Unusual seismic signal detected in 2025 central Alaska earthquake

Unusual seismic signal detected in 2025 central Alaska earthquake

An unusual seismic signal recorded less than a minute before a widely felt magnitude 4.2 earthquake in central Alaska may have given scientists a rare look at how earthquakes begin.

Reliable observations of activity seconds to minutes before an earthquake occurs are rare. That's what makes the Aug. 20, 2025, event special.

Analysis led by graduate student researcher Amanda McPherson at the University of Alaska Fairbanks Geophysical Institute revealed a very-low-frequency earthquake of magnitude 3.8 that occurred 24 seconds before the main earthquake, which struck 40 miles west of Fairbanks and 12 miles north of Nenana.

The very-low-frequency earthquake's epicenter was near that of the main earthquake, occurred at a similar depth and had the same type of ground movement.

"This is a special event in a special area," she said.

The research was published June 12 in Geophysical Research Letters. McPherson is the lead author. Co-authors include UAF professor Carl Tape and professor Yoshihiro Kaneko of Kyoto University in Japan.

Most earthquakes are not preceded by a clear signal on seismograms, though some are preceded by smaller high-frequency quakes known as foreshocks that are only identified as such after a larger quake occurs.

McPherson found that the Aug. 20 event consisted of three components: a high-frequency foreshock, a very-low-frequency earthquake, and the final rupture.

Earthquakes release elastic strain energy that has accumulated as rocks slowly bend under tectonic forces until they suddenly break or slip along a fault.

The process that initiates an earthquake is poorly understood, however. Seismologists refer to the brief moment before an earthquake's main shock as its nucleation, which typically is not detectable by seismometers at Earth's surface.

Very-low-frequency earthquakes release energy slowly, over tens of seconds rather than a few seconds. They are typically too weak or too gradual for people to feel.

The U.S. Geological Survey calls these earthquakes the "most enigmatic," because they are difficult to reliably detect and are poorly understood.

These slow signals are useful because they can give scientists a clearer picture of how a fault moved.

High-frequency signals, by contrast, consist of many short bursts of energy that often do not show a clear enough pattern to reveal the fault motion.

"This type of earthquake doesn't usually happen away from plate boundaries,"

McPherson said. "We have linked the one in the Minto Flats fault zone as a potential nucleation signal of a larger earthquake."

Very-low-frequency earthquakes are found mainly in subduction zones, places where one tectonic plate dives beneath another, such as along the Aleutian Trench. Scientists believe subduction zones provide the unique conditions for these slow-release earthquakes.

Here's the Aug. 20 earthquake's rapid sequence of events:

Bursts of a high-frequency magnitude 3 foreshock began 42.5 seconds before the main quake. The very-low-frequency earthquake began as the foreshock neared 20 seconds and continued for 24.3 seconds while the foreshock also continued.

At that point, 44 seconds after the foreshock began, the mainshock rupture occurred at the same location as the very-low-frequency earthquake.

The shared location, fault movement and fault type of the very-low-frequency earthquake and the mainshock strongly suggest a causal link, the authors write.

"These VLFEs tend to be noticed in subduction zones, where very big earthquakes occur," McPherson said. "Not only have we found it outside of a subduction zone, in the Minto Flats fault zone, but we found it occurring prior to a much smaller earthquake."

"That suggests that high-frequency foreshock and very-low-frequency earthquake signals may actually be earthquake nucleation signals," she said.

The Minto Flats fault zone is a major area of interest for seismologists, who have long viewed it as part of a broader central Alaska pattern known as "bookshelf faulting." In that pattern, smaller faults oriented northeast to southwest run between two much larger fault systems: the Denali fault system to the south and the Kaltag-Tintina system to the north.

Tape said the Aug. 20 event gives researchers an unusually clear view of the seconds before an earthquake.

"We had this window into what's happening right before an earthquake," he said. "Whatever we can learn from it may give us an idea of what to look for in other settings."

"You can squeeze two rocks together in a lab and then try to understand when a rupture will start," he said, "but it's a difficult experiment and not easy to relate to the real-world setting."

ADDITIONAL CONTACTS: Amanda McPherson, [email protected]; Carl Tape, [email protected]

006-27