A M4.9 Jolt in Interior Alaska

Tonight’s M4.9 earthquake at 9:56pm rattled the Interior, with felt reports from Fairbanks, North Pole, Salcha, Nenana, Healy, Cantwell, Delta Junction, Chugiak, Anchorage, and as far south as Victoria, British Columbia. The epicenter was 8 miles east of Salcha, with a depth of 2.4 miles. In Fairbanks, due to the close proximity, the motion felt like a series of short jolts rather than the rolling motion a more distant earthquake might generate. There were reports of items falling off walls in the North Pole area. Looking at the distances and the depth, and considering the likely rupture size, this earthquake was directly under Harding Lake. The seismic station at Harding Lake recorded a ground acceleration of 10% g, the highest acceleration recorded for this event. At that intensity, there is the possibility of minor structural damage near the earthquake origin. 

This recent event was only about 3 miles from the July 22, 2021 M4.7 earthquake near Salcha. They are both in the region known as the Salcha Seismic Zone, a well-known area of seismicity with no notable fault traces on the surface. This zone has produced many earthquakes, including a M7 on July 22, 1937. These two 2021 earthquakes are undoubtedly related in a seismic sequence, however there is no reason to think that these earthquakes portend anything larger in the days or months to come (though statistically, there is always a small possibility of this).

Earthquakes like tonight’s M4.9 are not unexpected and serve as reminders that larger-magnitude earthquakes can occur in the Interior. In fact, ShAKe, a virtual exhibit at the University of Alaska Museum of the North, is dedicated to a century of Interior Alaska earthquakes. The exhibit contains newspaper clippings, photos, interviews and more, including those from the 1937 M7 Salcha earthquake.

map with dots representing earthquakes in central Alaska.

Map of interior seismicity with the location of the July 22, 1937, July 22, 2021, and September 13, 2021 earthquakes noted. (Click to enlarge)