On February 28, 1979, a 7.7 magnitude earthquake rumbled the Chugach and St. Elias mountains in Southcentral Alaska. The event — stronger than the 7.1 magnitude earthquake that struck Cook Inlet last year — is believed by geologists to be the result of complex tectonic movements in the area, where the vast Pacific and North American plates meet and accumulate pressure.
Now, scientists are studying another element that may also influence the region's seismic activity: glacial melting.
Researchers Jeanne Sauber with NASA's Goddard Space Flight Center and Bruce Molnia from the U.S. Geological Survey sought to find out if glacial fluctuations had any relation to earthquakes in the area around the Malaspina and Bering glaciers, south of the Wrangell-St. Elias National Park and north of Yakutat. While their 2004 study concluded that moving tectonic plates had the largest role in major earthquakes, they also acknowledged that ice movements close to these sites may have also had an impact.
From 1993 to 1995, the Bering Glacier advanced rapidly as large amounts of ice from the top Bagley Ice Valley were transported to the edge terminus region, a movement known as a surge. In the five years that followed the surge, the newly formed mass of ice retreated and thinned, a response to rising temperatures.
When the ice thickened during the surge, the number of earthquakes decreased in the region, Sauber and Molnia said. During the thinning, they added, the number of small quakes increased, hovering around 1 to 2 magnitude on the Richter scale.
The researchers also calculated the amount of pressure that would have built up under the glaciers in the Icy Bay region, between the Bering and Malaspina glaciers, from 1899 to 1979. The large mass of a glacier can help keep things stable — but once that melts away, the tectonic plates are freer to move and create friction beneath the surface.
Between 1899 and 1979, the researchers wrote, the glaciers lost enough ice that the weight loss could have contributed to the St. Elias earthquake.
Molnia notes that southern Alaska is a unique location to study these type of interactions: Few places have a rapidly melting mass interacting with plates tens of miles beneath the Earth's surface.
"It's a mecca for glacier tectonicists," he said in a phone interview.
In a later study released in 2008, Sauber and Natalia Ruppert, from the Alaska Earthquake Center at the University of Alaska Fairbanks, found that between 2002 and 2006, the number of small tectonic movements in the Icy Bay region increased when compared to the seismic rate between 1988 and 1992.
"We hypothesize that this was due to a significant increase in the rate of ice wastage in 2002-2006," the study says.
While the researchers acknowledged that the 7.9 magnitude Denali earthquake itself may have triggered more seismic activity in the region, they noted that the rate of seismicity in the region didn't change the month after that main event.
Mike West, director of the Alaska Earthquake Center, acknowledged that these two studies are based on limited data. But, he said, "the theory behind this is reasonably strong. The data is suggestive that this really happens."
Although there have been studies looking at other glaciers in Alaska and Greenland triggering seismic activity, West said that these are different events than those being studied in the Bering Glacier region.
A variety of glacial activities — including crevassing, calving and moving along the underlying rock — trigger seismometer readouts, but West emphasized that these are not related to tectonic movements.
"These are just big old things happening at the surface of the Earth," rather than at a tectonic level, West said in a phone interview. The type of surface-level shakes picked up by seismic stations range in size, according to West, from mining blasts and chemical explosions to landslides.
Since 2006, there has been an increase of earthquake monitoring in the Icy Bay region, according to West, providing scientists with a richer data set to study these events.
Sauber and Ruppert are using this data to revisit their 2008 study and update it with new readings. The research is in its beginning stages, so the exact dates under scrutiny aren't set, but Sauber said in an email that they will certainly look at recent years.
For now, West says that the Alaska Earthquake Center is currently not incorporating these glacial-tectonic interactions into hazard models — the small, 1 to 3 magnitude earthquakes that glacial melt may produce are usually not felt, and are happening in Alaska "all the time," he said.
"That doesn't mean that this is a crackpot theory," West said.