Seismic Swarm PS20110311.1: Analysis of Activity Offshore Eastern Japan

A seismic swarm designated PS20110311.1 was recorded from 06:12 on 11 March 2011 to 20:14 on 12 March 2011. The events occurred approximately 261 km east-southeast of Ofunato, Japan. Within 38 hours and one minute, a total of 28 earthquakes were registered, with magnitudes ranging from 5.0 to 7.7 and focal depths between 4 km and 47 km.

The sequence began with a magnitude 6.2 event at 06:12:36 on 11 March, followed rapidly by a 6.3 at 06:15:23. The largest shock, magnitude 7.7, occurred at 06:25:50 at a depth of 18 km. Subsequent activity included multiple events above magnitude 6.0 through the afternoon of 11 March, then transitioned to predominantly magnitude 5.0–5.5 shocks that continued into 12 March. The final recorded event was a magnitude 5.0 at 20:14:32 on 12 March at a depth of 4 km.

Regional Geological Setting

The swarm location lies within the Japan Trench subduction zone, where the Pacific Plate converges with the Okhotsk Plate at a rate of approximately 8–9 cm per year. This tectonic boundary produces frequent megathrust earthquakes and associated aftershock sequences. The offshore Tohoku region features a well-developed accretionary prism and complex faulting in the overriding plate, contributing to varied focal depths observed in the swarm.

Seismicity History

Instrumental records since 2000 document only one prior swarm in the immediate area, occurring in 2011. Notable large earthquakes nearby include a magnitude 7.7 event on 11 March 2011 located 76 km from the swarm center and a magnitude 7.0 shock on 14 November 2005 at a similar offset. These events underscore the persistent seismic hazard along this segment of the subduction interface.

Swarm Characteristics and Implications

The temporal clustering and magnitude distribution of PS20110311.1 are consistent with triggered seismicity in a subduction setting. Depths span the seismogenic zone, indicating slip on both the plate interface and subsidiary crustal faults. Such swarms can precede or accompany major ruptures by redistributing stress, although each sequence requires separate evaluation for forecasting potential.

• SeismoSight internal swarm catalog (PS20110311.1 parameters)
• Historical earthquake database, 2000–present (regional events)
• Japan Trench tectonic framework summaries from national geological surveys