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Glaciers such as 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 landslide in Lituya Bay in Southeast Alaska, creating a tsunami that traveled 1,700 feet up the mountainside before sinking flee to 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 article, scientists at the Fairbanks Geophysical Institute at the University of Alaska found that the loss of ice near Glacier Bay National Park influenced the timing and location of earthquakes in ‘magnitude 5.0 or greater in the region over the past century.
Scientists have known for decades that melting glaciers caused earthquakes in otherwise tectonically stable regions, such as the interior of Canada and Scandinavia. In Alaska, this pattern was 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 reach thousands of feet thick and cover hundreds of square kilometers. The weight of the ice causes the land to sink underneath, and when a glacier melts, the ground bounces like a sponge.
An earthquake-triggered tsunami stripped vegetation from the hills and mountains above Lituya Bay in 1958. The treeless areas are visible as lighter ground surrounding the bay in this photograph taken shortly after. the event. Credit: Photo by Donald Miller, US Geological Survey
“Uprising has two parts,” said Chris Rollins, lead author of the study who conducted the research at the Institute of Geophysics. “There is what is called the ‘elastic effect’, which is when the earth immediately rises after a mass of ice has been removed. Then there is the prolonged effect of the mantle rising upwards under the empty space.
In the study, the researchers associate the expanding mantle motion with large earthquakes in southeast Alaska, where glaciers have been melting for more than 200 years. Over 1,200 cubic miles of ice have been lost.
Southern Alaska sits on the border between the North American Continental Plate and the Pacific Plate. They intersect at about two inches per year – about twice the rate of the San Andreas fault in California – resulting in frequent earthquakes.
The disappearance of glaciers, however, also caused Southeast Alaska’s land elevation of about 1.5 inches per year.
Rollins has performed models of earth movement and ice loss since 1770, finding a subtle but unmistakable correlation between earthquakes and the rebound of the earth.
When they combined their ice loss and shear stress maps with seismic records dating back to 1920, they found that most large earthquakes correlated with long-term earth rebound stress.
Unexpectedly, the greatest amount of stress from the loss of ice occurred near the exact epicenter of the 1958 earthquake that caused the Lituya Bay tsunami.
Although melting glaciers are not the direct cause of earthquakes, it likely modulates both the timing and severity of seismic events.
When the earth bounces after the retreat of a glacier, it does so like bread rising in an oven, spilling out in all directions. This effectively unblocks non-slip faults, such as the Fairweather in Southeast Alaska, and makes it easier to slide on both sides.
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. This effect and the loosening effect brought the defect closer to the failure.
“Plate movement is the primary driver of seismicity, uplift and deformation in the region,” Rollins said. “But the postglacial rebound adds to that, kind of like defrosting on the cake. This makes it more likely that the faults that are in the red zone will reach their stress limit and slip during an earthquake. “
Reference: “Stress Promotion of the 1958 Mw∼7.8 Fairweather Fault Earthquake 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, JGR solid earth.
DOI: 10.1029 / 2020JB020411
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