Seismic Swarm S20020715.1: Analysis of Earthquake Activity Near Karluk, Alaska

Seismic swarm S20020715.1 was recorded 88 km NNW of Karluk, Alaska, on Kodiak Island. The sequence began at 08:07 on 14 July 2002 and concluded at 08:34 on 15 July 2002, encompassing 27 earthquakes over 24 hours and 27 minutes. This event occurred within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. The region experiences frequent shallow seismicity due to crustal stresses along the subduction interface and overlying faults.

The swarm consisted predominantly of microearthquakes, with magnitudes ranging from 0.2 to 3.0 and focal depths between 0 and 6 km. Notable events included a magnitude 3.0 earthquake at 07:03:54 on 15 July at 1 km depth, alongside several magnitude 1.9 events at shallow depths. Activity intensified between 07:03 and 07:14 on 15 July, with multiple events clustered within minutes, followed by a decline toward the swarm's termination. Such patterns align with typical swarm behavior, featuring no dominant mainshock but rather distributed energy release over a short period, often linked to fluid migration or stress triggering in fractured crust.

Geological context for the Kodiak region includes its position on the overriding plate above the subducting Pacific slab. Historical records document major events such as the 1964 Mw 9.2 Great Alaska Earthquake, which ruptured a segment extending through this area and produced significant aftershock sequences. Updated seismic monitoring by regional networks confirms ongoing background seismicity, with swarms occasionally occurring in volcanic and tectonic settings around Kodiak Island. Depths in this swarm indicate activity within the upper crust, consistent with faulting above the deeper megathrust.

Insights from the temporal distribution reveal a rapid onset followed by a peak in rate and magnitude on the second day, suggesting possible pore-pressure changes or aseismic slip facilitating the sequence. All events remained below damaging thresholds, reflecting the swarm's limited energy release compared to mainshock-aftershock sequences common in the subduction zone.

• Alaska Earthquake Information Center historical data
• USGS Earthquake Catalog for regional tectonics
• SeismoSight internal swarm classification records