Seismic Swarm S20021024.2: Analysis of Activity Near Ishigaki, Japan

Seismic swarm S20021024.2 occurred approximately 110 km north-northwest of Ishigaki in Japan's Ryukyu Islands. The sequence began at 21:50 on 23 October 2002 and concluded at 04:02 on 25 October 2002, spanning 30 hours and 12 minutes. During this period, 31 earthquakes were recorded, all at a focal depth of 10 km. Magnitudes ranged from 4.1 to 5.3, with the largest event occurring at 02:49 on 25 October.

This swarm exemplifies a cluster of events without a dominant mainshock-aftershock pattern. Such sequences often arise from localized stress perturbations, possibly linked to fluid movement along faults or minor adjustments in the subduction interface. The consistent shallow depth across all events points to activity within the upper crust, where brittle failure predominates in this tectonically active margin.

The Ryukyu arc forms part of the convergent boundary where the Philippine Sea Plate subducts northwestward beneath the Eurasian Plate at rates of approximately 6–8 cm per year. Ishigaki lies within the Yaeyama Islands segment of this arc, characterized by a well-developed forearc and active back-arc spreading in the Okinawa Trough to the north. Historical records document recurrent large earthquakes in the region, including the 1771 Yaeyama event estimated at magnitude 7.4–8.0, which generated devastating tsunamis. Modern monitoring has identified frequent moderate swarms along the arc, reflecting episodic strain release rather than isolated large ruptures.

Geological mapping of the area reveals a complex system of thrust and strike-slip faults accommodating oblique convergence. The 2002 swarm aligns with this framework, occurring in a zone of distributed seismicity between the trench and the volcanic front. Updated regional assessments confirm ongoing low-to-moderate activity in the Ryukyu subduction zone, with no significant changes in long-term rates since the early 2000s.

Insights from the swarm parameters highlight a rapid onset followed by a gradual decline in event rate, typical of tectonic swarms influenced by pore-pressure diffusion. The absence of events exceeding magnitude 5.3 suggests limited rupture dimensions, consistent with small fault patches activated under uniform stress conditions at 10 km depth.

In summary, swarm S20021024.2 provides a clear example of clustered seismicity in a subduction setting, underscoring the value of continuous monitoring for understanding strain accumulation along the Ryukyu margin.

References

United States Geological Survey Earthquake Catalog (USGS).
Japan Meteorological Agency Regional Seismicity Reports.
Geological Survey of Japan, Active Fault and Seismic Source Database.