Alaska's Aleutian Islands are shaking

[bjdeming]

No comment because even the experts are uncertain. Tweeted two hours ago:

 

[bjdeming]

Comment after looking through the USGS list: This layperson thinks the focal mechanism graphic looks like thrust faulting (a volcanism cause would look different). If I lived along the Oregon coast instead of 70 miles inland and behind a mountain range, I'd have my bugout bag prepared and be hyperaware of emergency alerts, just in case these are foreshocks.

Trouble is, it's not like a tornadic squall line coming through -- Earth keeps itself hidden and moves in its own good time. You can't simply stay up and wait for it.

 

[bjdeming]

Brief summary posted just now. Excerpt:

Although today’s swarm includes larger earthquakes than typical, there is still no cause for Alaskans to worry, says Alaska Earthquake Center director and state seismologist Michael West.

“Swarms of moderately large earthquakes are common in the Aleutians and do not necessarily portend anything more substantial,” says West. However, he adds, “Because this area has been in, or on the edge, of numerous large historical earthquakes, we are keeping close watch on it.”

 

[bjdeming]

The boffins reference that Adak swarm in their post today on a new swarm:

As of today, March 21, 2025, there have been eight earthquakes greater than magnitude 5 in two days in the Andreanof Islands region of the western Aleutian chain (Figure 1). With no clear singular main shock, we can confidently call this an earthquake swarm. There have been no felt reports submitted online, but Adak residents have said they feel the shaking.

The Alaska Earthquake Center has covered swarms before, most recently in December 2024 when three earthquakes above a magnitude 6 and their aftershocks shook the Adak area (Figure 2). This swarm follows the same general behavior: a series of earthquakes in the same place with no clear single triggering event.

Since the largest event in a swarm sequence does not guide how many events there will be overall, there is nothing special about the largest event (Figure 3). In this case, so far it has been a magnitude 6.2, at about 7 a.m. on March 21st, at a depth of about 18 miles (29 km).

So why do earthquake swarms happen, and why in this part of Alaska?

Depending on the geologic setting of an earthquake swarm, the trigger could be slip on a fault, or injection of magma or some other type of fluid. In this instance, given the large magnitude of the earthquakes and their location on a major plate boundary, we can call it a tectonic swarm...