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Posted at 11:15 AM ET, 07/22/2010

Did melting ice cause D.C. earthquake?

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The news rang out across the nation: Mild earthquake shakes D.C. area. A real earthquake it was -- not the rumblings so often emanating from "inside-the-Beltway" political battles. Although the largest earthquake in the area since tracking of such began in 1974, it was minor in intensity (3.6 on the Richter scale) and no injuries or damage were reported.

But, could there be more to this? I ask in the context of two other recent noteworthy items. The first concerns a story on the disappearance of beaches along the Eastern seaboard, "Buh-bye East Coast Beaches." As the article notes, once popular day trips from Washington to Chesapeake Beach, Md., lead now not to wide sandy beaches, but rather to "a seven-foot-high wall of boulders protecting a strip of pricey homes marked with 'No Trespassing' signs." The second item concerns the breakup and retreat of one of the largest glaciers in Greenland. Water from melting glaciers contribute to rising sea levels, and thereby could enhance the threat to beaches on the East coast and elsewhere.

But what, you might ask, does the earthquake have to do with any of this? The short answer, perhaps surprisingly, is just what the other two items have in common -- sea-level change.

The primary contributors to sea level rise are generally believed to be thermal expansion of warming oceans and addition of water to oceans from melting glaciers and ice caps atop Greenland and Antarctica. However, local changes in sea level are relative the land such that, if the land subsides, sea level rises even if there is no additional water. In fact, the land is subsiding along the East Coast, and this might very well be tied to earthquakes such as the one just experienced.

At the end of the last ice age, when the massive ices sheets that once covered much of North America retreated, removal of the weight that had been depressing the underlying land surface led to a slow -- and still ongoing -- rising of the land. More directly relevant is that regions outside the periphery of the ice sheet, which includes much of the U.S. East Cost, have responded in an opposite manner -- these lands have been sinking.

According to environmental scientists at the University of Pennsylvania, sea level along the Atlantic Coast is rising faster now than at any time in the past 4,000 years, largely due to land being lost under the waves due to post ice-age subsidence. Additionally, the researchers find that the mid-Atlantic coastlines of New Jersey, Delaware and Maryland are subsiding twice as much as areas to the north and south. Hence, these regions are subject to an increased threat of further beach erosion, wetlands converting to open water, and coastal flooding. (Note: where the earth continues to rebound, most notably in Alaska, sea level relative to land is actually falling).

It's well known that, while major earthquakes are rare, mild earthquakes are not especially out of the ordinary along the Eastern seaboard. Mark D. Petersen, a seismologist with the USGS, says the ancient rock underlying this part of the country is riven with fissures and faults. On the other hand, according to the USGS, known faults east of the Rockies seem to have nothing to do with modern earthquakes. "It's just part of the ongoing creaking and grinding of the stable part of the continent," said Scott Southworth, a USGS scientist in Virginia, of the local quake last week.

Bottom line: exactly what geological factors caused last week's quake, and earlier (and likely future) ones, is unclear. Nevertheless, it's certainly possible that earthquakes along the East Coast are tied to the continuing subsidence of land following the last ice age. However, they occur too infrequently to firmly establish whether this or any alternative mechanism explains the quakes with certainty.

Independent of the explanation for the quakes, it's important to take account of the rise and fall of land masses when considering the question of sea-level rise associated with climate change. As reported by the bulletin of the American Congress on Surveying and Mapping (ACSM), sea level has been rising at the rate of about 3 mm/yr -- a rate nearly twice the 20th century average. Further, based on long-term observations by NOAA, the rate of sea-level rise due to regional subsidence of the coastal plain of mid-Atlantic states is 3 to 5 mm/yr. It follows that localized land subsidence, possibly linked to earthquakes, can considerably magnify sea-level rise resulting from climate change.

Postscript: According to scientists at the University of Miami, just as the underlying land rebounded as the ice age glaciers retreated, Greenland is rising as the Greenland ice sheet melts and glaciers over land are shed into the sea. The uplift apparently started in the late 90s with some coastal areas rising by nearly one inch per year (I'm unaware of any information concerning subsidence on or around the periphery of Greenland). The report goes on to state that, "what's surprising, and a bit worrisome, is that the ice is melting so fast that we can actually see the land uplift in response. Even more surprising, the rise seems to be accelerating, implying that melting is accelerating."

By  |  11:15 AM ET, 07/22/2010

Categories:  Environment, Environment, Environment

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