Location:

Period:

20 Jan 2005 14:16:21 - 21 Jan 2005 18:10:35 (1 day 3 hours 54 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

9 swarms found nearby.

2000

PS20000629.1(162.0km)

28 Jun

1 day 0 hours

10 earthquakes

2005

PS20050119.1(22.0km)

19 Jan

10 hours

6 earthquakes

PS20050726.1(124.4km)

26 Jul

1 day 19 hours

17 earthquakes

PS20050729.1(128.8km)

29 Jul

16 hours

6 earthquakes

2011

PS20110311.2(141.2km)

11 Mar

21 hours

21 earthquakes

PS20110313.1(188.5km)

13 Mar

3 days 13 hours

14 earthquakes

PS20110322.1(199.2km)

22 Mar

2 days 5 hours

14 earthquakes

2016

PS20160923.1(52.7km)

22 Sep

18 hours

8 earthquakes

2022

PS20220804.1(192.7km)

4 Aug

14 hours

9 earthquakes

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Seismic Swarm PS20050121.1: Analysis of Activity Offshore Katsuura, Japan

The seismic swarm designated PS20050121.1 occurred in a tectonically dynamic region 166 km southeast of Katsuura, Japan. This area lies within the influence of the Philippine Sea Plate and Pacific Plate boundary, where subduction processes drive recurrent seismic events along the Izu-Bonin-Mariana arc system. The swarm commenced at 14:16 on 20 January 2005 and concluded at 18:10 on 21 January 2005, spanning 27 hours and 54 minutes during which eight earthquakes were recorded.

Geological conditions in this offshore zone reflect ongoing plate convergence, with the Pacific Plate subducting westward beneath the Philippine Sea Plate at rates of approximately 5–7 cm per year. Historical seismic records indicate elevated activity linked to both interplate thrust faults and intraplate deformation within the downgoing slab. Depths ranging from 10 km to 80 km align with typical patterns observed in this segment of the subduction zone, where shallow crustal events often coexist with deeper intraslab seismicity.

The sequence included the following events: a magnitude 5.4 earthquake at 10 km depth on 20 January at 14:16:21, followed minutes later by another magnitude 5.4 at 51 km depth. On 21 January, activity intensified with a magnitude 5.7 at 10 km at 12:45:31, a magnitude 5.2 at 80 km at 12:45:41, a magnitude 4.5 at 10 km at 12:58:11, a magnitude 5.8 at 10 km at 17:58:56, a magnitude 5.4 at 60 km at 17:59:05, and a final magnitude 5.0 at 10 km at 18:10:35. These closely spaced occurrences, clustered in both time and space, characterize swarm behavior rather than a single mainshock-aftershock sequence.

Since 1 January 2000, only two such swarms have been documented in the region, with the prior instance occurring in 2000 (one event) and this 2005 swarm representing the second. Such infrequent clustering underscores the episodic nature of seismic release in this portion of the margin, where stress accumulation and release follow variable patterns influenced by fluid migration and fault interactions at depth.

This activity contributes to broader understanding of subduction-related hazards in eastern Japan, where monitoring networks continue to track similar phenomena for improved forecasting of potential larger events.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (historical regional data)
Japan Meteorological Agency seismic reports