Current Understanding of Intraplate Seismicity and Surface Deformation Associated with the August 9th, 2020 Mw 5.1 Earthquake Near Sparta, North Carolina

Wednesday, 28 October 2020
1:30 PM - 5:30 PM
Auditorium

SURFACE RUPTURE FROM THE 9 AUGUST 2020, MW 5.1 SPARTA, NORTH CAROLINA EARTHQUAKE AND CONNECTIONS WITH BEDROCK GEOLOGY

Program: Late Breaking Session
Time: 1:55 PM – 2:15 PM

Authors

Abstract

An Mw 5.1 earthquake near Sparta, NC, occurred at 8:07AM on 9 August 2020 and was felt across the Southeastern US. In the meizoseismal area, over 500 buildings and infrastructure were damaged. The initial focal mechanism indicated an oblique reverse fault (tensor solutions 299/59, rake 51 or 176/48, rake 136) at ~7.6 km depth, which places the hypocenter near the base of the Blue Ridge thrust sheet. The epicenter is located near the contact of the Ashe and Alligator Back Metamorphic Suites, which is mapped as a fault in SW Virginia, but the determined fault planes for the earthquake are nearly orthogonal to regional structural trends in the eastern Blue Ridge.

Surprisingly, this moderate quake produced surface rupture, the surface expression of which stretches for ~2.5 km. The rupture was initially recognized at two locations: at Rivers Edge Road (RER) where a water main ruptured and the road buckled, and at a business park on Greenway Drive (GD). At GD the rupture, which we name the Little River fault (LRF) is a small thrust fault oriented ~115/45 and with 10 cm top-to NNE displacement. Between GD and RER, rockfalls on steep slopes of the Little River Valley and a small area of liquefaction on the river were noted. Several segments of the rupture were traced on foot and with UAV reconnaissance from RER across a gentle upland topography with a well-developed soil profile and bedrock at ~1.5 m depth. At places, the LRF rupture is subtle but is manifested as a 1–1.5 m wide earthen swell, ridge, or bulge that is steeper than the surrounding slope. Longitudinal extensional cracks are common along the swell. The rupture varies from a displacement of upslope material 1-4 cm over downslope material, to an extensional scarp with 10–20 cm displacement on the downslope side (east) of the swell. The features consistently indicate uplift of the SW-side in agreement with the kinematics of the fault at GD. Surface rupture features die out ~100 m WNW of GD and gradually to the SE of Chestnut Grove Church Road (~1 km ESE of RER).

A shallower hypocenter places the initial rupture at a depth much closer to the Ashe-Alligator Back contact, and better explains the surface deformation, but the LRF is orthogonal to and transects the contact. Pre-existing regional jointing and faulting(?) may have played a significant role in the formation of the surface rupture.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-361227

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