Location:

Period:

23 Oct 2002 23:17:58 - 24 Oct 2002 09:28:00 (10 hours 10 minutes)

Volcanoes in 100km radius:

Submarine Volcano NNE of Iriomotejima(71km)

Earthquakes:

6 swarms found nearby.

2002

S20021024.2(4.8km)

23 Oct

1 day 6 hours

31 earthquakes

2004

PS20041108.1(138.6km)

8 Nov

4 hours

5 earthquakes

2007

PS20070420.1(162.0km)

20 Apr

4 hours

8 earthquakes

2013

S20130417.2(72.1km)

17 Apr

1 day 3 hours

41 earthquakes

2015

PS20150420.1(180.3km)

20 Apr

10 hours

5 earthquakes

2020

S20200615.2(55.2km)

14 Jun

1 day 20 hours

30 earthquakes

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Seismic Swarm PS20021024.1: Analysis of Activity Near Ishigaki, Japan

A seismic swarm designated PS20021024.1 occurred approximately 102 km north-northwest of Ishigaki in Japan's Ryukyu Islands. The sequence began at 23:17 on 23 October 2002 and concluded at 09:28 on 24 October 2002, encompassing 11 earthquakes over a period of 10 hours and 10 minutes. All events originated at a focal depth of 10 km, with magnitudes ranging from 4.9 to 5.4.

The swarm initiated with a magnitude 5.0 event at 23:17:58 on 23 October, followed rapidly by a magnitude 5.1 shock at 23:51:09. Activity intensified on 24 October, featuring multiple events above magnitude 5.0, including a pair of magnitude 5.2 earthquakes at 02:27:52 and 02:32:31, a magnitude 5.4 event at 04:35:13, and another magnitude 5.4 at 06:07:04. The sequence ended with a magnitude 5.3 event at 09:28:00. These closely spaced occurrences reflect typical swarm behavior, where numerous earthquakes of similar magnitude cluster temporally and spatially without a dominant mainshock.

Ishigaki lies within the Ryukyu Arc, a tectonically active region shaped by the northwestward subduction of the Philippine Sea Plate beneath the Eurasian Plate at rates of approximately 5–7 cm per year. This convergent margin generates frequent seismicity, including both shallow crustal events and deeper Wadati-Benioff zone activity. The arc's geology features volcanic islands, coral reefs, and fault systems influenced by oblique subduction and back-arc spreading in the Okinawa Trough to the north. Historical records document recurrent moderate earthquakes in the area, with the 1771 Yaeyama earthquake (estimated magnitude 7.4–8.0) producing devastating tsunamis that underscore the region's long-term hazard potential.

Since 1 January 2000, only one swarm has been recorded in this sector according to internal classification data, with the prior swarm also occurring in 2002. Such swarms provide insights into fluid migration or stress transfer along subduction-related faults, often without leading to larger mainshocks. Depths consistently near 10 km suggest activation within the upper crust, consistent with the tectonic regime of the central Ryukyu Islands.

This event highlights the persistent seismic productivity of the Philippine Sea-Eurasian plate boundary. Ongoing monitoring by regional networks continues to track similar sequences, aiding in refined hazard assessments for the Ryukyu Islands.

References

SeismoSight internal classification records for swarm PS20021024.1.
Geological Survey of Japan, AIST: Tectonic framework of the Ryukyu Arc (updated regional summaries).
USGS Earthquake Catalog: Historical seismicity in the Ishigaki region.