GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 208-7
Presentation Time: 3:10 PM


GALLEN, Sean F., Department of Geosciences, Colorado State University, Colorado State University, 400 University Ave., Fort Collins, CO 80523 and THIGPEN, J. Ryan, Earth and Environmental Sciences, University of Kentucky, 121 Washington Ave., Lexington, KY 40506

Earthquakes commonly occur far from plate boundaries, but they do not conform to plate tectonics theory. While intraplate seismicity is recognized as a relevant seismic hazard to heavily populated regions such as the central and eastern U.S., its driving mechanisms remain unclear. Here, we present a new geomorphic model of the Eastern Tennessee Seismic Zone (ETSZ), the second most seismically active region in North America east of the Rocky Mountains. Previous studies demonstrate that the Upper Tennessee basin, which lies directly above the ETSZ, is in a transient state of adjustment to ~150 m of base level fall that occurred in the Late Miocene. We show that the erosional response to base level fall is strongly controlled by spatial variations in bedrock erodibility, which vary by more than a factor of five within the basin. Relatively erodible rocks in the central axis of the basin allowed for preferential erosion of ~3,500 km3 of rock in a 70 km wide by 350 km corridor above the ETSZ. The lithologically controlled erosional response of the basin also produced a unique distribution of modern erosion rates; a 20 km wide by 300 km long NE-SW trending swath of elevated erosion rates, approximately three times higher than background, is located ~30 km southeast of the center of the seismic zone. Stress modeling indicates that preferential removal of less competent rocks reduced clamping stress on faults at 15 km depth by >3 MPa and that the modern zone of elevated erosion reduces clamping stress on faults within the seismic zone at rates of 0.3 – 0.4 Pa yr-1. We use this evidence to argue that the lithologically controlled redistribution of surface loads augmented the local stress field allowing preferentially oriented, pre-existing weaknesses in the mid-crust to slip in the regional stress field. Furthermore, we suggest that modern differential erosion, perhaps coupled with changes in Coulomb stress and fault weakening due to ongoing earthquakes, are sustaining activity of ETSZ today. This model for erosion-induced intraplate seismicity is consistent with some models for the New Madrid Seismic Zone and is generally transferable to other settings where landscape dynamics and bedrock geology conspire to focus erosion and perturb the stress field in the mid-to-upper crust.