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Earthquakes are a common occurrence in the eastern United States

EastUS_t.jpgBOONE—Approximately 2,000 earthquakes have been recorded in the eastern United States in the past 38 years, but it’s the one that occurred Aug. 23 that has people talking.

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Earthquakes that have occurred in the East in the past 38 years are shown on this map created by Appalachian State University geology major Julia Irizarry and geology assistant professor Dr. Scott Marshall. Nearly 2,000 earthquakes have been recorded in the East since 1973, including the recent earthquake in northern Virginia. (Image courtesy of Appalachian State University Department of Geology)

Dr. Scott Marshall and Dr. Sarah Carmichael, both assistant professors in Appalachian State University’s Department of Geology, have created a website answering several commonly asked questions following the 5.8 magnitude earthquake that occurred near Mineral, Va., in the Central Virginia Seismic Zone (www.geology.appstate.edu/virginiaquake2011.htm).

“The fact that this earthquake was felt so far from the source is completely typical for Eastern U.S. earthquakes,” Marshall said. According to the U.S. Geological Survey (USGS), the Virginia earthquake was felt as far south as Georgia and as far north as Canada. It was one of five magnitude 5 or greater earthquakes recorded across the globe that day.

Marshall said ongoing movement of the Mid Atlantic Ridge, which is located at the bottom of the Atlantic Ocean, pushing against the North American Plate is most likely to have triggered the quake.

It’s the geologic composition of the eastern U.S. that resulted in the broader seismic wave shaking compared to seismic patterns in the West.

“The reason why earthquakes are felt over a much larger distance in the East as opposed to the West has everything to do with seismic wave attenuation, or how quickly a seismic wave dies out,” Marshall said.

The West Coast is comprised of active fault zones and the subsurface rocks are generally much more broken and cracked at depth. When an earthquake occurs in California, for example, the broken up rocks at depth tend to dampen or reduce the wave’s effects, Marshall explained. Since the earth’s crust in the East is comprised of less broken, stiffer rocks at depth, the seismic waves aren’t scattered or absorbed as much and as a result shaking can be felt as much as 10 times farther from the earthquake source.

“The scary thing about East Coast earthquakes is that we are kind of all in it together,” Marshall said. “It’s not as important where earthquakes happen in the East, because they are going to shake a very broad region,” he said. The East Coast has at least four known active seismic zones, Marshall said.

“The fact that an earthquake occurred in the Central Virginia Seismic Zone is actually not all that surprising; however the magnitude of the earthquake was pretty surprising. A magnitude 5.8 earthquake is really big for the typical East Coast earthquake but certainly not unheard of. The East has seen magnitude 7-plus earthquakes before, but such events are quite rare.”

Earthquakes are measured both in terms of magnitude, or how much energy is released, as well as intensity or how much ground shaking or structural damage occurs, Marshall explained.

Magnitude is determined by data collected by seismograph stations located across the world. Intensity is a qualitative measurement based on first-person accounts of ground shaking and reports of damage following an earthquake and is known as the Modified Mercalli Intensity Scale. Intensity also is measured in Roman numerals to avoid confusion with magnitude levels.

The Virginia earthquake resulted in a III intensity level or less reported in Boone to V and VI intensity levels in the Washington, D.C., and Maryland area.

“It’s the intensity that’s important to humankind,” Marshall said. For that reason, the USGS has a “Did You Feel It” website where anyone can self-report the location and effects felt or observed during an earthquake (http://earthquake.usgs.gov/earthquakes/dyfi/). The responses are used to create an intensity estimate and a Mercalli map.

“That’s really important data. Seismologists, city planners and building engineers need to know how far the shaking was felt. Even reports of people that did not feel the shaking are useful,” Marshall said. “This is the largest event we have had in the East Coast since we have been measuring seismic events. It’s the best opportunity to get an idea of how much shaking there will be if a bigger event occurs,” he said.

For more information about earthquakes and the Virginia earthquake, visit www.geology.appstate.edu/virginiaquake2011.htm. In addition, a PowerPoint presentation about East Coast earthquakes is on view in the Department of Geology’s McKinney Geology Teaching Museum in Rankin Science Building.

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