What hap­pens dur­ing a gla­cial earth­quake

Study re­sults could help sci­en­tists track the loss of the Green­land Ice Sheet, which is shrink­ing fast.

Los Angeles Times - - THE WORLD - By Amina Khan amina. khan@ latimes. com Twit­ter: @ am­i­nawrite

Glaciers might move slowly but they have their dra­matic mo­ments. Sci­en­tists track­ing gla­cial earth­quakes in Green­land have man­aged to crack open the mys­te­ri­ous dy­nam­ics of calv­ing ice­bergs.

The re­sults, pub­lished in the jour­nal Science, could help sci­en­tists track the loss of the Green­land Ice Sheet, which is shrink­ing even faster than Antarc­tica.

Gla­cial earth­quakes are caused by the calv­ing of glacier ice — when a mas­sive shard cracks like a gun­shot and sloughs off the frozen wall.

Breath­tak­ing as these events are, they’re caused by very dif­fer­ent dy­nam­ics than your stan­dard earth­quakes, which oc­cur sud­denly af­ter stresses build­ing in the ground fi­nally re­lease. Gla­cial earth­quakes, by con­trast, can take min­utes to play out, and do so grad­u­ally ( and of­ten al­most im­per­cep­ti­bly).

“You can­not feel it,” said study coau­thor Mered­ith Net­tles, a seis­mol­o­gist at Columbia Univer­sity’s Lamont- Do­herty Earth Ob­ser­va­tory. Net­tles and col­leagues f irst dis­cov­ered gla­cial earth­quakes in 2003, and she has ex­pe­ri­enced a few of the icy tem­blors in per­son.

“If you’re lifted up for 60 sec­onds and grad­u­ally put back down, you won’t no­tice it,” she said. “It’s a lit­tle bit like if you ride in an el­e­va­tor — what you no­tice is when the el­e­va­tor stops and starts; you don’t re­ally feel the mo­tion in be­tween.”

These events also have sig­nif­i­cant con­se­quences for the loss of gla­cial ice that’s hap­pen­ing in the con­text of ris­ing sea lev­els and cli­mate change.

“The Green­land Ice Sheet is los­ing mass very quickly right now; about half of it is from calv­ing pro­cesses,” Net­tles said. “Calv­ing is ac­tu­ally not as well un­der­stood as one would hope, and in or­der to make bet­ter mod­els of how the ice sheet works and un­der­stand bet­ter what the con­tri­bu­tions to sea level change are likely to be in the fu­ture, [ it] re­ally re­quires a bet­ter phys­i­cal un­der­stand­ing of those pro­cesses.”

The in­ter­na­tional team of sci­en­tists sus­pected that gla­cial earth­quakes were linked to these calv­ing events, but there was lit­tle data to sup­port that the­ory. So they placed a wire­less net­work of GPS sen­sors around the calv­ing mar­gin of Hel­heim Glacier, which is a ma­jor out­let of the Green­land Ice Sheet, and mon­i­tored the net­work for 55 days.

They also placed two cam­eras in front of the calv­ing bor­der, which took pic­tures on an hourly ba­sis. These cam­eras acted like two eyes, al­low­ing the re­searchers to re­con­struct a three- di­men­sional model of the glacier’s front and the calved ice­bergs.

Back in the lab, the re­searchers ran sim­u­la­tions in a cylin­dri­cal tank mim­ick­ing the fjords, which are thin fin­gers of sea­wa­ter that ex­tend into land ( and where the wa­ter meets the Hel­heim Glacier wall). They used a rec­tan­gu­lar box with the same den­sity as the ice in or­der to mimic the move­ment of a calv­ing ice­berg and mea­sure the forces it gen­er­ated in the wa­ter.

The sci­en­tists found that as the calv­ing ice fell, it would of­ten f lip back­ward and push the glacier so hard that it com­presses the front of it like a spring, caus­ing it to brief ly re­verse di­rec­tion. For the Hel­heim Glacier, that’s quite a sur­prise, Net­tles said.

“It’s typ­i­cally mov­ing in the for­ward di­rec­tion some­thing like 100 feet per day. It’s one of the fastest glaciers in the world,” Net­tles said. “So part of the rea­son it’s so im­pres­sive to see it mov­ing back­ward is that nor­mally it’s mov­ing for­ward so fast.”

Then, when the ice­berg plunged into the wa­ter, the wa­ter pres­sure be­hind it would plum­met, caus­ing the main glacier to go down about 4 inches while pulling the earth up­ward.

This gen­er­ated the ver­ti­cal forces ob­served in gla­cial earth­quakes, the sci­en­tists found.

Karl Rit­ter As­so­ci­ated Press

RE­SEARCHERS, bot­tom, work on a GPS sen­sor that is mon­i­tor­ing the Hel­heim Glacier in Green­land.

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