Seismic Swarm PS20110311.4: Analysis of Intense Aftershock Activity off Eastern Japan

On 11 March 2011, a dense seismic swarm designated PS20110311.4 was recorded approximately 95 km east of Hasaki, Japan. The sequence began at 06:03 UTC and concluded at 14:56 UTC, encompassing 57 earthquakes within an 8-hour-52-minute window. This activity occurred in the immediate aftermath of the magnitude 9.0 Tohoku-Oki mainshock and formed part of its extensive aftershock sequence along the Japan Trench subduction zone.

The swarm exhibited a broad magnitude range, with the largest event reaching 7.9. Depths varied from very shallow crustal levels near 4 km to intermediate depths exceeding 50 km, reflecting rupture along the megathrust interface and within the overriding plate. Multiple events clustered between 20–40 km depth, consistent with the geometry of the subducting Pacific Plate beneath the Okhotsk Plate. The temporal distribution showed peak intensity in the first two hours, followed by a gradual decline, characteristic of aftershock decay modulated by static and dynamic stress changes from the mainshock.

Japan’s eastern margin lies within the Pacific Ring of Fire, where the Pacific Plate converges with the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic setting has produced repeated great earthquakes, including the 2011 Tohoku-Oki event whose rupture extended more than 400 km along strike. The region near Hasaki and the adjacent Japan Trench experiences frequent moderate-to-large aftershocks due to ongoing viscoelastic relaxation and afterslip on the megathrust.

Historical records since 2000 indicate four prior seismic swarms in the broader area, occurring in 2005 and 2008. The 2011 swarm stands out for its rapid onset and inclusion of multiple magnitude-6+ events within hours of the mainshock. A notable magnitude-7.9 aftershock on the same day, located 47 km east of Oarai and roughly 56 km from the swarm centroid, further highlights the elevated seismic productivity along this segment of the subduction interface.

Such swarms provide critical data for refining aftershock forecasts and understanding stress transfer in subduction zones. Continued monitoring by the Japan Meteorological Agency and international networks remains essential for assessing ongoing seismic hazard in this tectonically active region.

References

• Japan Meteorological Agency (JMA) seismic catalog and aftershock reports (2011–2024 updates)
• USGS Earthquake Hazards Program: Tohoku-Oki sequence documentation
• Tectonic framework from the Geological Survey of Japan (AIST) subduction zone studies