Glaciers such as the Yakutat in southeastern Alaska, shown here, have been melting since the end of the Little Ice Age, affecting earthquakes in the region. Credit: Photo by Sam Herreid
In 1958, a magnitude 7.8 earthquake shook Lithuania Bay in southeastern Alaska, creating a tsunami that ran 1,700 feet on a mountain slope before rushing to the sea.
Researchers now believe the huge loss of glaciers in the region has helped establish the stage for the earthquake.
In a recently published research article, scientists from the University of Alaska Fairbanks Geophysical Institute found that ice loss near Glacier Bay National Park over the past century has affected the timing and location of earthquakes with a magnitude of 5.0 or greater in the area. .
Scientists have known for decades that melting glaciers have caused earthquakes in otherwise tectonically stable regions, such as the interior of Canada and Scandinavia. In Alaska, this pattern is more difficult to detect, as earthquakes are common in the southern part of the state.
Alaska has some of the world’s largest glaciers, which can be thousands of feet thick and cover hundreds of square miles. The weight of the ice causes the land to sink beneath it, and when a glacier melts, the ground bounces back like a sponge.
A tsunami caused by an earthquake ravaged vegetation from the hills and mountains above Lituya Bay in 1958. The treeless areas are visible as lighter ground around the bay in this photo taken shortly after the event. Credit: Photo by Donald Miller, U.S. Geological Survey
“There are two components to the increase,” said Chris Rollins, lead author of the study, who did the research while at the Geophysical Institute. ‘There is what is called the’ elastic effect ‘, that is when the earth springs up again immediately after an ice mass has been removed. Then there is the long-lasting effect of the mantle flowing backwards under the evacuated space. ”
In the study, researchers linked the growing movement of the mantle to major earthquakes over southeast Alaska, where glaciers have been melting for more than 200 years. More than 1,200 cubic miles of ice were lost.
Southern Alaska sits on the border between the continental North American plate and the Pacific plate. They grind about two inches a year past each other – about twice the rate of the San Andreas Fault in California – which regularly results in earthquakes.
However, the disappearance of glaciers has also caused Southeast Alaska’s land to rise about 1.5 inches per year.
Rollins has been performing models of earthquake and ice loss since 1770 and has found a subtle but unmistakable correlation between earthquakes and earth-rebound.
When they combined their maps of ice loss and shear stress with seismic records up to 1920, they found that most large earthquakes were correlated with the stress due to the long-term recovery of the earth.
Unexpectedly, the greatest tension of ice loss occurred near the exact epicenter of the 1958 earthquake that caused the tsunami in Lithuania Bay.
Although the melting of glaciers is not the direct cause of earthquakes, it probably modulates the timing and severity of seismic events.
When the earth bounces back after a glacier’s retreat, it holds like bread rising in an oven and spreading in all directions. This effectively eliminates slip errors, such as the Fairweather in Southeast Alaska, and makes it easier for the two sides to slip past each other.
In the case of the 1958 earthquake, the post-glacial rebound teased the crust around the fault, in a way that also increased tension near the epicenter. Both this and the unraveling effect brought the error closer to failure.
“The movement of plates is the main driver of seismicity, upliftment and deformation in the area,” Rollins said. ‘But a postglacial rebound contributes to that, like the cherry on the cake. This increases the chance that faults in the red area reach their voltage limit and slip into an earthquake. ‘
Reference: “Stress Promotion of the 1958 Mw∼7.8 Fairweather Fault Earthquake and Others in Southeast Alaska through 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