Melting glaciers contribute to Alaska earthquakes

IMAGE: Glaciers like Yakutat in southeast Alaska, shown here, have been melting since the end of the Little Ice Age, affecting earthquakes in the area. Show more

Image source: Sam Herreid

In 1958, a 7.8-magnitude earthquake triggered a rockslide in Lituya Bay in southeast Alaska, causing a tsunami of 1,700 feet above the mountainside before it set off into the sea.

Researchers now believe that the area’s widespread loss of glacial ice helped pave the way for the earthquake.

In a recently published research article, scientists from the University of Alaska’s Fairbanks Institute of Geophysics found that the loss of ice near Glacier Bay National Park affected the timing and location of earthquakes of magnitude 5.0 or more in the region over the past century.

Scientists have known for decades that melting glaciers have caused earthquakes in other tectonically stable regions, such as inland regions of Canada and Scandinavia. In Alaska, this pattern has been 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 earth to sink beneath it, and when a glacier melts, the earth returns like a sponge.

“There are two components to the uplift,” said Chris Rollins, lead author of the study who conducted the research while at the Geophysical Institute. “There is what is called the” elastic effect, “which is when the Earth immediately returns to an altitude after the removal of an ice mass. Then there is the prolonged effect of the mantle that flows back upward under the empty space.”

In this study, researchers linked the expanding movement of the cover to large earthquakes across southeastern Alaska, where glaciers have been melting for more than 200 years. More than 1,200 cubic miles of ice have been lost.

Southern Alaska straddles the boundary between the North American continental plate and the Pacific Plate. They grind each other about two inches a year – nearly twice the rate of the San Andreas Fault in California – resulting in frequent earthquakes.

However, the disappearance of glaciers has also caused the lands of southeast Alaska to rise by about 1.5 inches per year.

Rollins has modeled Earth’s motion and ice loss since 1770, and found a subtle but unambiguous link between earthquakes and Earth’s recovery.

When they combined maps of ice loss and shear stress with seismic records dating back to the 1920’s, they found that most of the large earthquakes were related to long-term stress from Earth’s recoil.

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

While melting glaciers is not the direct cause of earthquakes, it has the potential to modulate the timing and intensity of seismic events.

When the ground bounces after a glacier retreat, it does a lot like bread rising in the oven, spreading out in all directions. This effectively eliminates slip faults, such as Fairweather in southeast Alaska, and makes it easier for both sides to slide one another.

In the case of the 1958 earthquake, the rebound following the Ice Age caused torque in the crust around the fault in such a way that the pressure increased near the epicenter as well. Both this and the effect of uninstalling brought the error akin to failure.

“Plate movement is a major driver of earthquakes, uplift and deformation in the region,” Rollins said. “But the rebound after an ice age adds to it, kind of like defrosting the cake. It makes it more likely that faults in the red zone are stressful and slid in an earthquake.”

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