Paper No. 182-9
Presentation Time: 10:25 AM
GEOMORPHIC PALEOGEODESY AND INTRAPLATE DEFORMATION ASSOCIATED WITH THE MINERAL, VA EARTHQUAKE AND SURROUNDING CENTRAL VIRGINIA SEISMIC ZONE (CVSZ)
The 2011 M 5.8 Mineral, VA earthquake was a sobering reminder that the central and eastern U.S. (CESU) plate interior deforms, but unlike plate boundaries, plate interiors lack a systems-level model that describes their various components, deformation processes, process linkages, and feedbacks. Similarly, we know little about the sources of seismic stresses and why earthquakes appear to be concentrated in specific zones of low-level, but persistent activity such as the CVSZ. Earthquakes deform the crust and as this deformation propagates to the surface it should be geodetically recorded. Lying in the Piedmont of Virginia, the CVSZ is traversed by numerous entrenched bedrock streams flanked by Pleistocene fluvial terraces, geomorphic markers commonly used for geomorphic paleogeodesy in active tectonic regions. Detailed surficial mapping along the South Anna River in the Mineral epicentral region has defined a middle-late Pleistocene fluvial stratigraphy and channel morphologic changes that are distinct in the hanging wall and footwall of the fault that ruptured during the earthquake. River terraces are defined using textural, compositional, and soil stratigraphic criteria. OSL and IRSL geochronology provides ages for a key, ~70 ka terrace that we use as the anchor of a regional correlation that is further informed by dated terraces on the nearby James River and upland gravels preserved on the inner Coastal Plain. The long term incision rate in the footwall of the fault that ruptured during the Mineral earthquake is ~40-50 m/Ma, indistinguishable from the background, regional Pleistocene rate of Piedmont fluvial incision. However, in the hanging wall, the incision rate may be as much as double at ~90-100 m/Ma and the terraces are arched up and over the surface projection of the fault plane that ruptured in 2011. The Mineral earthquake has been geodynamically modeled to have generated ~ 0.07 m of surface deformation distributed over a wavelength consistent with the terrace arch. Generating ~50 m of incision in the hanging wall in a million years would require ~700 Mineral-sized events, with a mean recurrence interval of ~1500 years. Although crude and carrying huge uncertainties, these estimates offer some insights into the rates and scale of stress concentration and release in the CEUS.