The new detection method aims to warn of landslide tsunamis

A cascade of rocks—180 million tons of them—flowed into Alaska's Tan Fjord near the head of a retreating glacier on a mid-October night in 2015 (Figure 1). A landslide caused by the collapse of a ridge that was backed by the glacier generated a dirty, rocky tsunami that rolled hundreds of feet over the fjord's shoreline in places.

The landslide generated seismic waves equivalent to those as strong as a 4.9 magnitude earthquake, but the event was not identified as a landslide for several hours. Evidence of a tsunami came later through visual observation.

There is currently no system to quickly detect large landslides and determine whether they are close to open water and pose a tsunami risk.

Two researchers from the Alaska Earthquake Center have devised a way to do just that.

“A warming climate is causing glaciers to retreat, leaving behind valleys whose mountainsides and slopes have lost their support,” said seismologist Ezge Karasuzun. “This is important, especially in areas like south coastal Alaska, because huge masses of land can spill into the water and cause tsunamis.”

Karasozin and Michael West, director of the Alaska Earthquake Center at the Geophysical Institute, detailed their method in a paper published February 9 in the journal The Seismic Record.

The two scientists write that their method of determining a landslide's location, size, and potential impact is fast enough to support NOAA's goal of issuing a tsunami warning within 5 minutes of a landslide.

Their paper draws attention to the risk of tsunamis from landslides by pointing to the landslide in Tan Gorge, which triggered a tsunami that stripped vegetation up to 620 feet above the waterline.

A prototype of a system from Karasözen and West capable of real-time detection has been in place since August in the Barry Arm area of ​​Prince William Sound. It uses data from seismic stations already in Alaska's network.

State and federal agencies fear a landslide and tsunami could occur at Barry Arm, where the Barry Glacier has retreated and left behind an unreinforced wall of the fjord that has dropped about 650 feet in recent decades. Retrospective analysis of Barry Arm seismic station data revealed three landslides that occurred in 2020 and 2021 (Figure 2).

The instability “raised concerns that a catastrophic failure could generate a tsunami several meters high into neighboring communities within just 20 minutes,” Karasuzin and West wrote.

The USGS leads multifaceted, interagency monitoring of the area.

“In the event of an earthquake, there are instruments that measure the height of ocean waves, and tsunami warning centers are on alert after an earthquake,” Karasozin said. “But landslides are not systematically monitored in Alaska or anywhere else in the world. If there was a tsunami triggered by landslides, we wouldn't know about it. That's a big concern.”

Karasozin and West's method involves rapidly identifying long-duration landslide waves amid a seismic record crowded with short-duration waves generated not only by landslides, but also by earthquakes, nearby glaciers, and human-caused activity.

The initial onset of a landslide is generally recorded as short-term waves; Identifiable long-period waves will only materialize after the slide is accelerated (Figure 3). Landslides produce disproportionately long-lasting energy compared to other sources. Most earthquake ruptures last only seconds, while landslides usually last a minute or more.

Coastal fjords present a major challenge for landslide detection because glaciers can create hundreds of prominent seismic events daily, the researchers wrote.

Karasuzin and West created an algorithm to continuously scan seismic data from multiple seismic stations to search for a crushing wave signature. When a match is found, their system will estimate the location and size of the chip. In areas with good seismic station coverage, the location can be estimated to within a few miles.

The goal is to have the system alert tsunami and seismological agency staff, but more work remains before that happens.

To create the algorithm, the two researchers analyzed data from the last three landslides on Barry Glacier and six additional landslides—five in southeastern Alaska and one on the western side of the lower Cook Inlet, on the other side of the Kenai Peninsula (Figure 4).

Other efforts have been tried in recent decades. Many researchers have shown that landslide seismograms can be used to estimate location and magnitude, but such efforts are usually unique to an area, require a great deal of fine-tuning, and are not designed for real-time purposes.

Locating a landslide from distant seismic stations does not allow for real-time assessment, due to the time it would take for seismic waves to reach those stations.

West said the research reinforces ongoing monitoring and alerting efforts.

“The ability to monitor large landslides in real time is an important component of the ongoing interagency effort to address Alaska's landslide challenge,” he said.

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