Seismic Swarm PS20180905.2: Analysis of Activity Near Chitose, Japan

Seismic swarm PS20180905.2 occurred 34 km east-southeast of Chitose, Hokkaido, Japan. The sequence began at 18:07 on 5 September 2018 and concluded at 21:11 the same day, spanning three hours and three minutes. Five earthquakes were recorded during this interval, providing a focused window into short-term clustered seismicity in a tectonically active subduction setting.

The events unfolded as follows: the initial shock at 18:07:59 reached magnitude 6.6 at 35 km depth; a magnitude 5.3 event followed at 18:17:10, also at 35 km; a magnitude 5.2 shock occurred at 18:20:11 at the same depth; a magnitude 4.7 earthquake registered at 21:04:32 at 43 km depth; and the final magnitude 5.3 event took place at 21:11:28 at 29 km depth. Depths remained largely consistent within the upper to mid-crust, consistent with regional faulting patterns.

This swarm exemplifies a rapid succession of events without a single dominant mainshock-aftershock progression. Such clusters often reflect fluid migration or stress redistribution along pre-existing structures rather than progressive rupture of a single fault plane. The tight temporal spacing and moderate magnitudes underscore the value of dense monitoring networks in capturing these transient episodes.

Hokkaido lies at the convergent margin where the Pacific Plate subducts beneath the Okhotsk Plate along the Kuril-Kamchatka Trench system. Chitose occupies a position within the Ishikari Lowland, an area underlain by sedimentary basins and influenced by both trench-parallel and arc-normal faults. Historical records indicate recurrent moderate-to-large earthquakes driven by slab dehydration and plate coupling variations. The 2004 swarm represents the sole prior cluster identified in the instrumental catalog since 2000, highlighting the infrequent nature of swarm-type activity in this specific locale compared with more persistent aftershock sequences elsewhere in the arc.

Regional geology features Quaternary volcanism and active folding, with the Hidaka Collision Zone to the east contributing additional complexity through oblique convergence. Updated tectonic models emphasize variable locking along the megathrust and the role of intraslab events at depths of 30–50 km, aligning closely with the observed focal depths of the 2018 swarm.

Continued seismic surveillance remains essential for distinguishing swarm behavior from foreshock sequences that might precede larger mainshocks in this high-hazard setting.

References:
SeismoSight internal swarm classification PS20180905.2
USGS Earthquake Catalog (regional Hokkaido tectonics)
Geological Survey of Japan (subduction framework of Hokkaido)