Paper No. 43-9
Presentation Time: 4:00 PM
A TALE OF TWO QUAKES: COMPARING THE 2011 M5.7 MINERAL VA AND THE 2020 M5.1 SPARTA NC EARTHQUAKES FROM A GEOLOGIC MAPPING VIEWPOINT
CARTER, Mark1, MERSCHAT, Arthur J.2, HORTON Jr., J. Wright1, BURTON, William2, COUNTS, Ronald3, TUTTLE, Martitia4, GREEN, Russell A.5, PAZZAGLIA, Frank J.6, HILL, Jesse7 and FIGUEIREDO, Paula8, (1)U.S. Geological Survey, Florence Bascom Geoscience Center, 926A National Center, Reston, VA 20192; U.S. Geological Survey, Florence Bascom Geoscience Center, MS 926A National Center, Reston, VA 20192, (2)U.S. Geological Survey, Florence Bascom Geoscience Center, MS 926A National Center, Reston, VA 20192; U.S. Geological Survey, Florence Bascom Geoscience Center, 926A National Center, Reston, VA 20192, (3)Mississippi Mineral Resources Institute/Dept. of Geology and Geological Engineering, University of Mississippi, Brevard Hall, Oxford, MS 38677, (4)M Tuttle & Associates, PO Box 345, Georgetown, ME 04548-0345, (5)Virginia Tech, The Charles E. Via, Jr. Department of Civil and Environmental Engineering, 120B Patton Hall, Blacksburg, VA 24061, (6)Lehigh University, Earth and Environmental Sciences, 1 W Packer Ave, Bethlehem, PA 18015, (7)North Carolina Geological SurveyDepartment of Environmental Quality, 2090 US Highway 70, Swannanoa, NC 28778-8211, (8)Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, 2800 Faucette Dr., Marine, Earth and Atmospheric Sciences 1, Raleigh, NC 27695-8208
Two moderate earthquakes have struck the eastern US (EUS) during the last 11 years and were felt over a broad area of the EUS and central U.S. Both events highlight the risk of damaging EUS earthquakes and lessons that can be learned about identifying past quakes in the geologic record. The 23 August 2011 M5.7 Mineral, VA earthquake occurred in the Central Virginia seismic zone (CVSZ) at a depth of 8 km. A Modified Mercalli intensity (MMI) of VIII was felt within 5 km of the epicenter. The quake likely resulted from a complex rupture on a N30°E, 50°SE reverse fault (Quail fault, Qf) with no known fault rupturing the surface. The 9 August 2020 M5.1 Sparta, NC earthquake occurred in western NC and is not associated with a known seismic zone. This quake had a hypocenter depth of 3.5 km or less and produced the first recorded surface rupture in the EUS along a N70°W, 45°SW fault (Little River fault, LRf). The quake had a MMI of VII within 2.5 km of the rupture trace.
Mapping in the CVSZ post-2011 identified the N45°E striking, steeply SE dipping Harris Creek fault (HCf) as a possible updip projection of the Qf. The HCf offsets NW-trending Jurassic dikes; excavations show an array of mm- to cm-thick clay- and manganese-filled and slickenlined brittle faults that offset Piedmont saprolite. Post-quake surveys in the meizoseismal area revealed 4 sand blows and 1 dike, within 7 km of the epicenter. River surveys identified 45 paleoliquefaction sand dikes, sills, and soft sediment deformation structures (SSDS) at 26 sites in the CVSZ and surrounding region. Dating of these features indicates at least three liquefaction-inducing quakes occurred during the last 9,000 yrs. Warping of terraces straddling the Qf also suggest Pleistocene crustal deformation.
In Sparta, the LRf reactivated cm-thick clay- and Mn-coated fault and fracture zones in eastern Blue Ridge rocks along a ~3 km rupture oblique to the regional Paleozoic trend. Lateral spreading and ejection of fluidized sand occurred on a small sandbar near the trace of the LRf. Five SSDS sites near Sparta have also been identified; their origin and age are under investigation. Kinematics on the LRf and subsidiary faults suggest a history of shallow crustal deformation since the Mesozoic. Our research on these quakes demonstrates that continued study of these rare events is warranted.
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