If you've paid much attention to earthquakes in Alaska over the last few years, you've probably heard the phrase “earthquake swarm.” In 2014, a swarm near Noatak rattled residents with five magnitude 5.3-5.7 earthquakes spread out over two months. In 2015, a swarm off St. George Island shook the normally quiet Pribilofs. And in 2018, a swarm in the eastern Brooks Range accounted for more than 2,000 of the year's record 55,000 earthquakes in Alaska.

But what is a swarm, and how is it different from other earthquake activity?

Most Alaskans are well acquainted with the classic mainshock-aftershock sequence: a large earthquake strikes, followed by a series of smaller aftershocks that taper off over time. When a smaller earthquake comes before the mainshock, we call it a foreshock-mainshock-aftershock sequence, but it's essentially the same thing. In both cases, you have one larger earthquake and then a series of smaller aftershocks as the earth adjusts to changes in stress caused by the mainshock. These are the most common types of earthquake sequences.