Location:

Period:

5 May 2011 15:41:11 - 8 May 2011 23:39:50 (3 days 7 hours 58 minutes)

Volcanoes in 100km radius:

Dana(97km)

Earthquakes:

6 swarms found nearby.

2007

PS20071002.1(93.4km)

2 Oct

2 hours

5 earthquakes

2020

PS20200722.1(93.0km)

22 Jul

15 hours

6 earthquakes

S20200722.1(11.9km)

22 Jul

13 days 17 hours

332 earthquakes

S20200722.2(21.9km)

22 Jul

5 days 1 hours

126 earthquakes

PS20201019.2(81.3km)

19 Oct

1 day 4 hours

12 earthquakes

2025

S20250716.2(29.4km)

16 Jul

1 day 7 hours

103 earthquakes

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Seismic Swarm S20110505.2 Near Sand Point, Alaska

A seismic swarm designated S20110505.2 occurred approximately 51 km south-southeast of Sand Point, Alaska, from 15:41 UTC on 5 May 2011 until 23:39 UTC on 8 May 2011. Over this 79-hour and 58-minute period, 50 earthquakes were recorded, with magnitudes ranging from 1.3 to 4.4 and focal depths between 13 km and 58 km. The sequence began with a magnitude 3.5 event at 21 km depth and included several events above magnitude 3.0, culminating in the largest shock of magnitude 4.4 at 58 km depth on 6 May.

The swarm exhibited typical characteristics of clustered seismic activity without a dominant mainshock-aftershock pattern. Early activity on 5 May featured events clustered around 20–25 km depth, with magnitudes mostly between 1.4 and 3.8. Activity continued into 6 May with a notable magnitude 4.4 event at greater depth, followed by additional moderate events through 7 and 8 May that gradually decreased in both frequency and magnitude. Depths remained predominantly in the upper to mid-crust, consistent with tectonic processes in the overriding plate and along the subduction interface.

The Sand Point region lies within the Shumagin Islands segment of the 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 tectonic setting produces frequent seismicity, including both interface thrust earthquakes and intraslab events. The Shumagin gap, located nearby, has been identified as a seismic gap with potential for large-magnitude ruptures, although historical great earthquakes have occurred both east and west of the area. The 1964 Great Alaska Earthquake and the 1938 and 1946 events underscore the region’s capacity for significant seismic release.

Since 1 January 2000, only one prior swarm has been documented in this immediate vicinity, occurring in 2007. The 2011 swarm therefore represents a relatively infrequent clustered sequence in the local catalog. Such swarms may reflect fluid migration, stress transfer along the plate interface, or minor adjustments within the overriding crust, although precise causative mechanisms require further geophysical investigation.

This event highlights the persistent seismic hazard along the Aleutian arc and the value of continuous monitoring for understanding subduction-zone dynamics. Ongoing instrumentation by regional networks continues to improve detection and characterization of similar sequences.

References:
USGS Earthquake Catalog
Alaska Earthquake Center reports
SeismoSight internal swarm classification S20110505.2