Seismic Swarm PS20110313.1: Post-Tohoku Activity near Asahi, Japan

The seismic swarm designated PS20110313.1 occurred 14 km south-southeast of Asahi in Chiba Prefecture, Japan. It began at 00:32 on 13 March 2011 and concluded at 13:39 on 16 March 2011, spanning 85 hours and 6 minutes during which 14 earthquakes were recorded. This sequence unfolded in the immediate aftermath of the devastating 11 March 2011 Tohoku mainshock and its associated M7.9 event 75 km from the swarm centroid.

Japan’s eastern margin lies within a complex subduction system where the Pacific Plate descends beneath the Okhotsk and Philippine Sea plates. The Kanto region, encompassing the swarm location, experiences frequent seismicity driven by this convergence, slab dehydration, and crustal faulting. Historical records document repeated episodes of clustered activity superimposed on the background rate of plate-boundary events. The 2011 Tohoku earthquake, one of the largest instrumentally recorded, released immense stress across a broad area and triggered widespread aftershock sequences and swarm-like clusters in surrounding prefectures, including Chiba.

Within the 85-hour window, the swarm produced events ranging from M4.6 to M5.7. The largest shocks reached M5.7 on 15 March at 20:29 (depth 19 km) and again on 16 March at 03:52 (depth 10 km). Hypocenters varied between 4 km and 84 km, indicating activity across both shallow crustal levels and deeper portions of the subducting slab. Earlier events on 13 March included five shocks of M5.0 or greater within the first 24 hours, demonstrating rapid energy release without a single dominant mainshock.

Since 1 January 2000, nine comparable swarms have been identified in the same source region. Prior episodes occurred in 2000 (one swarm), 2005 (two swarms), 2008 (two swarms), and 2011 (four swarms, including the present sequence). These clusters typically exhibit similar characteristics: short duration, multiple events of comparable magnitude, and depths distributed through the upper 50 km of the crust and slab.

The March 2011 swarm illustrates how large megathrust ruptures can rapidly alter the local stress field, promoting transient clusters of moderate-magnitude earthquakes. Such swarms differ from classic aftershock sequences by lacking a clear magnitude decay relationship and instead showing sustained, nearly equal-sized events over several days. In tectonically active subduction zones, these patterns provide valuable indicators of stress redistribution along secondary faults and within the overriding plate.

Seismic monitoring networks operated by the Japan Meteorological Agency and international partners continue to track residual activity in the region. The geological framework of eastern Japan ensures that similar swarm episodes will recur, underscoring the importance of sustained observation for hazard assessment.

References:
USGS Earthquake Catalog
Japan Meteorological Agency Seismic Database
Global CMT Project
SeismoSight internal swarm classification records