17 December 2012
As air columns collapse, glaciers tremble
Posted by kramsayer
One moment, a block of ice about the size of a 15-passenger van plummets from the edge of a melting glacier to the water below. Seconds later, seismic vibrations shake the glacier and surrounding rock. For years, scientists have been puzzled over why glaciers quake while losing ice. Now, a new study has uncovered how the icequakes and ice loss are connected, which may help glaciologists and climate scientists track retreating ice throughout the Arctic.
From a camp at Yahtse Glacier in Alaska, researchers pointed a camera at the 60-meter-high glacier cliff and monitored seismographs they placed around the glacier. They synchronized the clocks in the camera and seismograph using a GPS system and let the equipment run for several days, capturing the precise timing of falling ice and the icequakes.
It’s rare to see the source of seismicity in action, said Timothy Bartholomaus, a glaciologist at the University of Alaska Fairbanks.
“I think an earthquake or volcano seismologist would be extremely excited if they could see the rock fracturing beneath the ground,” he said. “We’re actually quite lucky that we get to watch.”
The research by Bartholomaus and his colleagues will be published 22 December in the Journal of Geophysical Research – Earth Surface, a journal of the American Geophysical Union.
The scientists found that the ice quakes began when the ice hit the water at the foot of the cliff, sending seismic waves through the water and into the glacier and surrounding rock. Furthermore, some data showed that the intensity of certain icequakes sharply peaked while others did not. By looking over the behavior of the falling ice during peaking icequakes, the researchers discovered that how the ice hits the water is at least as important as the size of the falling icebergs.
If the ice that falls off a glacier is boulder-shaped, as opposed to a flat slab, and if it descends fast enough, it leaves a column of air in its path as it descends into the water. The walls of surrounding water then rush inward to fill the gap, pinching off the air column partway down. When the walls of the column meet, the momentum of the water fires a visible, upward stream of water called a Worthington jet. Bartholomaus and his colleagues noted several Worthington jets in their study, including one that was even taller than Yahtse glacier’s cliff and ejected bits of ice about 100 meters into the air.
When researchers studied the data, they realized that the icequakes’ intensity peaked at the moment of the Worthington jets, showing a direct relationship between the strongest icequakes and the collapsing air column. The momentum from the collapse doesn’t just create Worthington jets – it sends a horizontal pressure pulse that slams into the glacier underwater, causing the icequake, the scientists suggest.
This is the first research to relate the falling ice directly to the icequakes, said glaciologist Jason Amundson of the University of Alaska Southeast, who was not involved in the study. “Tim took it to sort of the next level,” he said. This explanation has eluded most icequake researchers before now, he explained, because seismic waves don’t travel well in water and scientists had assumed the water wasn’t important. But it turns out that if an ice block impacts the water hard enough, or in a specific way, it can send a seismic wave that travels through the water.
Such icequakes are common everywhere ice is falling from glacial cliffs, Bartholomaus noted, from Alaska to Greenland. Understanding the fundamental connection between falling ice and icequakes could give scientists a way to monitor ice loss there and elsewhere, which would in turn help scientists project sea-level rise. “We can’t have someone sitting at every terminus in the world,” said Bartholomaus, “but perhaps we can have seismometers at all of them.”
-Sean Treacy, AGU science writing intern