Hidden seismic signals hint at a tsunami threat in Alaska

By analyzing these seismic recordings, researchers identified a previously unrecognized type of signal. These events are marked by sharp, high-frequency pulses that become more common from late summer through mid-winter, then suddenly stop in late winter or early spring.

Strange Signals Linked to Freezing and Thawing Rock

Writing in Seismological Research Letters, Gabrielle Davy of the University of Alaska Fairbanks and her colleagues propose that the signals are caused by water freezing and thawing inside tiny cracks in the rock beneath the nearby Cascade Glacier. The research team is the first to carry out a systematic analysis of these short, impulsive seismic events near the Barry Landslide.

The scientists emphasize that these signals are not signs that the landslide itself is moving. However, they may still provide valuable insight into changes in underground water conditions behind the slope. Those changes could eventually play a role in triggering slope movement.

Why the Barry Landslide Poses a Serious Risk

Searching for seismic warning signs at Barry Arm is especially important because the landslide sits in a highly unstable setting. The slope is steep and underlain by weak, heavily fractured bedrock, making it prone to failure. It has also lost critical support from Barry Glacier, which has rapidly melted and retreated over the last century.

"What makes Barry Landslide especially concerning is the size of the landslide," Davy explained. "It's a large, slowly moving mass -- on the order of about 500 million cubic meters -- that has been creeping for decades."

"If a rapid collapse were to occur, the material would fall directly into the fjord, and that could generate a tsunami with potentially high wave heights," she added. "Barry Arm is visited by kayakers and cruise ships, and nearby communities such as Whittier could be affected, so understanding the hazard is important from both a scientific and a public-safety perspective."

Sorting Through a Year of Seismic Data

Because of these risks, the landslide area has been extensively instrumented since 2020. The study by Davy and her colleagues is among the first to carefully examine the large volume of seismic data collected by those instruments.

For this research, the team manually reviewed an entire year of continuous seismic waveform recordings. They searched for signals that might help determine when and where a landslide could occur.

This hands-on approach allowed the researchers to recognize the wide range of signals present in the data. These included vibrations from small earthquakes, glacier motion, slope deformation, and other sources of seismic background noise.

"We needed to build a clear baseline understanding of the types of signals that routinely occur in the area, so that any unusual or previously unrecognized signals would stand out. By spending time with the raw data, you train your eye to recognize what 'normal' looks like" before developing classification tools and detection algorithms, Davy explained.

Seasonal Patterns Reveal a Freeze-Thaw Process

After learning how to identify the unusual short-impulsive events in the seismic records, the researchers compared them with weather and rainfall data. They also used ground-based radar to track subtle changes in slope movement. This combination allowed them to study when and where the signals occurred.

The timing, location, and characteristics of the signals pointed to small, brittle events that happen seasonally as water freezes and thaws inside cracks in the rock.

"Similar seismic signals have been documented in other settings, although they are not widely reported," Davy said. She pointed to a recent study from Norway that observed comparable signals near an unstable rock slope and "suggested that their signals may be linked to freeze-thaw processes acting on cracks within the bedrock."

Toward Better Landslide Early Warning Systems

Co-author Ezgi Karasözen said the Alaska Earthquake Center is now testing a regional landslide detection system at the Barry Landslide site. According to Karasözen, the system "will alert us to any slope failures in this area."

"As research on landslide seismology grows, there's increasing recognition that precursor seismic activity -- when it does occur -- can be an important source of early warning," Karasözen said. "That motivates broader investigations not only at Barry Arm, but also at other sites in southern Alaska where similar hazards exist."