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Source Parameters of the M5.1 Sparta, NC Earthquake of 9 August 2020

Session: Tectonics and Seismicity of Stable Continental Interiors I

Type: Oral

Date: 4/23/2021

Presentation Time: 10:15 AM Pacific

Description: 

The M5.1 Sparta earthquake was strongly felt locally, with light to moderate damage (intensity VI), and potential surface rupture observed along a ~1km trend with azimuth about 106 degrees. A source radius of ~1 km is compatible with the stress drop of 22 MPa estimated for this event. The closest permanent station was ~50km distant (CERI deployed 4 real-time stations in the local area by August 12.) leading to main-shock location estimates that were poorly constrained and not generally compatible with the potential surface rupture; being either too deep or too far away. We employ a two-step method to improve the main shock location estimate. In step 1, P- and S-wave travel times at regional stations are forward modeled over a horizontal grid of potential earthquake locations while holding depth fixed. A best fitting horizontal location (minimum RMS residual) is determined for each discrete depth between 0.1 and 13km below ground surface. First motion focal mechanisms are then calculated for each depth-location pair. In step 2, regional waveforms are modeled for each step-1 location and focal mechanism to obtain the best fitting depth. Our best fitting main shock location is 36.488 N, 81.106 W with depth 600 meters below ground surface. The first motion focal mechanism for the main shock has strike, dip and rake of 108, 60 and 24 degrees. This location and first motion focal mechanism are both consistent with left-lateral strike slip and a little vertical motion on a SW dipping fault that is parallel to and surfaces at the proposed surface rupture. Most early aftershocks (on August 12 and 13, the first two days after our temporary network was deployed) appear to align along this SW dipping plane within the top 2km below the ground surface. The distribution of later epicenters is more scattered with the potential ground rupture located in the midst of the distribution, although a large number of these later events also appear to align along the SW dipping plane.

Presenting Author: Stephen Horton

Student Presenter: No

Authors