Fewer Quakes in Alaska, More Landslides in ‘24

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Created on Wednesday, 26 February 2025 12:41

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By YERETH ROSEN

Alaska Beacon

Alaska, one of the world’s most seismically active locations, just completed a relatively quiet year for earthquakes but a busy year for landslides in the Southcentral region, according to an annual report issued by seismic experts at the University of Alaska Fairbanks.

There were 39,836 seismic events in Alaska and nearby regions in 2024, according to the Alaska Earthquake Center’s newly released 2024 Alaska Seismicity Summary. 

That is about 5,700 fewer than in 2023 and about 15,000 fewer than in 2018, when a record was set, according to the center. Still, there were 50 quakes of magnitude 5.0 or greater, equating to about one a week, the center said.

Seismic events are defined as earthquakes or any other earth-shaking phenomena.

The year’s seismic record was notable because, for the first time since the major Nov. 30, 2018, earthquake that damaged the Anchorage and Matanuska-Susitna Borough areas, there were no detected aftershocks from that magnitude-7.1 event, the center said.

There were, however, more than 1,000 aftershocks throughout the year from the magnitude-7.8 Simeonof earthquake that struck on July 21, 2020, off the Alaska Peninsula, the report said. The most powerful of those was measured at magnitude 5.6.

The most powerful earthquakes in 2024 were two magnitude-6.3 events that occurred as part of a swarm of quakes that struck the central Aleutians in early December, the report said. The first of the magnitude-6.3 quakes occurred on Dec. 8 in the Andreanof Islands region, and the second occurred the following day, the report said.

While there were fewer earthquakes detected in 2024 than in past years, a relatively new effort at the center has resulted in stepped-up monitoring of landslides in a region considered to be at elevated risk: Prince William Sound.

That program uses an array of instruments to track the seismic signals of landslides, which are different from those produced by earthquakes. Seismic detections of landslides are followed by visual confirmation, under the new system.

In 2024, the monitoring system identified 31 landslides, including some events that produced localized tsunamis.

Five separate landslides occurred within the span of about an hour on Sept. 20 at Surprise Glacier, which is near Harriman Fjord in Prince William Sound. Together, they produced over 3.5 million cubic meters of debris, or 4.6 million cubic yards, the report said. They also produced a small but measurable tsunami, at 4 centimeters, or a little over 1.5 inches, the report said.

These were the first Prince William Sound landslides with debris that reached the water since dedicated instrument-based landslide monitoring in the region started in 2020, the report said.

A more significant tsunami resulted from an Aug. 7 landslide at Pederson Lagoon southwest of Seward. It dumped about 2 million cubic meters of material and generated a localized tsunami of about 17 meters, or nearly 56 feet, that damaged boardwalks at a local lodge and reached a Kenai National Park campsite, the report said.

Another cluster of landslides detected by the instruments was at Denali National Park and Preserve, the report said. Other slides were detected elsewhere in Southcentral Alaska, mostly in coastal regions,.

The landslide monitoring program is still being developed, said Elisabeth Nadin of the Alaska Earthquake Center. The hope is to have a landslide-related tsunami warning system, she said.

The seismic-detection program is limited to an approximately 200-mile radius, so it does not cover the vast majority of Southeast Alaska. It was not used for the Aug. 25 Ketchikan landslide that killed one person, injured three others and damaged several houses.

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