduction settings. Monitoring efforts by agencies such as the Japan Meteorological Agency continue to track activity patterns that may precede or follow major events.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Data
Geological Survey of Japan, AIST
International Seismological Centre Bulletin