Seismic Swarm Analysis: Rat Islands, Aleutian Islands, November 2003

The Rat Islands region of the western Aleutian arc lies along the convergent boundary where the Pacific plate subducts beneath the North American plate. This tectonically active zone produces frequent large-magnitude earthquakes due to the release of accumulated strain along the megathrust interface. The arc's geology features volcanic islands, deep oceanic trenches, and a history of both megathrust and intraslab events, with the subduction angle steepening westward and introducing oblique components that influence rupture patterns.

SeismoSight internal records classify swarm PS20031117.1 as occurring in this setting. The sequence initiated at 06:43 on 17 November 2003 with a magnitude 7.8 event at 33 km depth and concluded at 11:55 on 18 November 2003. Over 29 hours and 11 minutes, 18 earthquakes were recorded, the majority at depths of 33 km with a single event at 40 km. The initial magnitude 7.8 shock was followed by events ranging from magnitude 5.0 to 5.8, consistent with clustered seismic energy release along the subduction interface.

The temporal distribution shows intense activity in the first several hours after the mainshock, with nine events exceeding magnitude 5.0 occurring before noon on 17 November. Subsequent activity tapered but included additional magnitude 5.5 and 5.8 shocks on 18 November. Depths remained predominantly shallow to intermediate, aligning with typical megathrust seismicity in the Rat Islands segment.

Regional historical data since 2000 indicate limited swarm occurrences, with this 2003 sequence representing the sole documented swarm in the catalog. Strong earthquakes in the vicinity include the magnitude 7.9 event of 23 June 2014 located 83 km from the swarm center, the magnitude 7.8 shock of 17 November 2003 itself only 15 km away, and the magnitude 7.1 event of 17 March 2003 positioned 35 km distant. These events underscore the persistent seismic productivity of the area driven by ongoing plate convergence at rates exceeding 6 cm per year.

Swarm sequences such as PS20031117.1 provide insight into stress transfer and fluid migration processes within the subduction zone. The concentration of moderate aftershocks following the magnitude 7.8 mainshock reflects localized fault segment interactions rather than a single prolonged rupture. Continued monitoring of the Aleutian arc remains essential given its capacity for great earthquakes exceeding magnitude 8.

References

USGS Earthquake Catalog
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification database