Seismic Swarm PS20070113.2 in the Kuril Islands: Analysis and Context

On January 13, 2007, a seismic swarm designated PS20070113.2 was recorded in the Kuril Islands. The sequence began at 04:23 and concluded at 12:10, encompassing eight earthquakes over seven hours and forty-six minutes. This event occurred within a tectonically active subduction zone where the Pacific Plate converges with the Okhotsk Plate at rates exceeding 8 cm per year.

The swarm initiated with a magnitude 8.1 earthquake at 04:23:21 UTC, located at a depth of 10 km. Subsequent events included a magnitude 5.1 quake at 05:34:41, a magnitude 5.0 at 06:09:51, and two events at 08:00:53 with magnitudes 4.7 and 5.0 at depths of 10 km and 2 km respectively. Additional activity comprised a magnitude 5.3 event at 08:20:25, a magnitude 5.8 at 09:18:31, and a final magnitude 5.0 at 12:10:03, all predominantly at 10 km depth. These shallow-focus events reflect brittle failure along the plate interface and associated crustal faults.

The Kuril Islands form part of the Pacific Ring of Fire, characterized by intense volcanism and seismicity driven by oblique subduction. The arc extends approximately 1,200 km from Hokkaido to Kamchatka, featuring numerous active volcanoes and frequent megathrust earthquakes. Geological records indicate that the region has experienced repeated great earthquakes, with recurrence intervals on the order of centuries along major segments of the subduction zone.

Historical data since 2000 show nine prior swarms in the area, with notable clusters in 2003 (one swarm), 2005 (one swarm), and 2006 (seven swarms). These sequences typically precede or accompany larger mainshocks, highlighting episodic stress release in the forearc region. The 2007 swarm coincided closely with the magnitude 8.1 Kuril Islands earthquake of the same day, centered 62 km from the swarm epicenter. This mainshock ranks among the strongest events recorded in the region since 2000 and triggered widespread aftershock activity consistent with the observed swarm pattern.

Such swarms provide critical insights into fault mechanics and stress transfer along the subduction interface. Monitoring of similar sequences aids in assessing short-term seismic hazards in this remote but strategically important arc.

References:
USGS Earthquake Catalog
SeismoSight internal swarm classification database