Location:

Period:

11 Apr 2011 08:30:04 - 13 Apr 2011 01:08:02 (1 day 16 hours 37 minutes)

Volcanoes in 100km radius:

Nasudake(44km), Takaharayama(58km), Adatarayama(75km), Bandaisan(76km), Azumayama(85km), Nantaisan(86km), Omanago Group(86km), Numazawa(92km), Nikko-Shiranesan(95km)

Earthquakes:

6 swarms found nearby.

2011

PS20110312.2(63.0km)

11 Mar

20 hours

6 earthquakes

PS20110314.1(77.6km)

13 Mar

13 hours

14 earthquakes

PS20110319.1(23.8km)

18 Mar

1 day 7 hours

6 earthquakes

PS20110322.3(54.8km)

22 Mar

12 hours

6 earthquakes

PS20110411.1(11.8km)

10 Apr

1 day 7 hours

7 earthquakes

2016

PS20161121.1(81.6km)

21 Nov

20 hours

11 earthquakes

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Seismic Swarm S20110411.2 Near Ishikawa, Japan: Event Analysis and Regional Context

Seismic swarm S20110411.2 was recorded between 08:30 on 11 April 2011 and 01:08 on 13 April 2011, centered 21 km south-southeast of Ishikawa, Japan. Over 40 hours and 37 minutes, 35 earthquakes were detected, with magnitudes ranging from 4.0 to 5.3 and focal depths between 7 km and 68 km. The sequence began with two magnitude-4.3 events at depths of 40 km and 35 km, followed by a rapid succession of events that peaked at magnitude 5.3 at 01:08 on 13 April at 32 km depth.

The swarm exhibited typical characteristics of clustered aftershock activity, with the majority of events occurring within the first 24 hours. Early events clustered around magnitudes 4.3–4.9 at mid-crustal depths of 35–52 km. Later activity included shallower events, such as a magnitude-4.5 quake at only 7 km depth on 12 April, alongside deeper occurrences reaching 68 km. This vertical distribution reflects stress redistribution along multiple fault segments in the overriding plate following the March 2011 Tohoku mainshock.

Japan’s eastern margin lies within the Pacific Ring of Fire, where the Pacific Plate subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year along the Japan Trench. This tectonic setting produces frequent moderate-to-large earthquakes, with the 11 March 2011 Tohoku event (Mw 9.0) generating extensive aftershock sequences across northeastern Honshu. The Ishikawa swarm occurred roughly one month later, consistent with ongoing post-seismic relaxation and triggered slip on secondary structures.

Since 1 January 2000, five earthquake swarms have been identified in the region according to SeismoSight internal classification, with this event marking the first recorded swarm in 2011. Subsequent swarms documented through the same system illustrate recurring episodic seismicity linked to the same subduction-related stress field.

The swarm’s temporal evolution shows an initial high-rate phase decaying over two days, a pattern commonly observed in subduction-zone aftershock zones. No surface rupture or significant damage was associated with these events, though they contributed to the prolonged seismic unrest following the Tohoku mainshock.

References

United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
Japan Meteorological Agency Seismological Data (www.jma.go.jp)
SeismoSight internal swarm classification records