Surface deformation associated with the Mw 5.1 Sparta, North Carolina earthquake and evidence of recurrent Quaternary surface ruptures in an intraplate setting

Paula M. Figueiredo, DelWayne R. Bohnenstiehl, Arthur J. Merschat, Mark W. Carter, Karl W. Wegmann, Matthew C. Ricker, & Lewis A. Owen

Published September 11, 2022, SCEC Contribution #12353, 2022 SCEC Annual Meeting Poster #095

The shallow Mw 5.1 Sparta, North Carolina 2020 earthquake was triggered on an unknown fault in an intraplate setting with low tectonic stress. Surprisingly, initial field surveys identified a surface rupture, expressed by reverse scarps and/or folding/flexures with heights often reaching 8-10 cm, along segments of 10-100s of meters occasionally en echelon, and recognized along a ~N110° trend for ~2 km SE of Sparta. These features are indicative of an upthrown southern block and consistent with a focal mechanism solution with nodal plane N108°, 60° SW. Reverse faulting was associated with pre-existent discontinuities in metamorphic bedrock. These discontinuities seem to be inherited “fabrics” that were reactivated prior to 2020 as brittle structures, during timing(s) poorly constrained and trending obliquely to the regional structural framework.

A post-earthquake lidar survey imaged a ~4 km rupture length and a wider yet subtle co-seismic flexure, which increased locally the total vertical deformation up to 35-50 cm. Ground-Penetrating Radar and Electrical Resistivity surveys (GPR and ERT) across the rupture highlighted a ~10 m-wide fault zone with SW dipping GPR reflectors and ERT anomalies consistent with a fault, now called the Little River fault. Two 30 m-long and 2 m deep paleoseismologic trenches were excavated across the fault trace to investigate deformation of Quaternary soils and sediments. The trenches exposed weathered metamorphic bedrock covered by a sequence of soil horizons; all units are deformed by several fault strands in a 10-15 m wide fault zone. The 2020 earthquake caused a cumulative displacement along fault of ~50 cm recognized in 3 strands in association with a ~30 cm vertical step in the topography. A deformed soil sequence of likely late to mid-Pleistocene age provides evidence for at least one paleoearthquake with a similar amount of deformation, indicating that the Little River fault has been active and generating surface ruptures before the 2020 earthquake.

The Mw 5.1 Sparta earthquake was an unexpected event that generated an amount of surface deformation larger than estimates from empirical relationships. A shallower depth may explain the larger surface deformation; however, the trigger(s) for this shallow seismicity in pre-existent structures is not yet recognized, nor is it possible to identify which structures are more favorable to reactivation in similar regions with low tectonic stress.

Key Words
Intraplate Setting, Eastern USA, surface rupture, surface deformation

Citation
Figueiredo, P. M., Bohnenstiehl, D. R., Merschat, A. J., Carter, M. W., Wegmann, K. W., Ricker, M. C., & Owen, L. A. (2022, 09). Surface deformation associated with the Mw 5.1 Sparta, North Carolina earthquake and evidence of recurrent Quaternary surface ruptures in an intraplate setting. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology