Melting glaciers contribute to Alaskan earthquakes and cause the land to rise 1.5 inches per year

Yakutat glacier

Glaciers like the Yakutat in Southeast Alaska, shown here, have been melting since the end of the Little Ice Age, influencing earthquakes in the region. Credit: Photo by Sam Herreid

In 1958, a 7.8 magnitude earthquake triggered a rockslide in Lituya Bay in southeastern Alaska, creating a tsunami that raced 1,700 feet down a mountainside before rushing into the sea.

Researchers now believe that the widespread loss of glacial ice in the region helped set the stage for the earthquake.

In a recently published research paper, scientists at the University of Alaska Fairbanks Geophysical Institute found that ice loss near Glacier Bay National Park has influenced the timing and location of earthquakes with a magnitude of 5.0 or more in the area during the last century.

Scientists have known for decades that melting glaciers have caused earthquakes in tectonically stable regions, such as inland Canada and Scandinavia. In Alaska, this pattern has been more difficult to detect, as earthquakes are common in the southern part of the state.

Alaska has some of the largest glaciers in the world, which can be thousands of feet thick and cover hundreds of square miles. The weight of the ice causes the land below it to sink, and when a glacier melts, the ground sprouts again like a sponge.

Bay of Lituya 1958

An earthquake-triggered tsunami stripped vegetation from the hills and mountains above the Bay of Lituya in 1958. The treeless areas are visible as lighter terrain surrounding the bay in this photograph taken shortly after the event. Credit: Photo by Donald Miller, US Geological Survey.

“There are two components to improvement,” said Chris Rollins, lead author of the study who conducted the research while at the Geophysical Institute. “There is what is called the ‘spring effect’, which is when the earth sprouts instantly again after an ice mass is removed. Then there is the lingering effect of the mantle flowing up under the vacant space. “

In the study, the researchers link the expanding movement of the mantle to major earthquakes in southeast Alaska, where glaciers have been melting for more than 200 years. More than 1,200 cubic miles of ice have been lost.

Southern Alaska is on the boundary between the North American continental plate and the Pacific plate. They grind side by side at about two inches per year, about twice the speed of the San Andreas fault in California, resulting in frequent earthquakes.

However, the disappearance of the glaciers has also caused the land of Southeast Alaska to rise about 1.5 inches per year.

Rollins ran models of earth movement and ice loss since 1770, finding a subtle but unmistakable correlation between earthquakes and ground rebound.

When they combined their ice loss and shear maps with 1920 seismic records, they found that most large earthquakes were correlated with long-term ground rebound stress.

Unexpectedly, the greatest amount of stress from ice loss occurred near the exact epicenter of the 1958 earthquake that caused the Lituya Bay tsunami.

While melting glaciers is not the direct cause of earthquakes, it likely modulates both the timing and severity of seismic events.

When the land rebounds from the retreat of a glacier, it does so like bread that rises in an oven and spreads out in all directions. This effectively frees up slip faults, like the Fairweather in Southeast Alaska, and makes it easier for the two sides to intersect each other.

In the case of the 1958 earthquake, the postglacial rebound tightened the crust around the fault in a way that also increased stress near the epicenter. Both this and the unlock effect brought glitch closer to glitch.

“The movement of the plates is the main driver of seismicity, uplift and deformation in the area,” Rollins said. “But the postglacial rebound adds to that, kind of like a melting cake. It makes faults in the red zone more likely to reach their stress limit and slip in an earthquake. “

Reference: “Stress Promotion from the 1958 Mw∼7.8 Fairweather Fault and Others in Southeast Alaska by Glacial Isostatic Adjustment and Inter-earthquake Stress Transfer” by Chris Rollins, Jeffrey T. Freymueller, and Jeanne M. Sauber, December 11 2020, Solid Ground JGR.
DOI: 10.1029 / 2020JB020411

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