Seismic Swarm S20060614.1: Analysis of Earthquake Activity Near Attu Station, Alaska

The seismic swarm designated S20060614.1 occurred in the remote Aleutian region of Alaska, centered 284 km east-southeast of Attu Station. Registered from 04:26 on 14 June 2006 to 05:38 on 15 June 2006, the event sequence lasted 25 hours and 12 minutes and included 136 earthquakes. This activity marked the first swarm recorded in the area since systematic monitoring began in 2000.

The Aleutian Islands form part of the circum-Pacific seismic belt, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. Convergence rates average 7–8 cm per year, generating frequent earthquakes and volcanic activity across the arc. Depths of events in this swarm ranged primarily from 4 km to 36 km, consistent with shallow crustal and upper-plate seismicity typical of the subduction interface and overlying wedge. The largest event reached magnitude 6.0 at 29 km depth, while other notable shocks included magnitudes 5.4, 4.8, and multiple events above 4.0, all concentrated within the first several hours.

Analysis of the initial 100 events reveals a rapid onset with clustered shallow activity. Early shocks included a magnitude 4.7 at 4 km depth followed closely by the magnitude 6.0 mainshock. Subsequent events showed magnitudes predominantly between 2.5 and 3.8, with depths varying between 5 km and 35 km. Activity decayed steadily after the initial peak, transitioning to lower-magnitude events distributed across a broader depth range. This pattern indicates a classic swarm sequence driven by fluid migration or stress transfer along pre-existing faults rather than a single mainshock-aftershock cascade.

Geological records confirm the Aleutian arc’s long history of intense seismicity. Major historical events include the 1965 magnitude 8.7 Rat Islands earthquake and the 1957 magnitude 8.6 Andreanof Islands earthquake, both occurring along the same subduction segment. Such large ruptures demonstrate the region’s capacity for megathrust events, while smaller swarms like S20060614.1 reflect ongoing strain accumulation and release within the overriding plate and along the plate interface.

Since 2000, only two swarms have been identified in this sector, underscoring the relative rarity of such clustered sequences compared with background seismicity. Continued monitoring by regional networks provides critical data for refining hazard assessments in this tectonically active margin.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries.
Alaska Earthquake Center historical seismicity database.