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 DEFORMATION ASSOCIATED WITH THE MW 5.1 SPARTA, NC, EARTHQUAKE

Program: Late Breaking Session
Time: 2:15 PM – 2:35 PM

Authors

Abstract

The August 9, 2020 Mw 5.1 Sparta, North Carolina earthquake had a likely focal depth of ~3 km (preferred depth), with a reverse-oblique focal mechanism with nodal planes striking ~300° dipping north and ~165° dipping south. The earthquake generated surface ruptures north of the epicentral area. The surface rupture (fault) is a narrow zone of deformation that is traceable for ~2.2 km. The western termination of the surface rupture, immediately south of Sparta, has several ground fissures and small scarps trending ~100o with right stepping en-echelon geometry. The surface rupture extends ESE, crossing the Little River where it is poorly exposed, and then is present as a series of straighter segments that cross open fields and wooded terrain in a mid-slope position. Along the length of the surface rupture, the reverse displacement and folding/flexure of the hanging wall (south side) reveal a maximum vertical displacement of ~20 cm and averaging 8–10 cm. Unmanned aerial vehicle and terrestrial laser scan surveys highlight the minor changes along the surface rupture reflecting strain partitioning. Two trenches perpendicular to the surface rupture show different deformation mechanisms, including faulted and distributed deformation in the sediment layers near the surface. At one trench, the faulted sub horizontal layers are offset 10 cm by a reverse fault striking 100° and dipping 19° S, with 4 cm of vertical displacement expresses as a small scarp. Approximately 20 cm below the surface, this fault roots in a pre-existing plane dipping 50°S into weathered Neoproterozoic metasediments, which show evidence of past ductile and brittle deformation (faulting) although timing is unknown. Subtle deformation was recognized at the second trench, with flexure of the upper colluvium. Sub-surface ground penetration radar acquisition across the surface rupture in the open fields shows a south-dipping low-angle structure near the surface, consistent with reverse displacement and deformation of sediments layer near the surface. The preliminary InSAR analysis and field observations are consistent with the USGS focal mechanism, suggesting the existence of an NW-SE structure dipping to the SW and causing relative uplift of the SW hangingwall block and relative subsidence of the NE footwall block over an area of 20 km2.

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

doi: 10.1130/abs/2020AM-361234

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