![]() Source: US Geological Survey (USGS), Advanced National Seismic System Virginia experiences shaking from earthquakes that occur as far away as Missouri However, the less-intense eastern earthquakes can cause damage further away from their origin. The United States east of the Mississippi has many fewer earthquakes than does the west, and western quakes are stronger. Source: US Geological Survey (USGS), Earthquakes in Virginia and Vicinity 1774 - 2004 Giles County and Central Virginia seismic risk areas in Virginia If that change results in a greater stress than the strength of the rock (or the friction holding the fault still), then you get movement, and that generates an earthquake. Changes in how much weight is pushing down on the rock can result in changes to the stress field that rocks (or pre-existing fractures in rocks) are subject to. In addition, erosion of the crust redistributes weight and creates additional isostatic stress: 3Īs the east coast is eroded, rock material is removed, and that changes how much the crust weighs in that area. The realignments could generate stress that propagates to the surface and is released through an earthquake. The crystals in such crustal fragments will realign into different minerals as heat and pressure increases, while the crystals in mantle minerals may also realign as the cooler crustal fragment slides by. The Taconic and Neo-Acadian orogenies, or the collision of the African and North American plates 250 million years ago, may have left behind chunks of crust that are still sliding down to the mantle. Subduction of oceanic crust beneath continental plates may continue long after the evidence of an orogeny has eroded away. Sea level rise in the Chesapeake Bay is magnified by the sinking crust, and the vertical shifts of the land mass create stress that can trigger earthquakes. Further east a bulge that once uplifted the surface is dropping down. The land underneath the former ice sheet is rising up. Melting of the massive ice sheet that reached into Pennsylvania 18,000 years ago had removed a weight on the crust. The tectonic stresses in the North American plate are caused by motion to the west, but also by isostatic rebound. Thus, the earthquakes felt in the Commonwealth today generally have no relationship with faults seen at the surface. Another difference is that California earthquakes often break the ground surface, while earthquakes in Virginia usually occur on faults at depths of from 3 to 15 miles. ![]() Virginia is located near the center of the North American plate and, thus, experiences a much lower rate of seismicity than California. As noted by the Virginia Department of Energy: 2 Since Virginia is in the relatively-stable inner portion of a plate, Virginia does not experience the large-magnitude earthquakes that affect Los Angeles, Alaska, Haiti, Japan, Chile, or other places that are on a tectonic plate's edge. As the land moves into the new alignment, everything shakes nearby in the earthquake - and buildings, highway bridges, etc. Intermittently, the North American/Pacific plates break free of each other, and the edges spring into a new alignment. Earthquakes occur when the pressure to move exceeds the capacity of the rocks to resist motion. The North American and Pacific plates are moving in different directions, and the rocks bend under the strain - up to a point. That boundary is marked by the San Andreas Fault and many other named/unnamed faults. In Southern California, the western edge of the North American Plate rubs against the eastern edge of the Pacific Plate. In contrast, California/Oregon/Washington are at the edge of the North American Plate. Virginia is located far from the edge of the North American Plate. Source: Library of Congress, Seismicity of the earth, 1960-1980 The Mid-Atlantic Ridge marks the eastern edge of the North American Plate At that ridge, magma rises slowly and pushes Virginia (and the rest of the North American Plate) towards China, at the rate of about 2-3 centimeters/year or about 14 miles every million years. Instead, the eastern edge of the North American Plate is in the middle of the Atlantic Ocean at the Mid-Atlantic Ridge. ![]() The Eastern Shore/Virginia Beach are at the edge of the continent, but are not located at the edge of the continental plate. The eastern coast of the United States marks the boundary between continental and oceanic crust, but the North American Plate includes both continental and oceanic crust. The plate includes both continental crust and heavier (iron- and magnesium-rich) oceanic crust. The North American Plate is one of the 15 or so major "chunks" of crust that float on top of the hot mantle. Virginia and the eastern side of the North American continent are in the middle of a tectonic plate. Source: US Geological Survey (USGS) World Plate Boundaries Animation Earthquakes in Virginia Virginia Earthquakes
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