Page 4 - i1052-5173-32-3-4

The M 5.1, 9 August 2020, Sparta
w
Earthquake, North Carolina: The First

Documented Seismic Surface Rupture

in the Eastern United States

Paula M. Figueiredo, Dept. of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina
27695, USA; Jesse S. Hill, North Carolina Geological Survey, Swannanoa, North Carolina 28778, USA; Arthur J. Merschat, U.S.
Geological Survey, Florence Bascom Geoscience Center, Reston, Virginia 20192, USA; Corey M. Scheip, Dept. of Marine, Earth and
Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695, USA, and North Carolina Geological Survey,
Swannanoa, North Carolina 28778, USA; Kevin G. Stewart, Dept. of Geological Sciences, University of North Carolina at Chapel
Hill, Chapel Hill, North Carolina 27599, USA; Lewis A. Owen, Dept. of Marine, Earth and Atmospheric Sciences, North Carolina
State University, Raleigh, North Carolina 27695, USA; Richard M. Wooten, North Carolina Geological Survey, Swannanoa, North
Carolina 28778, USA; Mark W. Carter, U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, Virginia 20192, USA;
Eric Szymanski, Dept. of Earth and Environmental Sciences, University of Michigan, Michigan 48109, USA; Stephen P. Horton,
Center for Earthquake Research and Information, The University of Memphis, Memphis, Tennessee 38152, USA; Karl W. Wegmann,
DelWayne R. Bohnenstiehl, Dept. of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North
Carolina 27695, USA, and Center for Geospatial Analytics, North Carolina State University, Raleigh, North Carolina 27695, USA;
Gary W. Thompson, North Carolina Geodetic Survey, Raleigh, North Carolina 27607, USA; Anne Witt, Virginia Dept. of Mines,
Minerals and Energy, Virginia 22903, USA; Bart Cattanach, Thomas Douglas, North Carolina Geological Survey, Swannanoa,
North Carolina 28778, USA

ABSTRACT INTRODUCTION AND BACKGROUND the 1916 M 5.2 Skyland, and the 1926 MMI
At 8:07 a.m. EDT on 9 Aug. 2020 a M The M 5.1 Sparta earthquake was the V–VI Mitchell County had intensities com-
w
w
5.1 earthquake located ~3 km south of largest in North Carolina in nearly 100 parable to the Sparta earthquake. However,
Sparta, North Carolina, USA, shook much years (Stover and Coffman, 1993) and the there is insufficient information to infer
of the eastern United States, producing the strongest in the eastern United States since which fault(s) generated them (Reinbold
first documented surface rupture due to the 2011 M 5.8 Mineral, Virginia, earth- and Johnston, 1987; Stover and Coffman,
w
faulting east of the New Madrid seismic quake. The maximum intensity was VI–VII 1993). The seismic catalog of Reinbold and
zone. The co-seismic surface rupture was (MMI) at Sparta and was widely felt across Johnston (1987) documents 166 earth-
identified along a 2-km-long traceable zone the eastern and central United States quakes since 1776, strong enough to be felt
of predominantly reverse displacement, (USGS, 2020a). Most notably, the earth- and interpreted to have their epicenters in or
with folding and flexure generating a scarp quake generated the first documentated co- near North Carolina. However, instrumen-
averaging 8–10-cm-high with a maximum seismic surface rupture by faulting in the tal seismicity records low magnitude
observed height of ~25 cm. Widespread eastern United States (Fig. 1). (M ≤ 4) earthquakes, and in the Blue Ridge
deformation south of the main surface Moderate (5 < M <6) to large (M ≥7) province, the mean hypocenter depth is
rupture includes cm-dm–long and mm-cm– earthquakes in intraplate settings, such as 12 km (Bollinger et al., 1985).
wide fissures. Two trenches excavated the North American–Atlantic passive mar- The tectonic framework in the southern
across the surface rupture reveal that this gin, are rare (Wolin et al., 2012). Notable Appalachians preserves multiple Paleozoic
earthquake propagated to the surface along earthquakes in the eastern and central U.S. orogenic events recorded by NE-trending
a preexisting structure in the shallow include the 1755 Cape Ann (M 5.9; Ebel, regional structures (Hatcher et al., 2007).
bedrock, which had not been previously 2006), the 1811–1812 New Madrid sequence Crossing these structures are several poorly
identified as an active fault. (three ≥M7; Hough and Page, 2011), the 1886 understood WNW to E-W topographic
Surface ruptures by faulting are rarely Charleston, South Carolina (M 6.8–7.2; lineaments. Their genesis is speculated to
reported for M <6 earthquakes, and hence Chapman et al., 2016), and the 2011 Mineral, result from early-to-mid-Mesozoic extension
the Sparta earthquake provides an opportu- Virginia (M 5.8; Horton et al., 2015). (e.g., Garihan and Ranson, 1992), Cenozoic
w
nity to improve seismic hazard knowledge Earthquakes occurring within the East mantle reorganization, which may account
associated with these moderate events. Tennessee, central Virginia, Giles County, for Cenozoic regional uplift (Weems and
Furthermore, this earthquake occurred in a and coastal Charleston seismic zones con- Edwards, 2007; Gallen et al., 2013; Hill,
very low strain rate intraplate setting, where tribute to North Carolina seismic hazard. 2018), or tectonic inheritance from Iapetian
earthquake surface deformation, regardless In the Blue Ridge physiographic province rifts (Thomas, 2011). In addition, some
of magnitude, is sparse in time and rare to of North Carolina, historical earthquakes WNW lineaments have brittle deformation
observe and characterize. such as the 1861 Wilkesboro MMI V–VII, of unknown age and are roughly normal to

GSA Today, v. 32, https://doi.org/10.1130/GSATG517A.1. CC-BY-NC.

4 GSA TODAY | March-April 2022