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

SEISMIC OVERVIEW OF THE M=5.1 SPARTA, NC EARTHQUAKE OF AUGUST 9, 2020

Program: Late Breaking Session
Time: 1:35 PM – 1:55 PM

Authors

Abstract

The M5.1 Sparta NC earthquake occurred early on a Sunday morning (08/09/2020 08:07 EDT). It was strongly felt in Sparta, with light to moderate damage to structures (intensity VI), and lightly felt as far away as Cincinnati Ohio and Wilmington NC. The NC Geological Survey observed potential surface rupture near the main shock epicenter. We estimate a stress drop of 22 MPa compatible with a source radius of ~1 km and comparable with other values in the region. This relatively high stress drop contributes to the more intense shaking for this event. The M5.1 was preceded by 8 foreshocks (1.8 <= md <= 2.6) over ~24 hours and followed by more than 184 locatable aftershocks (-0.8 <= md <= 2.9) into September. A first motion reverse-slip focal mechanism was determined for the main shock with strike, dip and rake of 155°, 45° and 83° for one nodal plane and 345°, 45° and 97° for the other. This focal mechanism was similar to SLU and USGS moment tensors. The depth estimate (7.8km) of the main shock was poorly constrained because the closest permanent station was 50 km distant. Therefore, the Center for Earthquake Research and Information at the University of Memphis deployed 4 temporary stations within 56 hours to help improve earthquake location estimates. A subset of 57 aftershocks between 08/12-08/22 have been relocated using only the 4 temporary stations and using a program (HYPOELLIPSE) that allows depths above sea level. The relocated aftershocks show a significant scatter in map view and linear trends are not pronounced. In cross-section, the hypocenters occur largely between the earth’s surface and 3km below sea level. This is reasonable evidence that the main shock fault is also shallow and contained within the aftershock distribution. The distribution has a cone shape in cross-section, wide at the top and narrowing towards depth, and it encompasses the potential surface rupture features in map view. The pattern of intensity reports is similar to the 2011 M5.7 Mineral, VA earthquake with more attenuation of shaking to the NW and less attenuation to the NE and SW. Unlike the Mineral earthquake, the Sparta earthquake shows shaking attenuated the least to the east. Ground motion observations confirm this pattern of less attenuation to the east and more attenuation to the west, even before reaching the amplifying soils of the Atlantic Coastal plain.

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

doi: 10.1130/abs/2020AM-361224

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