Seismic Swarm S20050614.1 Near Adak, Alaska: Characteristics and Regional Context

Seismic swarm S20050614.1 occurred 279 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 08:07 on 14 June 2005 and concluded at 07:47 on 15 June 2005, spanning 23 hours and 39 minutes. During this interval, 36 earthquakes were recorded, with magnitudes ranging from 2.6 to 4.8 and focal depths between 0 and 56 km. The largest event reached magnitude 4.8 at a depth of 50 km.

The swarm exhibited a typical clustered pattern without a dominant mainshock. Early activity on 14 June included several events of magnitude 3.5–3.6 at shallow to moderate depths, followed by a series of stronger shocks between 11:44 and 19:56, including multiple magnitude 4+ events. Later phases showed continued moderate activity tapering toward the end of the sequence. Depths varied notably, with some events occurring near the surface and others extending into the mid-crust, consistent with tectonic processes along the subduction interface.

The Adak region lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This setting produces frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes. The Andreanof Islands segment, encompassing the swarm location, has a well-documented history of large megathrust ruptures, such as the 1957 magnitude 8.6 Andreanof Islands earthquake and subsequent aftershock sequences.

Swarm activity in this area is not uncommon. Since 1 January 2000, three swarms have been documented in the broader region: two in 2003 and one in 2005. Such clusters may reflect stress redistribution along the plate boundary or fluid migration within the subduction system, though precise causative mechanisms require further geophysical analysis.

This event provides additional data on the spatiotemporal distribution of seismicity in a high-hazard arc environment. Continued monitoring supports improved understanding of precursory patterns and long-term seismic hazard assessment for the Aleutian Islands.

References

United States Geological Survey Earthquake Catalog
Alaska Earthquake Center historical records
SeismoSight internal swarm classification database