Seismic Swarm Activity Southeast of Iwaki, Japan: March 2011 Analysis

The seismic swarm designated PS20110319.1 occurred 27 km southeast of Iwaki in Fukushima Prefecture, Japan, from 02:47 on 18 March 2011 to 09:56 on 19 March 2011. Over 31 hours and 9 minutes, six earthquakes were recorded, with magnitudes ranging from 4.6 to 5.7 and focal depths between 6 km and 66 km. This sequence unfolded in a tectonically active region shaped by the subduction of the Pacific Plate beneath the Okhotsk Plate along the Japan Trench.

Japan's eastern margin lies within the Pacific Ring of Fire, where convergent plate boundaries generate frequent seismic events. The Iwaki area experiences both interplate thrust earthquakes and intraplate activity due to crustal stresses transmitted from the trench. Depths recorded in the swarm reflect this duality, with shallower events near 6 km indicating possible activation of local faults and deeper ones around 65 km consistent with slab-related seismicity.

The swarm took place one week after the magnitude 9.0 Tohoku earthquake of 11 March 2011, whose epicenter lay approximately 80 km from the swarm center. That mainshock, followed by a magnitude 7.9 aftershock 47 km east of Ibaraki, substantially altered regional stress fields, promoting clusters of moderate events. Historical records since 2000 show 12 swarms in the vicinity, including one in 2004 and two in 2008, with nine occurring in 2011 alone, underscoring the elevated post-Tohoku activity.

The six events progressed as follows: a magnitude 5.0 quake at 33 km depth on 18 March at 02:47, followed by a magnitude 5.3 event at 65 km depth later that evening. Two magnitude 5.1 shocks occurred minutes apart near midnight at depths of 34 km and 40 km. Early on 19 March, a magnitude 4.6 event registered at 66 km, concluding with a magnitude 5.7 shock at only 6 km depth. Such variation in depth and magnitude within a short window typifies swarm behavior, where no single mainshock dominates but rather distributed energy release occurs along multiple fault segments.

Geological studies of the Fukushima region highlight its position above the subducting slab, where fluids and temperature gradients influence seismicity patterns. The 2011 sequence demonstrated how megathrust events can trigger secondary swarms through dynamic and static stress changes, a phenomenon documented in subduction zones worldwide.

References:
USGS Earthquake Catalog (earthquake.usgs.gov)
Japan Meteorological Agency Seismic Database (jma.go.jp)
Geological Survey of Japan, AIST tectonic summaries