Earthquakes are fairly common in Southwest Virginia

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By Marty Gordon

 

New River Valley residents felt the effects around 8 a.m. Sunday of an earthquake that occurred just south of the Virginia border near Sparta, N.C., and measured 5.1 on the Richter scale.

Perhaps the surprising news for NRV residents is not that an earthquake occurred but that earthquakes are rather common in this area though the intensity of this particular quake is not. People typically think of earthquakes happening in California and Alaska, but eastern Tennessee and southwest Virginia are in one of the most active areas in the nation in terms of the number of earthquakes recorded.

According to Martin Chapman, director of the Virginia Tech Seismological Observatory, Virginia has had more than 160 earthquakes since 1977, though only about two of them are felt at the surface every year. The New River Valley is right in the line of fire, so to speak. Over the past 50 years, earthquakes have occurred in Giles, Pulaski, Montgomery and Bland counties in Virginia and in Monroe, Mercer and Summers counties in West Virginia.

According to Chapman, seismic activity has been known for several decades to be the strongest in and around Giles County with the Giles County seismic zone extending for almost 50 miles along the West Virginia border. This zone is laced with known faults and many smaller and deeper ones that remain undetected. The largest historical earthquake in the Giles County seismic zone was magnitude 5.0-5.5 and occurred on May 31, 1897.

According to the U.S. Geological Survey, some 45,000 people from West Virginia to Georgia felt Sunday morning’s quake. Thus, seismic officials at Virginia Tech have been closely monitoring the recent earthquake activity and warning that aftershocks with a 57 percent chance of a magnitude of at least three are possible this week. According to Chapman, there is a five percent chance of an aftershock greater than five emanating from the epicenter about 37 miles from Boone, N.C., and 46 miles from Lenoir, N.C.

The United States Geological Survey also recorded a smaller quake with a 2.6 rating just before 2 a.m. Sunday

A quake of 4.0 was recorded in Pearisburg in 2017, the area’s largest since 1968. Witnesses said it was loud and dramatically shook the earth with reports coming in from as far as Radford and Pulaski. Items were knocked off shelves, and several buildings suffered cracks in their foundations and walls.

“Damaging earthquakes of 5.0 and larger appear to happen (here) at a rate such that the average time between them is about 150 years,” Chapman said shortly after the Pearisburg quake.

In 2011, Mineral, Louisa County (Va.), recorded a quake with a magnitude of 5.8 centered near Mineral that was the largest and most damaging earthquake in the United States since 1886. Significant damage was reported from the quake to structures within a 100-mile radius including Washington, D.C., with as many as 876 aftershocks.

“Shocks like Mineral can be expected to occur along the east coast of North American about once per century on average,” Chapman said. “But at any specific spot, a quake could occur  tomorrow or a thousand+ years from now.

The observatory director did say it is unlikely that Virginia will experience a magnitude 7.1 event within our lifetimes.

“But in 2011, we did experience an earthquake very similar to the first shock in a recent California sequence (M 6.4),” Chapman said. “The ground shaking during the August 23, 2011, Mineral earthquake (M 5.8) near the epicenter was essentially the same as that experienced during their initial Ridgecrest California shock. This has to do with the fact that rocks in Virginia transmit seismic waves much more efficiently than those in California, and the energy per unit area of fault rupture in the Virginia shock was much greater than in any of these recent California shocks.”

The San Andreas Fault in California is the best-known problem area in the country with frequent tremors. Much smaller quakes can be felt on the east coast in a larger circle. For example, one at a 4.0 magnitude can be felt as far away as 60 miles from where it occurred.

Chapman said the recent California earthquakes are by no means a surprise. “They are not happening along the main coastal plate boundary faults (e.g., San Andreas) but instead east of the Sierra Nevada mountains in a region known to seismologists as the ‘eastern California shear zone,’” the observatory director said. “It has a long history of major earthquakes like we are seeing now.”

The earthquakes on the east coast are different from California’s tremors.

We are in a very different tectonic environment than California,” Chapman said. “The fault that ruptured during the 2011 Virginia shock may have been created during that earthquake.

California shocks tend to occur on faults that are easily mapped at the surface.  The faults causing the earthquakes in eastern North America are not easily mapped and may not even be faults until the earthquakes themselves occur.  In other words, new faults are being created in earthquakes like the Mineral shock in 2011.”

Most of eastern Tennessee, northwestern Georgia and northeastern Alabama have earthquakes as rates comparable to the Giles county seismic zone. That area is known as the Eastern Tennessee seismic zone. Central Virginia is also an area of earthquakes of moderate intensity and is known as the Central Virginia Seismic zone.

Chapman did provide some advice to homeowners by suggesting they buy earthquake insurance.

“The lack of such [insurance] was a problem in 2011 in central Virginia,” he said. “Also, communicate with your state representatives about the lack of earthquake hazard mitigation efforts in Virginia. That includes both scientific research and building code enforcement.”