Progress in Understanding the Geodynamics of the Eastern Tennessee Seismic Zone

Date: 4/25/2019

Time: 04:30 PM

Room: Pike

The Eastern Tennessee Seismic Zone (ETSZ) is an enigma. No large events have occurred historically, and no major rupture is documented paleoseismologically, but the natural rate of moment release—from small-magnitude earthquakes—is the second highest in the central and eastern United States. This presentation examines the ETSZ in the context of the tectonic and geodynamic setting of eastern North America as a whole, from the perspective of long-term stress accumulation. Stress inversions of focal mechanisms demonstrate that the ETSZ is anomalous, characterized by oblique-normal faulting that differs from the thrust faulting to the east and dominantly pure strike-slip faulting to the north and west. Additionally, in 3D density models of the crust and upper mantle (derived by simultaneously fitting gravity, topography, and seismic velocity), the ETSZ appears to host the most buoyant lower crust anywhere in the Southeast. This density model of eastern North America is fed into finite-element models of gravity-derived stress, which show that this buoyant material does indeed stress the overlying seismogenic crust in a way that encourages extension. By adding a regional, far-field tectonic stress—determined by optimizing fit to 3D stress tensors inverted from focal mechanisms—to this gravity-derived stress, it is possible to estimate spatial variations in long-term stress across the entire region. The ETSZ does host a modest high-stress anomaly, but preliminary models show that this anomaly alone does not account for the high rates of seismicity. Other factors such as low-viscosity lower crust or upper mantle, upper mantle dynamics, inherited crustal damage, or focused post-Miocene erosion may also contribute to the high rates of seismicity. Joint Pn/Sn tomography is underway to examine the former two possibilities. The remaining mystery notwithstanding, the ETSZ is demonstrably a distinct geodynamic feature, and future seismicity does appear to be most likely to recur in or near the known seismic zone.

Presenting Author: Will Levandowski

Authors