Over the past two weeks, there has been increased earthquake activity near Makushin Volcano. The activity started on June 15 with a magnitude 4.2 earthquake, followed by several additional significant earthquakes ranging in magnitude from 3.1 to 4.1 over the next 24 hours.
These larger earthquakes have been accompanied by over 850 smaller ones located by the Alaska Volcano Observatory since mid-June.
John Power, a geophysicist with the AVO, said two more earthquakes on June 28 — a magnitude 3.0 and 3.8 — suggests that there's a number of processes that might be taking place.
"We could be looking at some kind of volcanic activity, possibly some interaction between faults in the geothermal field, or possibly, just some protracted activity caused by faulting in the area. It's a little unclear what we're seeing at this point, but the earthquake activity is continuing at the present time."
The rate of occurrence and size of the earthquakes initially declined for about a week following the magnitude 4.2 earthquake, according to the AVO. However, since June 24, the rate and size of the earthquakes has fluctuated with the most recent increase associated with the two earthquakes this past Sunday.
All of the earthquakes are clustered about six miles east of the summit of the volcano at a depth of about five miles below the surface of the earth. The largest earthquakes have been felt strongly by residents of Unalaska.
The exact cause of this increased earthquake activity, in terms of possible volcanic or tectonic processes, is uncertain at this time, according to the AVO. The earthquakes may be related to a number of scenarios, including the movement of magma beneath the east side of Makushin Volcano, interactions between an earthquake fault and the nearby geothermal field, the activation of more than one fault in the area, or a combination of this activity.
Powers said while there's no immediate concern for Unalaskans, the AVO is maintaining its aviation color code at "yellow" and alert level at "advisory."
"We're watching Makushin at this point all the time and keeping track of things like steaming, gas emission, ground deformation, and of course, ongoing earthquake activity," he said.
Because of the distance and depth of the earthquakes in relation to the summit of Makushin, and the absence of other forms of volcanic unrest such as increased steaming, gas emission, or ground deformation, there is no indication that the present earthquake sequence will necessarily lead to a volcanic eruption. Powers said the AVO will continue to monitor the volcano closely.
"We're watching with satellite, checking our webcam that looks at the summit of the volcano when it's cloud-free. And so far, we see no other indications of any kind of volcanic eruptive activity right now. It's simply an increase in earthquakes, and they are removed about six miles east of the summit of the volcano. And those are all good signs for us," Power said.
Makushin Volcano is located on northern Unalaska Island, about 16 miles west of the community of Unalaska. It is a broad, ice-capped stratovolcano that rises to an elevation of 6,680 feet. The summit caldera — which is slightly less than 2 miles in diameter — is the site of frequent steam and minor ash eruptions, according to the AVO. But no large eruptions have occurred in this century.
Makushin is monitored by a network of seismic stations, web cameras, GPS, satellite images, and regional infrasound and lightning detection data.