Seismic Swarm S20090502.1: Analysis of Activity Southeast of King Cove, Alaska

Seismic swarm S20090502.1 occurred in a tectonically active zone 83 km southeast of King Cove, Alaska, on the Alaska Peninsula. This region lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting megathrust fault system generates frequent earthquakes, ranging from shallow crustal events to deeper intraslab activity, and contributes to the formation of volcanic arcs along the peninsula.

The swarm initiated at 02:19 UTC on 2 May 2009 and concluded at 11:15 UTC on 3 May 2009, spanning 32 hours and 56 minutes. During this period, 54 earthquakes were recorded. The sequence featured a largest event of magnitude 5.6 at a depth of 9 km, followed by numerous smaller events with magnitudes between 1.6 and 3.6. Depths ranged from 1 km to 25 km, indicating a mix of shallow crustal and mid-crustal seismicity consistent with stress release along the subduction interface and overlying plate.

Earthquake swarms in subduction zones often reflect fluid migration, aseismic slip, or localized stress adjustments rather than a single mainshock-aftershock sequence. Here, the temporal clustering and magnitude distribution align with swarm characteristics observed in the Aleutian arc. The events likely occurred offshore or near the continental slope, where the subducting slab interacts with heterogeneous crustal structures.

Historically, seismic swarms have been documented in this sector of the Alaska Peninsula since at least 2000. Three such swarms occurred in the interval from 1 January 2000 onward, with the earliest recorded in 2007. These episodes underscore the persistent seismic productivity of the area, influenced by ongoing plate convergence and possible volcanic influences from nearby systems such as those on Unimak Island.

The 2009 swarm provides insight into the short-term variability of seismicity in the region. Initial high-magnitude activity gave way to a prolonged series of lower-magnitude events distributed across a range of depths, suggesting distributed faulting rather than rupture on a single plane. Such patterns aid in refining models of subduction zone hazards, including potential for larger thrust earthquakes.

References

• SeismoSight internal swarm classification data for S20090502.1
• USGS Earthquake Catalog (historical regional context)
• Alaska Earthquake Center reports on Aleutian arc tectonics