Seismic Insights into Swarm S20130513.1: Kokhanok, Alaska

Earthquake swarm S20130513.1 occurred approximately 83 km south-southeast of Kokhanok on the Alaska Peninsula. The sequence began at 06:06 on 12 May 2013 and concluded at 08:34 on 14 May 2013, spanning 50 hours and 27 minutes. During this interval, 31 earthquakes were recorded, with magnitudes ranging from 1.2 to 4.6 and focal depths between 4 km and 12 km.

The swarm initiated with a 2.3-magnitude event at 7 km depth, followed rapidly by additional small shocks. The largest event, magnitude 4.6, struck at 11 km depth roughly 28 minutes after onset. Subsequent activity included numerous events clustered between 6 km and 9 km depth. A second notable shock of magnitude 4.4 occurred near the end of the sequence at 12 km depth, accompanied by aftershocks of 2.9 and 2.5. Depths remained shallow throughout, consistent with crustal processes in the overriding plate.

This region lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates exceeding 6 cm per year. The Alaska Peninsula experiences frequent seismic swarms due to both megathrust locking and upper-plate faulting. Nearby volcanic centers, including Iliamna and Redoubt volcanoes, contribute to elevated background seismicity through hydrothermal and magmatic influences, though the 2013 swarm showed no clear volcanic signature.

Historically, the area has hosted significant events, including the 1964 magnitude-9.2 Great Alaska Earthquake and numerous moderate swarms linked to the subduction interface. Modern monitoring networks operated by the Alaska Earthquake Center have documented hundreds of similar low-magnitude sequences since 2000, aiding in refined hazard models for the Cook Inlet and Alaska Peninsula corridors.

The 2013 swarm illustrates typical swarm behavior: a rapid onset, lack of a single dominant mainshock-aftershock pattern, and energy release distributed across dozens of events. Such sequences help delineate active fault strands and stress conditions within the overriding plate, contributing to improved probabilistic seismic hazard assessments for nearby communities and infrastructure.

References:
USGS Earthquake Catalog
Alaska Earthquake Center annual reports
Alaska Division of Geological & Geophysical Surveys tectonic summaries