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Source Characterization of the 2020 Mw 5.1 Sparta, North Carolina, Earthquake Sequence

On August 9, 2020, a Mw 5.1 earthquake shook the Blue Ridge Mountains near Sparta, North Carolina, causing damage to roads, utility lines, and masonry structures. This earthquake is the strongest recorded in North Carolina since the 1916 M 5.2 earthquake near Asheville. Despite its moderate size, the Sparta mainshock generated the first documented surface rupture due to faulting in the eastern United States (EUS). The co-seismic surface rupture was identified along a ~2-km-long ESE-striking, SSW-dipping reverse fault. The mainshock was preceded by 8 foreshocks with duration magnitude (Md) ranging between 1.8 and 2.6 within the 25 hours before the mainshock and followed by hundreds of locatable aftershocks. The focal mechanism of the mainshock that we determined using moment tensor inversion reveals a very shallow (1 km depth) oblique reverse fault striking SE (N118°E) and dipping 52° to SW. This mechanism matches well with the observed surface deformation. Spectral analysis of the mainshock data indicates a corner frequency of 0.77 Hz, corresponding to a Brune stress drop of only 5.7 MPa, unusually small for an EUS mainshock, and the calculated source radius of 1.5 km and average slip of 20 cm both agree well with field observations. Preliminary relocations of ~300 events in the sequence using the double-difference algorithm greatly improved the spatial alignment of the seismicity compared to the original catalog. Notably, in addition to the NW-SE-striking fault associated with the mainshock fault plane and the surface rupture, the relocated events also delineate a conjugate fault trending NE, which intersects the mainshock fault near the northwest edge of the surface rupture. Ongoing work involves the construction of a rupture model for the mainshock, the detection and location of more events to construct a more complete catalog of the sequence, focal mechanism determination and stress drop estimation for smaller events in the sequence. Our results will help shed light on the source processes and seismic hazard assessment in this very low strain rate intraplate setting.

Session: Tectonics and Seismicity of Stable Continental Interiors [Poster Session]

Type: Poster

Room: Exhibit Hall

Date: 5/3/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Qimin Wu

Student Presenter: No

Additional Authors