Analysis of the September 2019 Earthquake Swarm near Karluk, Alaska

The earthquake swarm designated VS20190905.1 occurred approximately 86 km northwest of Karluk, Alaska, within a tectonically active region shaped by the subduction of the Pacific Plate beneath the North American Plate. This convergent boundary, part of the Aleutian megathrust system, accommodates convergence rates of roughly 6–7 cm per year and generates frequent seismic and volcanic activity across the Alaska Peninsula and Kodiak Island areas. The swarm initiated at 08:21 on 4 September 2019 and concluded at 11:49 on 27 September 2019, spanning 555 hours and 27 minutes during which 423 events were recorded. Examination of the first 100 events reveals predominantly low-magnitude seismicity, with magnitudes ranging from -1.1 to 2.4. The majority of events clustered between magnitudes -0.8 and 1.0, consistent with microseismic activity often observed in fluid-driven or stress-adjustment processes along the subduction interface. Depths remained shallow, typically between 0 and 7 km, indicating activity within the upper crust rather than deeper portions of the Wadati-Benioff zone. Temporal patterns show initial sparse detections on 4 September, followed by increased frequency on 5–7 September, including several events above magnitude 1.5. Notable clusters occurred around 6 September 00:29, with three events in quick succession (magnitudes 2.4, 1.6, and 1.3), and on 7 September 15:48–15:57, featuring multiple events between magnitudes 0.0 and 1.6. These bursts suggest episodic stress release possibly influenced by local fault interactions or hydrothermal fluid migration common in the volcanic arc setting. Geological context places the swarm in proximity to the Aleutian volcanic chain, where historical seismicity includes both mainshock-aftershock sequences and swarm-like episodes. Regional fault systems, including splay faults branching from the main megathrust, accommodate oblique convergence and can host such distributed microseismicity. The area's volcanic history, marked by andesitic stratovolcanoes and caldera complexes, further contributes to elevated background seismicity through crustal fracturing and magma-related processes. Historical records since 2000 indicate three prior swarms in the immediate vicinity: one in 2002 (single event noted in classification), one in 2016, and the present 2019 sequence. These recurrent swarms highlight episodic rather than continuous seismic release, potentially modulated by long-term tectonic loading along the subduction zone. No significant surface deformation or volcanic unrest was associated with the 2019 activity, aligning with the low-energy character of the recorded events. Overall, the VS20190905.1 swarm exemplifies typical background seismicity in this high-hazard subduction environment. Continued monitoring remains essential given the region's capacity for larger megathrust earthquakes.

References

USGS Earthquake Catalog Alaska Earthquake Center Annual Reports NOAA National Centers for Environmental Information Seismicity Database