February 2022 Virtual Meeting

Monday, February 14, 2021
6:00 - 7:30 pm

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February Presentation

Comparing the 2011 M5.7 Mineral, VA and the 2020 M5.1 Sparta, NC Earthquakes



Mark Carter, PG, CPG, USGS Florence Bascom Geoscience Center
With contributions from Arthur Merschat1, Wright Horton1, Bill Burton1, Martitia Tuttle2, Russell A. Green3, Frank Pazzaglia4, Jesse Hill5, Paula Figueiredo6


1USGS Florence Bascom Geoscience Center

2M. Tuttle and Associates

3Virginia Tech, Charles E. Via Dept. of Civil and Environmental Engineering
4Lehigh University, Dept. of Earth and environmental Sciences
5North Carolina Geological Survey
6North Carolina State University, Dept. of Marine, Earth, and Atmospheric Sciences


Two moderate earthquakes struck the eastern United States during the last 11 years. The 23 August 2011 M5.7 Mineral, VA earthquake occurred in the Central Virginia Seismic Zone (CVSZ) and was felt across the central and eastern US, from Florida to Canada and as far west as Denver Colorado. A Modified Mercalli intensity (MMI) of VIII was felt within 5 km of the epicenter. The quake, with a hypocenter depth of 8 km, occurred in crystalline rocks of the central Virginia Piedmont (Chopawamsic terrane) and likely resulted from a complex rupture on a N30E, 50 deg SE reverse fault (Quail fault) with no known coseismic surface rupture. USGS intensity maps show elongation of MMI contours parallel to the NE-SW Paleozoic structural grain of the Appalachians.

The 9 August 2020 M5.1 Sparta, NC earthquake occurred in western NC and is not associated with a known seismic zone, although it was proximal to Giles County (Virginia) and East Tennessee seismic zones to the north and west, respectively. It too was felt across broad areas of North America, including three reports of shaking as far west as Salt Lake City, UT. This quake had a hypocenter depth of 3.5 km or less and occurred in metasedimentary and metavolcanic rocks (Ashe and Alligator Back Metamorphic Suites) of the allochthonous eastern Blue Ridge province. This earthquake produced the first recorded coseismic surface rupture in the eastern US along a N70W, 45 deg SW fault (Little River fault). The quake had a MMI of VII within 2.5 km of the rupture trace; USGS intensity maps show elongation of MMI contours parallel to the NW-SE trace of the Little River fault, oblique to the regional Paleozoic trend, and directed toward the Piedmont.

Both events highlight the continuing risk of destructive eastern US earthquakes and the importance of documenting field evidence through which past earthquake activity may be reconstructed. For example, post-quake surveys in the CVSZ and surrounding region have identified 45 paleoliquefaction sand dikes, sills, and soft sediment deformation structures (SSDS) at 26 sites (e.g., Carter and others, 2020; Tuttle and others, 2021). Dating of these features indicates at least three liquefaction-inducing quakes occurred during the last 9,000 yrs. Warping of terraces straddling the Quail fault also suggest Pleistocene crustal deformation (e.g., Pazzaglia and others, 2021). Five SSDS sites near Sparta have also been identified; their origin and age are under investigation but kinematic indicators on unruptured portions of the Little River fault and subsidiary faults, marked by cm-thick clay- and Mn-coated fault and fracture zones, suggest a history of shallow crustal deformation since the Mesozoic.



Mark W. Carter is a USGS Research Geologist with the Florence Bascom Geoscience Center and co-project chief of the National Cooperative Geologic Mapping Program Piedmont and Blue Ridge Project. Mark is a licensed professional geologist (since 1996) and has also worked for the North Carolina Geological Survey and Virginia Survey. Mark's expertise is geologic mapping throughout the southern Appalachian crystalline core. He has produced geologic maps and reports in three states (Tennessee, North Carolina, and Virginia) and from four geologic provinces (Valley and Ridge, Blue Ridge, Piedmont, Coastal Plain).


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