Location:

Period:

29 Jul 2005 03:26:12 - 29 Jul 2005 20:25:00 (16 hours 58 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

6 swarms found nearby.

2005

PS20050119.1(150.9km)

19 Jan

10 hours

6 earthquakes

PS20050121.1(128.8km)

20 Jan

1 day 3 hours

8 earthquakes

PS20050726.1(4.8km)

26 Jul

1 day 19 hours

17 earthquakes

S20050727.1(5.4km)

27 Jul

1 day 15 hours

25 earthquakes

2016

PS20160923.1(177.9km)

22 Sep

18 hours

8 earthquakes

2022

PS20220804.1(79.3km)

4 Aug

14 hours

9 earthquakes

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Seismic Swarm PS20050729.1 Off the East Coast of Honshu, Japan

On July 29, 2005, a seismic swarm designated PS20050729.1 was recorded off the east coast of Honshu, Japan. The sequence began at 03:26 UTC and concluded at 20:25 UTC, spanning 16 hours and 58 minutes. During this period, six earthquakes were registered in the region, with magnitudes ranging from 3.7 to 5.5 and focal depths between 1 km and 80 km.

The events unfolded as follows: at 03:26:12, a magnitude 5.0 quake occurred at 80 km depth; six minutes later at 03:32:26, another magnitude 5.0 event struck at 10 km depth. Activity resumed around midday with a magnitude 5.5 earthquake at 12:51:00 (5 km depth), followed immediately by a magnitude 5.2 event at 12:51:06 (35 km depth). A smaller magnitude 3.7 shock at 14:59:44 occurred at just 1 km depth. The swarm concluded with a magnitude 5.4 event at 20:25:00, located at 11 km depth.

This swarm represents the first such episode recorded in the area since January 1, 2000. Historical statistics indicate a total of four swarms have occurred in the region since that date.

The east coast of Honshu lies along the Japan Trench, a major subduction zone where the Pacific Plate descends beneath the Okhotsk Plate. This tectonic setting drives frequent seismic activity through megathrust faulting and associated crustal deformation. Earthquake swarms in subduction environments often arise from stress redistribution along the plate interface or within the overriding plate, sometimes linked to fluid migration or aseismic slip episodes. Depths recorded in the 2005 swarm span both shallow crustal levels and intermediate depths typical of the subducting slab.

Japan’s geological history reflects ongoing convergence along the Pacific Ring of Fire, with the Japanese archipelago shaped by repeated subduction, accretion, and volcanism over millions of years. The Honshu margin has produced numerous large earthquakes, underscoring the persistent seismic hazard in the region.

SeismoSight internal classification identifies swarm PS20050729.1 as a distinct cluster without a dominant mainshock-aftershock pattern, consistent with swarm characteristics observed in subduction zones worldwide.

References

Japan Meteorological Agency seismic records
USGS Earthquake Catalog
Geological Survey of Japan tectonic framework reports