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Earthquakes

What is an earthquake?

Earthquakes are tremors in the ground resulting from the sudden displacement of rock within the Earth’s crust.  When stress that has accumulated over time eventually exceeds the rock’s strength, rupture occurs.  This is accompanied by a release of built-up energy, producing undulating forces in the rock known as seismic waves, and these waves generate an earthquake. The plane of rupture is known as a fault, and some faults can reach the Earth’s surface and offset the landscape.

Earthquakes vary greatly in strength.  Some are small and imperceptible, while large magnitude quakes can affect thousands of square miles with disastrous results such as collapsed structures, landslides, compromised water supplies, and widespread fires from ruptured gas and electrical lines, not to mention devastated infrastructure and near-complete disruption of the local economy.  Losses can run into the tens of billions of dollars.

Mercalli and Richter scalesEarthquakes are measured by either magnitude or intensity.  The Richter scale is a quantitative, logarithmic measure of energy released, or magnitude, and each unit corresponds to a ten-fold increase in wave amplitude.  Quakes less than 3.5 on this scale are generally not felt at the surface, but can be detected by sensitive instruments called seismographs.  Quakes from 3.5 up to 5.5 are felt, but there is little structural damage; above 6.0, damage increases dramatically.  The perceived intensity of an earthquake is measured using the modified Mercalli scale, which is based on qualitative descriptions, such as the type and extent of property damage, and changes in groundwater and surface water flows.  The Richter scale uses Arabic numerals, while Mercalli levels are typically described using Roman numerals, with I corresponding to imperceptible events up to XII for total destruction.  The Mercalli scale is a measure of the effects of an earthquake at a particular place and depends not only on the strength (magnitude) of the quake, but also the distance from the place of origin and the local geology at the observation point.  Thus, a given event will have only one magnitude, but many intensity values, which tend to decrease with distance from the origin, although local conditions can produce anomalies.

Intensities are considerably greater over soft soils than solid rock.  In loose material, the shaking can increase the pressure of shallow groundwater, mobilizing sand and silt deposits, a process known as liquefaction.  As a result, ground displacement increases by a factor of four or five.  As the liquefied earth loses strength, buildings sink or topple over and underground utility lines rupture.  Liquefaction is more likely to occur in loose, saturated granular soils with poor drainage, usually Holocene-age (less than 10,000 years old), alluvial deposits found along floodplains, or in other areas where thick, unconsolidated deposits of sand and silt have accumulated.  Areas of land reclamation are often prone to liquefaction, which was a major factor in the destruction in San Francisco’s Marina District during the 1989 Loma Prieta earthquake.

Other local factors can amplify incoming seismic waves.  Mountains and ridges may enhance ground vibrations by a factor of two or three as wavelengths become “tuned” to the distance between ridges.  The Coastal Plain near the Fall Line is a wedge of soft sediment forming a feather edge overlying hard basement rock, and such a circumstance focuses the destructiveness of seismic waves.  Many of Virginia’s taller building are located along the Fall Line.

Most earthquakes occur along plate boundaries where tectonic stress is greatest.  Unlike the West Coast, the East Coast is situated near the center of a tectonic plate and resides on what geologists call a passive margin.  This is not to say that earthquakes don’t occur in Virginia, but they are much different than in California.  West Coast quakes can be very shallow and often break the ground surface, while in Virginia they usually occur at depths of anywhere from three to fifteen miles and it is not always possible to associate a specific quake with a specific fault.  In general, East Coast earthquakes are less energetic than those on the West Coast, but due to the coherency of the basement rock (think concrete slab vs. brick patio) they are felt much farther away.  The affected area can be up to ten times larger for a similar magnitude event.

 

What earthquakes have occurred in Virginia?

Earthquake activity in Virginia generally has been, with a few exceptions, low-magnitude but persistent.  The first documented earthquake in Virginia took place in 1774 near Petersburg, and many others have occurred since then, including an estimated magnitude 5.9 (VII) event in 1897 centered near Pearisburg in Giles County.  This was the second largest earthquake in the East over the last two-hundred years, being felt across twelve states, an area of at least 280,000 miles.  A Roanoke attorney who was in Pearisburg said that for nearly fifty miles from that place he “saw hardly a sound chimney standing.”  In his opinion, “If the buildings throughout Giles had been largely of brick, the damage would have been very great, and serious loss of life would have occurred.”  Since 1977, when Virginia Tech expanded its seismograph array, more than 175 quakes have been detected as originating beneath Virginia.  Of these, at least twenty-eight were large enough to be felt at the Earth’s surface.  This averages out to about six earthquakes per year, of which one is felt.

Virginia’s past seismic activity is concentrated in two areas: the central Piedmont along the James River, and the New River Valley in Giles County.  The central Virginia seismic zone includes the counties of Fluvanna, Goochland, Cumberland, Powhatan, Louisa, Albemarle, and Buckingham.  The New River/Giles County seismic zone extends from Giles County to the southwest, and includes Bland, Tazewell, Russell, Scott, Pulaski, Wythe, Smyth, Washington, and parts of Buchanan, Dickenson, and Wise counties.  However, all parts of Virginia should be considered susceptible to earthquakes, because the entire state has experienced seismic activity in the past. 

Map shows the locations of earthquakes in or near Virginia. The Giles County seismic zone is shown in pink and the Central Virginia seismic zone is shown in yellow.

Map shows the locations of earthquakes in or near Virginia.
The Giles County seismic zone is shown in pink and the Central Virginia seismic zone is shown in yellow.

 

The exact mechanisms of Virginia’s earthquakes are not clearly understood.  The Piedmont has been assembled piece-by-piece through geologic time and it is laced with numerous faults of varying ages.  Residual stress may cause these faults to reactivate on occasion, but patterns are unclear.  The New River/Giles County seismic zone may be associated with the Narrows Fault, the Saltville Fault, and/or an extension of the Holston Valley Fault, or all three.  These faults trend toward eastern Tennessee, which is one of the most seismically active areas in the continental U.S.

The Virginia Tech Seismological Observatory (VTSO) is one of the primary sources for data on seismic activity in the central East Coast.  In 1963, as part of the worldwide program, seismographs were installed at Blacksburg, and in 1977 several more seismographs were stationed in the Commonwealth and operated by the Virginia Division of Geology and Mineral Resources.  Some of these instruments were stationed around the North Anna Nuclear Power plant, but in the 1990’s, due to budget cuts, most of the North Anna sensors were taken off line.  Along with other southeastern regional seismic networks and the U.S. National Seismic Network, VTSO contributes to seismic hazard assessment in the southeastern United States and compiles a Southeastern U.S. Earthquake Catalog. 

 

The Big Quake 2011

James R. Martin II, director of the Earthquake Engineering Center for the Southeastern United States, has said, “Recent seismological studies suggest that the southern Appalachian highlands have the potential for even larger earthquakes than have occurred in the past.  But now those events would take place in much more highly populated areas.”  He believes that “we are under a significant threat of large, damaging earthquakes.”  Martin goes on to say that earthquakes don’t occur as often in the East as along the West Coast because the tectonic strain rates are different and our region “tends to experience large earthquakes isolated by long periods of quiet.” 

There’s another difference. “The earth’s crust is stronger here,” explains Martin Chapman, director of the Virginia Tech Seismological Observatory. “So shock waves moving from the epicenter of an earthquake don't lose as much energy as during quakes in California.  When a magnitude 7.0 earthquake occurs in the Southeast, the waves affect a larger area and can cause more damage at a greater distance than when a similar shock hits California.”

 

What should I do in the event of an earthquake?

Most of Virginia’s recorded earthquakes have been magnitude 4.5 or less, and the associated damage has been minor (cracks in foundation, tumbling chimneys, etc.).  However, if Virginia today experienced an earthquake with a magnitude 5.0 or greater, such as the quake of 1897, the consequences could be serious.  Richmond, Charlottesville, Petersburg, and Lynchburg are situated on the periphery of the central Virginia seismic zone.  A worst-case scenario would include the collapse of bridges and tall buildings, flash-flooding from breached reservoirs, widespread electrical fires and exploding gas pipelines, and two compromised nuclear power plants at North Anna.  Damage is compounded as ruptured water lines hinder fire abatement and disrupted transportation systems delay the evacuation of seriously injured persons. 

Despite the potential for a damaging earthquake in the future, few engineering studies or emergency response plans have been devised specifically for our region.  Studies of features left by prehistoric earthquakes, called paleoseismology, can reveal a great deal about what to expect in the future.  Further research into the geologic control of earthquakes in Virginia could greatly lessen the impact of a destructive event by 1) improved predictability and characterization of damages, and 2) delineation of earthquake-susceptible substrates in urban areas such as Richmond, as well as for situating critical emergency response facilities throughout the State. 

The typical homeowner’s insurance policy does not cover damage from earthquakes.

Earthquakes In Virginia Fact Sheet

 

Web sites:

College of William & Mary Geology of Virginia Web site, details of the 2003 earthquake

ScienceBlog, “Virginia earthquake not a fluke in the seismically active Southeast.”

The USGS Earthquake Hazards Program

USGS web site specifically for Virginia earthquakes  
several links to DGMR’s web site no longer work.

Virginia Tech Seismological Observatory

Earthquake Engineering Center for the Southeastern United States

Site with map showing earthquakes in the eastern US within the last six months

 

 

References: Bollinger, G. A, 1978, Seismic Hazard in Virginia: Virginia Minerals, Vol. 24, no. 4.

Bollinger, G. A, 1981, The Giles County, Virginia, seismic zone — configuration and hazard assessment in J. E. Beavers, editor, Earthquakes and earthquake engineering: The Eastern United States, vol. 1, Ann Arbor Science Pub., Inc., Ann Arbor, Michigan, p. 277-308.

Bollinger, G. A., and Sibol, M. 1985, Seismicity, seismic reflection studies, gravity and geology of the central Virginia seismic zone; Part I, Seismicity: Geological Society of America Bulletin, v. 96, p.49-57.

Bollinger, G. A., Snoke, J. A., Chapman, M. C., and Sibol, M. S.,1989, Estimates of the occurrence and resulting effects of damaging earthquakes in Virginia: Virginia Minerals, v. 35, n. 3.

Bollinger, G. A., Johnston, A. C., Talwani, P., Long, L. T., Shedlock, K. M., Sibol, M. S., and Chapman, M. C., 1991, Seismicity of the southeastern United States: 1698 to 1986 in Neotectonics of North America, Geology of North America Decade Map Volume, p. 291-308.

Coruh, C., Bollinger, G. A., and Costain, J. K. 1988, Seismogenic structures in the central Virginia seismic zone: Geology, v.16, p.748-751

Mixon, R. B. and Newell, W. L., 1977, The Stafford fault system-structures documenting Cretaceous and Tertiary deformation along the Fall Line in northeastern Virginia: Geology, v. 5, p. 437-440.

Paliser, L. C., 1991, Earthquakes: U. S. Geological Survey pamphlet, 20 p.

Spears, D. B., and Bailey, C. M., 2002, Geology of the central Virginia Piedmont between the Arvonia syncline and the Spotsylvania high-strain zone: 32nd Virginia Geological Field Conference Guidebook, 36 p.

Stover, Carl W., and Coffman, Jerry L., 1993, Seismicity of the United States, 1568- 1989, USGS Professional Paper 1527..