Seismic Swarm Analysis: May 2009 Event Southeast of Old Harbor, Alaska

An earthquake swarm occurred 118 km southeast of Old Harbor, Alaska, from 19:23 on 15 May 2009 to 18:37 on 16 May 2009. In 23 hours and 14 minutes, five events were recorded, providing a snapshot of tectonic stress release along the Aleutian subduction zone.

The region lies where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin produces frequent seismicity, with Kodiak Island and surrounding offshore areas experiencing both shallow crustal events and deeper intraslab earthquakes. Historical records document major ruptures, including the 1964 Great Alaska Earthquake (Mw 9.2), whose aftershock zone extended into the Kodiak region and generated significant local tsunamis. Ongoing plate convergence continues to load faults, resulting in persistent background seismicity and occasional swarms.

The 2009 swarm comprised the following sequence:

• 15 May 2009, 19:23:13 UTC – magnitude 5.2 at 10 km depth
• 16 May 2009, 18:22:28 UTC – magnitude 5.8 at 30 km depth
• 16 May 2009, 18:22:29 UTC – magnitude 5.9 at 10 km depth
• 16 May 2009, 18:33:09 UTC – magnitude 2.2 at 10 km depth
• 16 May 2009, 18:37:40 UTC – magnitude 5.3 at 21 km depth

The near-simultaneous occurrence of the 5.8 and 5.9 events within one second suggests rapid triggering along adjacent fault segments. Depths ranging from 10 km to 30 km place activity within both the overriding plate and the subducting slab, consistent with the transition zone of the subduction interface. The modest total energy release and short duration align with swarm characteristics, where events cluster without a single dominant mainshock–aftershock decay pattern.

Such swarms in the Aleutian arc often reflect transient stress perturbations, possibly from aseismic slip or fluid migration along the plate interface. They contribute to long-term strain accommodation without necessarily preceding larger ruptures, although monitoring remains essential given the region’s capacity for great earthquakes.

Updated seismic hazard assessments for Kodiak Island incorporate both historical megathrust events and smaller swarm-type sequences to refine probabilistic forecasts. Continued instrumentation by regional networks supports real-time detection and improved understanding of subduction dynamics in this high-hazard setting.

References: USGS Earthquake Catalog (events 2009) Alaska Earthquake Center regional reports Plate Boundary Observatory GPS velocity fields for the Aleutian arc