Location:

Period:

25 Oct 2013 17:10:19 - 25 Oct 2013 21:27:34 (4 hours 17 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.1; off the east coast of Honshu, Japan; (25 Oct 2013) (7km from the swarm center)

8 swarms found nearby.

2011

PS20110311.6(169.8km)

11 Mar

2 days 8 hours

50 earthquakes

PS20110311.1(180.1km)

11 Mar

1 day 14 hours

28 earthquakes

PS20110312.1(136.6km)

11 Mar

1 day 3 hours

8 earthquakes

PS20110315.2(90.2km)

15 Mar

1 day 17 hours

6 earthquakes

PS20110322.2(119.4km)

21 Mar

18 hours

5 earthquakes

PS20110325.1(97.9km)

24 Mar

20 hours

6 earthquakes

2012

PS20121207.1(110.5km)

7 Dec

14 hours

10 earthquakes

S20121207.1(111.4km)

7 Dec

1 day 20 hours

39 earthquakes

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Seismic Swarm PS20131025.1 off the East Coast of Honshu, Japan

Seismic swarm PS20131025.1 was recorded off the east coast of Honshu, Japan, beginning at 17:10 on 25 October 2013 and ending at 21:27 the same day. In roughly four hours and seventeen minutes, six earthquakes occurred. The sequence opened with a magnitude 7.1 event at 35 km depth, followed by events of magnitude 5.1 at 35 km, 5.4 at 37 km, 4.8 at 17 km, 5.1 at 33 km, and a final magnitude 5.5 shock at 10 km depth.

The swarm lies within the Japan Trench subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This convergent margin produces frequent seismicity at shallow to intermediate depths, with the Wadati-Benioff zone extending to more than 500 km. Crustal stresses in the region accumulate rapidly because of the high convergence rate and the presence of seamounts and fracture zones on the incoming plate that locally alter coupling.

The October 2013 swarm occurred in a segment of the margin that had already experienced significant stress redistribution from the 2011 Tohoku earthquake and its aftershock sequence. Since 1 January 2000, eight swarms have been documented in the immediate vicinity. Earlier episodes took place in 2011 (six swarms) and 2012 (two swarms). The magnitude 7.1 mainshock of 25 October 2013 was located only 7 km from the swarm centroid, underscoring the proximity of larger events to these clustered sequences.

Swarm activity in subduction settings commonly reflects heterogeneous faulting, fluid migration along the plate interface, or afterslip following a larger rupture. The depth range observed here (10–37 km) spans both the upper plate and the plate interface, consistent with distributed deformation rather than slip on a single through-going fault. Such episodes supply useful constraints on the frictional properties of the megathrust and on the time scales of post-seismic relaxation.

Ongoing monitoring by regional networks continues to track microseismicity in the same corridor, providing an updated baseline for assessing whether future swarms deviate from the established pattern since 2000.

References
USGS Earthquake Hazards Program
Japan Meteorological Agency seismic bulletins
Geological Survey of Japan, subduction zone summaries