An earthquake is a shaking or trembling of the ground caused by energy coming from volcanic eruptions or from rocks breaking during the geologic process known as faulting. This process takes place as a result of differential movements of segments of the Earth's crust known as plates. The plate tectonics theory has identified seven major crustal plates, as well as several minor plates, which are thought to be colliding with one another, separating from one another, or sliding past one another like books on a tabletop. The sudden movements of these plates, as they separate, collide, or slide past one another, are believed to cause most of the planet's earthquakes.

Volcanoes can cause earthquakes too, but these earthquakes are generally minor and occur only during periods of heightened volcanic activity. Some additional causes of earthquakes include landslides that drop large quantities of earth suddenly on underlying surfaces, and human activities, such as blasting during surface or underground mining operations. The shock waves from underground nuclear tests can generate vibrations similar to earthquakes as well.

Concerns have been raised that climate change may lead to earthquakes in Alaska, or elsewhere, and these concerns may not be entirely unwarranted. Many glaciers and ice sheets are located in regions of past or continuing geologic activity, where crustal movements are still taking place. Removal of ice cover during periods of global warming might be sufficient to activate ancient faults in these areas and set off earthquakes of considerable magnitude. The rate at which the ice melts may not need to be rapid to cause such activity. Rocks can build up strains over a period of years without breaking at all, until the tension is suddenly released in a devastating shock. Scientists describe this process as elastic rebound. While the rocks on either side of a fault slowly slide past each other, the fault itself may remain "locked," with no movement taking place. Meanwhile, the rock masses on both sides of the fault begin to stretch, in a fashion similar to the stretching of an elastic band. When the strain in the rocks becomes too great, the fault will break, and the rocks on either side will snap back to their original dimensions.

There is clear evidence that the removal of ice from the Earth's surface following the ice ages is causing significant upward movement of the crust in several places. In parts of Scotland, Scandinavia, and Canada, the amount of rise has been dramatic. Interior Scandinavia has risen as much as 250 meters since the ice disappeared, and elevated beaches around Hudson Bay in northern Canada indicate the land there has risen more than 330 meters during the same period. Scandinavia is still rebounding at a rate of one meter per century, and some coastal cities have been uplifted so rapidly that docks constructed several hundred years ago are now inland, while harbors have grown so shallow that they can no longer accommodate the ships that once docked there.

Scientists refer to this uplifting process as isostatic rebound. When the weight of a continental ice sheet is added to the Earth's crust, the crust responds by subsiding. When the ice is removed, the crust begins to rise again, just as a cargo ship that sank lower in the water when it was loaded floats higher after its cargo is removed. The weight of the three-kilometer-thick ice sheets that once covered North America and Europe apparently caused down-warping of the crust by hundreds of meters. Now that the ice has been removed, the crust is rebounding to its original height.

Whether faults that were once active can be reactivated sufficiently by this rebounding process to cause earthquakes is difficult to determine, but such rebounding may well explain a series of mysterious earthquakes that plagued upper New York State during the late 1900's, as well as earthquakes that rattled Boston at the same time. Both areas were once buried beneath the North American ice sheet, and Boston's jolts have continued into the twenty-first century. By contrast, the continent of Antarctica still has its ice cover, and Antarctica's continental shelves, which would normally be found 120 meters below sea level, are now found 330 meters down. Presumably, the shelves were formed in water depths of 120 meters when Antarctica was ice free, and they will rise again to normal depths if the weight of the ice now covering the continent is someday removed.

Bibliography:

1)         Anderson, David E., Andrew S. Gaudie, and Adrian G. Parker. Global Environments Through the Quaternary: Exploring Environmental Change. New York: Oxford University Press, 2007.

2)         Hough, Susan Elizabeth. Earthshaking Science: What We Know and Don't Know About Earthquakes. Princeton, N.J.: Princeton University Press, 2002.

3)         Zeilinga de Boer, Jelle, and Donald Theodore Sanders. Earthquakes in Human History: The Far-Reaching Effects of Seismic Disruptions. Princeton, N.J.: Princeton University Press, 2005.

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