Research geologist says recent uptick in Aleutian earthquakes no cause for concern, but useful reminder to prepare
People living in Aleutian communities are no strangers to the occasional earthquake. But in recent years, the region has seen some ramped up seismic activity, including a magnitude-6.2 earthquake that hit just about 40 miles south of Unalaska earlier this month.
KUCB’s Maggie Nelson sat down with Rob Witter, a research geologist with the U.S. Geological Survey, to hear more about the island’s recent earthquake and how that fits into the larger picture of seismic activity in the Aleutians.
ROB WITTER: That area is known to be very active, seismically. And it's on the boundary between two tectonic plates, the Pacific plate out in the ocean, beneath the ocean and the North America Plate, which is where Unalaska sits.
MAGGIE NELSON: So basically, this is just tectonic plates, the Earth's crust is shifting around. And that's what's caused this recent earthquake?
WITTER: Yeah, you can kind of liken the earth to an egg, the relative thickness of the shell of the egg is about the same relative thickness of the outer crust of the earth. And an egg has some integrity in its shell, but the Earth's crust is broken up into many different pieces. And those pieces are moving with respect to one another because the heat generated within the earth is causing the upwelling and moving plates. And the Pacific Plate is moving quite rapidly at several inches per year and being shoved underneath the Aleutian Islands, shoved underneath Unalaska Island in fact. And so the remnants of the Pacific Plate are beneath your feet. And when those two plates get stuck, and the stresses build up, eventually it breaks along the boundary between subducting, we call it subducting be pushed down below North America, the subducting plate slips and breaks. And that's what causes an earthquake. And the shaking is the vibrations produced by that breaking.
NELSON: So, in June, around June of 2020, we saw some pretty notable earthquake activity near Makushin. And after this recent earthquake, I've heard people, you know, concerned or asking questions about whether or not this earthquake was related to Makushin Volcano. It did hit very close. But my understanding is that's not the case. The two aren't related. Tell me about what that relationship is and why those two are not necessarily related.
WITTER: Back in 2012, you guys got hit by a couple of earthquakes. You had a 4.2 and a 4.1. They were just three hours and 15 minutes apart, or so. And they were located at a much shallower depth at about eight kilometers. And they're very close to the Makushin summit. They were only 12 kilometers southeast of the volcano. So these were really close and felt very strongly in Unalaska. And it's no surprise that it was a shocker for the folks who live there. This earthquake was much farther away offshore, and it's near the boundary between two plates that are interacting along Alaska's plate boundary. They're not related because they're so far apart. And it's much easier to explain the earthquake as the release of stresses caused by the friction. And this colliding between the Pacific Plate and North America. There are little earthquakes happening all the time in your area. And most of these earthquakes or earthquakes you can’t even feel. These earthquakes occur at a variety of depths. And they occur because you have these two plates interacting, but you also have volcanoes nearby that are producing earthquakes. We live in a shaky world up here in Alaska. And that's because we're on a very active plate boundary where the Pacific Plate is colliding with North America.
NELSON: So we are in a pretty active area all the time. But recently, the Aleutians have seen some ramped up activity. Could you kind of just tell me a little bit about that recent uptick in activity here in the region?
WITTER: You have seen some ramped up activity, in particular over near the Shumagin Islands. There were two large earthquakes in the summer of 2020. And then just this past, late summer, there was a large earthquake near Chignik. That was a significant earthquake because it was a magnitude-8.2, largest earthquake in Alaska since 1965, I think. And these are significant earthquakes because of their size. There was the Simeonof earthquake in 2020, which was a magnitude 7.8, I believe, and then followed by this Chignik earthquake, it appears that there was some interaction between those two earthquakes and one may have triggered the other. And it's in a region that's been kind of quiet for a long, long period of time. The Shumagin Island region is a part of the plate boundary that didn't break in the 20th century, whereas almost the entire plate boundary in Alaska, this “subduction zone,” as geologists refer to it, almost the entire length of subduction zone broke in a series of large, very large earthquakes in the 20th century. Starting in 1938, there was a large earthquake near Kodiak Island. And then in 1946, there was a large earthquake near Sanak, followed by the 1957 earthquake in your region, which was magnitude-8.6, produced a really large tsunami that hit Hawaii. And then there was the great Alaska earthquake in 1964, which is the second largest instrumentally recorded earthquake, and the largest earthquake to hit Alaska. And then in 1965, there was an earthquake magnitude-8.6 way out west on the plate boundary. And then since that time has been relatively quiet along this plate boundary until the summer 2020. So yeah, it is seeming like there's kind of been ramped up seismicity lately. But this is a long plate boundary that's very active. We know it's active.
NELSON: So what does that mean for locals for people living in the region? It sounds like this is not necessarily reason to be concerned. But what can we take away from this information?
WITTER: Yeah, there's nothing to be alarmed about. There's also no reason not to use it as an opportunity to be prepared. And to make sure that you check your go-bag. You should have a backpack or a bag with some supplies, so that if there is a larger earthquake that you can get to a safe place soon, especially if you live down near the shoreline and you need to evacuate in case of a tsunami. One of the good things about feeling earthquakes that don't cause damage is that it keeps us aware that we live in earthquake country, and we always need to be prepared.