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How Common is the 5.8M Virginia Quake?

posted on Tuesday, August 23rd, 2011 at 3:05 pm

A 5.8 magnitude earthquake rattled metro Washington D.C., Philadelphia, New York and other East Coast cities at 1:51 EDT this Tuesday afternoon. The quake’s epicenter was located 84 miles southwest of Washington D.C.. The closest town to the epicenter was Mineral, Virginia which is 5 miles south-southwest of the epicenter. The earthquake occurred 3.7 miles beneath the earth’s surface. The last earthquake of this magnitude to occur in Virginia took place in 1897 in Blacksburg Virginia, near the Appalachian Mountain Range. The 1897 quake was felt from Georgia to Pennsylvania and from Indiana to the Atlantic Coast.

Virginia Earthquake, Image: US Geological Survey

Virginia Earthquake, Image: US Geological Survey

This map from the United States Geological Survey shows the areas that experienced shaking near the epicenter. The shaking ranged from weak to very strong with moderate damage occurring.  As you can see from this map, people in and around the D.C. metro area felt light to moderate shaking as a result of the quake. The US GS received close to 9000 reports from 17 cities.

Virginia Earthquake, Image: US Geological Survey

Virginia Earthquake, Image: US Geological Survey

Earthquakes in Virginia are rare. However, there are fault lines in the bedrock that formed hundreds of millions of years ago from continental collisions. These collisions formed the Appalachian Mountain Range, west of the earthquake epicenter.

According to the United States Geological Survey, “Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast.”

The US GS website also states that the largest damaging earthquake (4.8M) in the Central Virginia Seismic Zone occurred in 1875. Click here for more information on Virginia’s earthquake history.

Dawn Brown, FOX 8 New Orleans

Gulf Coast Seiche

posted on Sunday, February 28th, 2010 at 5:05 pm

The recent Chilean earthquake at a magnitude 8.8 had some pretty far reaching effects and we’re not just talking about across the Pacific!  Some effects were actually felt as far away as the Gulf Coast of the United States.  Something called a seiche was actually observed in Lake Pontchartrain.  What is a seiche? According to the American Meteorological Society Glossary, a seiche is an, “oscillation of a body of water at its natural period.”  In other words, it is some additional amount of water, or a “standing wave,” on top of the normal tidal cycle or water level.  In this case, this additional amount of water was caused by the seismic waves of Chilean earthquake 4,700 miles away traveling through the earth’s crust!  It was actually measured that tides were running 5″ above their predicted levels in Lake Pontachartrain 11 minutes after the earthquake occurred.  Seiches are nothing new.  The term was coined in 1955 and the phenomenon has been observed as far back as 1755 in England.

In fact in history, due to the massive Alaskan earthquake of 1964, seiches also occurred in the Gulf Coast area; some reported as high as nearly six feet!  This extreme height was probably aided by surface seismic waves that were in sequence with those moving through the earth’s crust.  The same earthquake caused bodies of water as small as home pools to overflow as far away as Puerto Rico.  Other seiches occurred in north and central New Mexico, eastern Kansas, and the region at the southern tip of Lake Michigan.

The occurrence of seiches seems to be more due to the make-up of the land that the seismic waves are traveling through, rather than the radial distance from the epicenter of the earthquake.  The density of seiches appears to be roughly proportional to the thickness of surface sediments.  For example, where there’s more opportunity for seismic waves to resonate through plenty of “loose” sediment and “rattle” bodies of water, like the Mississippi Delta region, there will be more seiches.  In places like the New England where the surface is more stable, there will be far fewer or no seiches.

More on seiches can be found on the USGS site.

1960's Seiche in Baraga County, Michigan, Image: Baraga County Historical Museum

1960's Seiche in Baraga County, Michigan, Image: Baraga County Historical Museum

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