When an earthquake occurs in the U.S. we think of California, but one just struck North Carolina. What gives? Presaged by a series of foreshocks, the M 5.1 quake occurred at 8.07 a.m. on Sunday, August 9, local time, about two miles from the small town of Sparta, close to the Virginia border.
Shaking was reportedly felt as far away as Atlanta, Georgia, and Washington, D.C., but the quake caused only minor damage locally and no injuries. Chimneys toppled, items fell from walls and shelves, and cracks appeared in pavement. Only minor damage to buildings was noted, for according to the USGS construction in the area is generally “resistant to earthquake shaking, though vulnerable structures exist.” But it was the location and the magnitude of the event that garnered so much attention.
The Earth’s crust and upper mantle form the rigid, strong lithosphere, which is divided into large plates that move very slowly relative to one another. Most of the world’s seismic and volcanic activity occur along boundaries between tectonic plates. About 90% of Earth’s earthquakes, for example, occur along the 24,900 mile-long Circum-Pacific Belt (the Ring of Fire), which is formed by the boundaries between several tectonic plates. But some earthquakes, like the one in North Carolina, strike well within tectonic plates, far from any boundaries, in what are known as stable continental regions (SCRs). (Another SCR is Australia.)
The Central and Eastern U.S. (CEUS), where Sparta is located, lies within the North America tectonic plate and is considered a relatively seismically quiet SCR. Most faults in the CEUS are buried and do not reach ground level and are consequently difficult to locate and study. The U.S. Geological Service (USGS) notes that “the most significant historical earthquakes in the Central and Eastern U.S., those that potentially did have a fault line at the surface, happened so long ago that any evidence of the fault line has since been obliterated by erosion and decomposition of the rocks and by sedimentary deposits that have covered the surface.”
For active deforming regions along plate boundaries, fault slip rates, paleoseismological data, and geodetic survey data for deformation rates along faults or fault zones can provide supplementary information to determine the rate of significant earthquakes even if the return periods of such earthquakes are longer than the historical record. But in SCR regions the crustal deformation rate is very low, and the scant historical earthquake data becomes the primary—if not the only—data source for seismic hazard analysis.
Modeling earthquake risk in regions of low seismic activity is always challenging because of this lack of data. Throughout most of the CEUS there is little geological or geodetic information about active faults, but there are a few seismic zones in areas with a history of large earthquakes. These seismic zones are understood by sparse historical (often pre-instrumental) events, some evidence of paleoearthquakes, and physical models of deformation and glacial rebound. They are generally modeled as zones or several potential fault sources to account for uncertainties until there is evidence to define an individual fault source.
Why and where earthquakes occur in SCRs is still poorly understood. While seismic activity in SCRs is generally low, when earthquakes do occur their consequences can be severe, in part because building codes adopted within them may not take full account of the risk. The Sparta earthquake is significant both because it was of comparatively large magnitude for an SCR and because it is a potent reminder that earthquake risk in the CEUS is real.
