Seismic Swarm S20230716.3 Near Sand Point, Alaska: Geological Context and Event Analysis

Seismic swarm S20230716.3 occurred approximately 83 km south-southwest of Sand Point, Alaska, in the Shumagin Islands region of the Aleutian arc. The sequence began at 07:00 on 16 July 2023 and concluded at 10:40 on 22 July 2023, spanning 147 hours and 40 minutes. During this period, 65 earthquakes were recorded, with magnitudes ranging from 1.3 to 4.7 and focal depths between 3 km and 46 km. The largest event reached magnitude 4.7 at a depth of 42 km on 16 July.

The swarm exhibited typical characteristics of clustered seismic activity, with multiple events occurring in rapid succession early in the sequence. Notable activity included several magnitude 3+ earthquakes on the first day, followed by a gradual decline in frequency and intensity over subsequent days. Depths were predominantly concentrated around 35 km, consistent with intermediate-depth seismicity in subduction environments, though shallower events as shallow as 3 km also occurred.

Sand Point lies within the tectonically active 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. This convergent margin produces frequent earthquakes, volcanic activity, and associated seismic swarms. The Shumagin Islands segment has a well-documented history of both shallow crustal events and deeper intraslab earthquakes driven by plate bending and dehydration processes.

Historically, the broader Alaska-Aleutian region has experienced major earthquakes, including the 1964 magnitude 9.2 Great Alaska earthquake and numerous events exceeding magnitude 7 in the 20th and 21st centuries. Seismic swarms are recurrent in this setting due to fluid migration, stress transfer, and episodic slip along the megathrust and subsidiary faults. Since 2000, seven prior swarms have been identified in the area, occurring in 2007, 2011, and five instances in 2020, underscoring the episodic nature of clustered seismicity here.

Analysis of swarm S20230716.3 reveals a temporal pattern of elevated activity on 16–17 July, with diminishing rates thereafter. Events clustered at mid-crustal to upper-mantle depths suggest involvement of the subducting slab interface. Such swarms often precede or accompany larger tectonic adjustments but do not necessarily indicate an imminent major earthquake.

This sequence provides valuable data for monitoring subduction zone dynamics. Continued observation through regional seismic networks remains essential for understanding stress evolution in one of Earth’s most active plate boundaries.

References USGS Earthquake Hazards Program – Aleutian Arc Tectonic Setting Alaska Earthquake Center – Regional Seismicity Reports Incorporated Research Institutions for Seismology (IRIS) – Subduction Zone Studies