Earthquake swarm occurs southwest of Adak Island

A magnitude 6.3 earthquake was recorded southwest of Atka Island in the Western Aleutian Islands on Sunday afternoon, December 8, 2024 (Figure 1). The earthquake attracted great interest due to its significant size. Since it was offshore and no damage was reported, there was no cause for concern for us. A handful of people reported feeling a slight tremor during the event, which occurred at a depth of about 18 kilometers. The National Tsunami Warning Center did not issue a warning.

The activity that followed within an hour – some events above magnitude 5 – was expected aftershock activity. But then, starting around 3 p.m., two more magnitude 6 earthquakes occurred within 20 minutes. This is not a typical mainshock-aftershock sequence; It is a swarm – a collection of several earthquakes of approximately the same magnitude.

Although today’s swarm includes several significant earthquakes, the likelihood that they are precursors to something larger is statistically quite low, says Michael West, director of the Alaska Earthquake Center and state seismologist.

“Swarms of moderate earthquakes are common in the Aleutian Islands and do not necessarily indicate anything more significant,” West says. However, he adds, “Because this area has been affected by or has been on the verge of numerous large historic earthquakes, we are keeping a close eye on it.”

Setting of the Atka swarm

The earthquakes effectively mark the tip of the subducting Pacific Plate. Aside from the number of large earthquakes in this swarm, these are fairly typical events that occur at the subduction boundary between the descending Pacific plate and the overlying North American plate.

The larger earthquakes in which seismologists determine the nature of fault movement were all caused by thrust movements in the subduction zone. This is also typical of the convergent movement between the Pacific and North American plates.

While small earthquake swarms occur on average every few years in the subduction zone along the Aleutian Islands, there was a similar earthquake swarm in May 2024, but about 350 miles (550 km) to the east. Uneven strain along a particular portion of the subduction interface could result in a swarm rather than a stress release in a major event. More often there is a single large earthquake followed by many aftershocks.

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Monitoring seismic events helps us understand the complex nature of earthquakes along this major plate boundary in Alaska. Due to the unusually high number of moderate earthquakes in this swarm, we are keeping an eye on the region. We expect aftershocks, particularly of magnitude 5 and below, to occur in the coming days and weeks.