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Faults and Earthquakes

Faults and Dikes

Faults and Dikes MapThe major faults of Western North Carolina discovered thus far are ancient, and none are known to be active. Most were formed during the Paleozoic mountain building episodes, around 480 to 300 million years ago. During this time, the Earth’s crust in the region was buckled (folded) and shifted many miles to the northwest along thrust faults. Some areas experienced up to three episodes of deformation and metamorphism that produced complex structures with multiple generations of folding and faulting. There is also evidence that a few faults in the region may be Mesozoic or younger in age, forming when the continents pulled apart as the modern Atlantic Ocean opened. With further detailed geologic mapping, more fault lines will likely be identified.

These faults are not visible as breaks in the ground surface due to their antiquity, and similarly the known faults do not correspond with the locations of historic earthquake epicenters. The most significant fault in the region is the Brevard fault zone, which extends from Alabama to Virginia across North Carolina, where it coincides with a long, linear topographic low. Another major fault in the region is the Linville Falls fault. During the Paleozoic, the Linville Falls fault carried approximately one-billion-year-old rocks over the younger ~700-million-year-old Grandfather Mountain Formation. Erosion through the billion-year-old rocks has exposed and created the rugged, scenic terrain of Grandfather Mountain that we enjoy today. This geologic feature, where erosion exposes younger rocks beneath older, fault-transported rocks, is called a “window.”

Rocks in fault zones are commonly thinly layered because their minerals were aligned and stretched by the tremendous pressures caused by fault movement. Closely spaced fractures are also common to fault zones. These characteristics of fault zones influence the quantity, quality, and flow directions of groundwater, can affect the stability of rock slopes, and can make them ideal sources of flagstone.

A dike is a tabular igneous intrusion that is younger than the existing rock it crosscuts. In Western North Carolina, several generations of igneous dikes are exposed at the surface today, but are too small to be displayed on the regional map. The most recent generation intruded the crust along mainly NW-SE trending fractures that formed around 150-250 million years ago, during Mesozoic rifting of North America. Many streams in the region have similar trends because their downward erosion exploited these weakened fracture zones in the bedrock. Fractures along the borders of the dikes can also influence groundwater quantity, quality, and flow direction.

Earthquakes

North Carolina has experienced the effects of earthquakes throughout its history, although large and damaging earthquakes are rare, with most earthquakes having a magnitude of less than 3.0. This level of seismic activity is due to North Carolina’s presence upon a passive, not active, continental margin – unlike West Coast regions, which experience frequent earthquakes along known and active fault zones. All of the known faults exposed on the surface in North Carolina are ancient and remain inactive. Most of the earthquakes here are small, random, scattered movements of the Earth's crust. The locations of earthquake epicenters in North Carolina do not consistently line up with known faults (as they do in California), indicating that the inactive faults exposed on the surface are not responsible for the earthquakes in this region.

Although much less active than the West Coast, there are exceptions in and around North Carolina, where increased seismicity has been recorded. One example is the cluster of epicenters stretching from Tennessee through the edge of Western North Carolina and into northern Georgia and Alabama, called the Eastern Tennessee Seismic Zone. Research is underway to more accurately understand the seismic potential in this region.

Damaging Earthquakes Centered in Western North Carolina

  • Wilkes County Earthquake – August 31, 1861. Magnitude 5.1. The epicenter of this earthquake was near Wilkesboro, where intense shaking caused bricks to fall from chimneys.  The shock was felt from Washington, D.C., to Charleston, South Carolina, and Cincinnati, Ohio.
  • Skyland Earthquake – February 21, 1916. Magnitude 5.5. Damage occurred in Skyland, Waynesville, Tryon, and Forest City. Chimney tops were dislodged, and many windowpanes were broken. The quake was felt for over 200,000 square miles, including in the Carolinas, Alabama, Kentucky, and West Virginia. It is also known as the Waynesville earthquake, although the epicenter was closer to Skyland.
  • Mitchell County Earthquake – July 8, 1926. Magnitude 5.2. Centered in southern Mitchell County, this sharp local earthquake toppled one chimney and cracked several others. Other damage included cracked house foundations, dislodged foundations, broken water pipes, and fallen glassware from shelves. The area of significant damage was about 0.5 miles long and 900 feet wide, but it was felt over an area of about 40,000 square miles.

Damaging Earthquakes Centered Outside North Carolina

Earthquakes centered in other areas of the country can affect North Carolina. The great earthquakes of 1811–1812 (magnitude 7.3–8.3) were centered in the Mississippi Valley near New Madrid, Missouri, and were felt throughout the state. Effects such as broken windows and cracked plaster and masonry affected the western part of the state. The relatively small amount of damage in North Carolina, and in some other areas of the region, can be attributed to the low population density of that time. The 1886 Charleston earthquake occurred in the Charleston Seismic Zone, with an estimated magnitude of 6.7. Damage in North Carolina occurred mainly in the eastern and central parts of the state, but damage was reported in Waynesville. In Western North Carolina, the shaking produced effects such as broken windows, damage to masonry (especially weak masonry), ringing bells, and furniture being moved or broken. On December 9, 2003, a 4.5 magnitude earthquake near Richmond, Virginia, was felt as far south as Raleigh. This earthquake occurred in the Central Virginia Seismic Zone, an event similar to the magnitude 5.8 Virginia earthquake on August 23, 2011, which was felt throughout most of Western North Carolina.

Seismic Hazard Maps

The U.S. Geological Survey (USGS) produces National Seismic Hazard Maps that display potential earthquake ground motions for various probability levels across the United States. These maps, rather than maps of earthquake epicenters, are applied in seismic provisions of building codes, insurance rates, risk assessments, and other public policy. The USGS National Seismic Hazard Mapping Project develops these maps by incorporating information on known and potential earthquakes and the associated ground shaking obtained in science and engineering research and workshops, review by science organizations and state geological surveys, and advice from expert panels.

References

Bechtel, R., M.A. Medina, J.G. Nickerson, K.B. Taylor, J.C. Reid, R.M. Wooten, K. Snider, and T.W. Clark. 2007. “Map of earthquake epicenters in North Carolina and portions of adjacent states (1698-2006).” N.C. Geological Survey Geologic Hazards Map Series 2, map scale 1:1,500,000. Accessed from: http://nc-maps.stores.yahoo.net/gh2mapofeaep.html.

Hatcher, R.D., Jr., B.B. Bream, and A.J. Merschat. 2007. “Tectonic map of the southern and central Appalachians: A tale of three orogens and a complete Wilson cycle.” In R.D. Hatcher, Jr., M.P. Carlson, J.H. McBride, and J.R. Martinez Catalan, editors, 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 595-632, doi: 10.1130/2007.1200(29).

North Carolina Department of Crime Control and Public Safety, Division of Emergency Management. Earthquake History, Response and Emergency Kit. Accessed from: http://www.nccrimecontrol.org/.

North Carolina Geological Survey. 1985. Geologic Map of North Carolina, Scale 1:500,000 (out of print). Accessed from: http://gis.enr.state.nc.us/sid/bin/index.plx?client=zGeologic_Maps&site=9AM.

United States Geological Survey. National Earthquake Information Center. Accessed from: http://neic.usgs.gov.

United States Geological Survey. Seismic Hazard Maps. Accessed from: http://earthquake.usgs.gov/hazards/products/conterminous/2008/maps/ and http://pubs.usgs.gov/of/2008/1128/.