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An Interpretation of Ground Motions, Intensities and Stress-Drop for the M5.1 Sparta, North Carolina, Earthquake

Session: Tectonics and Seismicity of Stable Continental Interiors I

Type: Oral

Date: 4/23/2021

Presentation Time: 10:30 AM Pacific

Description: 

Observed ground motions, intensities, and mainshock stress-drop can provide important insights into the earthquake setting both geophysically and geologically, particularly from a seismic hazard point of view. The observed mainshock Brune stress drop of 22 +6/-4 MPa is consistent with published regional stress-drops for the eastern US and with the level of shaking and damage associated with the Sparta earthquake. The observed intensity 7 in the Sparta area suggests peak ground accelerations of 0.2-0.4 g and peak ground velocities of 6-8 cm/s using a recently updated Ground Motion vs. Intensity Correlation Equation for Central and Eastern North America. The regional intensity pattern from the Sparta earthquake reinforces observations from the 2011 M5.7 Mineral, VA earthquake of differences in shaking attenuation along and across regional structure. From the Sparta earthquake intensities, we see more shaking attenuation to the W and NW and less shaking attenuation to the E, even before reaching the Atlantic Coastal Plain. A factor of 6 decrease in ground motions to the W and NW relative to those to the E develops between 50 and 100 km from the epicenter and appears consistent out to 900 km. This increased seismic attenuation suggests a higher attenuation associated with the boundary between the Valley and Ridge sedimentary rocks and the Blue Ridge metamorphic rocks, possibly due to a strong velocity contrast reflecting energy back into the Blue Ridge. Intensities from the 1886 M7.0 Charleston, SC earthquake show a pocket of lower intensities associated with Eocene volcanic rocks and an area of mantle delamination near the West Virginia/Virginia border. Observations from the 2019 M4 Eastlake, OH earthquake show a region of higher crustal attenuation (Qo=213, h=0.55) in this same area extending into central Pennsylvania, possibly due to warmer crust over the shallower depth to mantle in the area due to the delamination.

Presenting Author: Chris H. Cramer

Student Presenter: No

Authors