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Location:
113 km ENE of Miyako, Japan
Period:
16 Sep 2011 19:26:40 - 18 Sep 2011 07:04:18 (1 day 11 hours 37 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
10
M 7.0+:
M 7.4; 100 km ENE of Miyako, Japan; (20 Apr 2026) (10km from the swarm center)
14 swarms found nearby.
2011
PS20110311.1(196.5km)
11 Mar
1 day 14 hours
28 earthquakes
PS20110311.7(42.0km)
11 Mar
3 days 15 hours
38 earthquakes
PS20110311.8(181.9km)
11 Mar
9 hours
5 earthquakes
PS20110315.1(35.9km)
14 Mar
1 day 10 hours
6 earthquakes
PS20110319.2(116.8km)
18 Mar
1 day 17 hours
8 earthquakes
PS20110323.1(54.3km)
22 Mar
23 hours
6 earthquakes
PS20110413.1(32.2km)
13 Apr
8 hours
6 earthquakes
2012
PS20120314.1(176.8km)
14 Mar
2 hours
6 earthquakes
PS20120520.1(61.8km)
19 May
1 day 3 hours
9 earthquakes
2015
PS20150611.1(60.3km)
10 Jun
20 hours
5 earthquakes
2016
PS20160820.1(76.1km)
20 Aug
1 day 7 hours
6 earthquakes
2025
PS20251108.1(62.1km)
8 Nov
1 day 13 hours
25 earthquakes
PS20251208.1(125.0km)
8 Dec
18 hours
9 earthquakes
2026
PS20260420.1(52.8km)
20 Apr
21 hours
9 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20110916.1: Geological Context and Event Analysis off Miyako, Japan

Seismic swarm PS20110916.1 occurred 113 km ENE of Miyako, Japan, beginning at 19:26 on 16 September 2011 and concluding at 07:04 on 18 September 2011. Over 35 hours and 37 minutes, the swarm produced 10 earthquakes with magnitudes ranging from 5.1 to 6.7 and focal depths between 10 km and 40 km. The sequence began with a magnitude 6.7 event at 30 km depth, followed by events at 19:40 (M5.1, 35 km), 20:11 (M5.7, 35 km), 21:08 (M5.9, 18 km), 21:36 (M5.8, 40 km), 22:40 (M5.7, 37 km), and additional shocks on 17 September at 07:33 (M5.3, 30 km), 07:34 (M5.7, 30 km), 17:54 (M5.2, 22 km). The final event registered M5.6 at 10 km depth on 18 September.

This activity unfolded within the Japan Trench subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. The offshore region near Miyako lies along the northern segment of this convergent margin, characterized by complex faulting and historical megathrust potential. Depths recorded during the swarm align with typical intraslab and interface seismicity in the area, reflecting brittle failure within the subducting slab and overlying crust.

Since 1 January 2000, seven swarms have been documented in the vicinity, with PS20110916.1 marking the earliest. Such clustered sequences indicate episodic stress redistribution along pre-existing fractures rather than isolated mainshock-aftershock patterns. The 2011 swarm followed the great Tohoku-oki earthquake by six months, suggesting possible delayed triggering through viscoelastic relaxation or fluid migration in the forearc.

A magnitude 7.4 earthquake occurred on 20 April 2026, centered 100 km ENE of Miyako and only 10 km from the swarm centroid. This event underscores the persistent seismic hazard in the region, where accumulated strain along the plate interface continues to produce significant ruptures.

Geological records indicate that the Miyako offshore area has experienced recurrent large-magnitude events throughout the Holocene, driven by the same subduction dynamics. Bathymetric features such as the trench axis and forearc basins further modulate rupture propagation and aftershock distribution.

References
SeismoSight internal classification records for swarm PS20110916.1.
Geological Survey of Japan, AIST: Tectonic framework of the Japan Trench.
Japan Meteorological Agency seismic catalogs (2000–2026).