INVESTIGATING THE PRESCENCE OF HISTORICAL SEISMICITY'S EFFECT ON LANDSCAPE EVOLUTION IN THE CENTRAL VIRGINIA SEISMIC ZONE, USA, USING FLUVIAL STRATIGRAPHY AND GEODESY

The passive east coast margin of North America has been historically characterized as being seismically quiet, with few earthquakes resulting in significant deformation or building of topography. However, the region has demonstrated its propensity for substantial seismic events, the most recent example being the 2011 M5.8 Mineral, VA earthquake. Although well-defined by the aftershock swarm, the previously unknown fault plane, which strikes N36°E and dips 49.5° SE with up to the SE reverse motion, does not exhibit any apparent surface rupture, leading us to hypothesize that it is a blind ramp coring an actively growing fault-propagation fold. Rates of deformation on the U.S. East coast may be slow enough, that rock uplift could be closely balanced by weathering and erosion, resulting in the masking of clear surface deformation. However, fluvial terraces preserved in the landscape are stratigraphic and geodetic markers, recording the presence and magnitude (if dated) of post-depositional deformation. Several flights of fluvial terraces are well preserved along the South Anna River that fortuitously flows above the Mineral earthquake epicenter, orthogonal to the relevant fault. Our study involves detailed (1:24,000) mapping and descriptions of fluvial deposits underlying terraces along ~40 km of the South Anna River with respect to the river’s current profile. Terraces are characterized by their morphology, texture, composition, provenance, pedogenic development, and elevation above local base level. A suite of OSL samples (pending) were collected from these terraces to aid in correlation and quantify river incision rates. We explore three possible patterns of terrace correlation that test the growing fold hypothesis: (1) the terraces descend downstream steeper than the modern river profile; (2) terraces descend downstream more gently than the river profile; or (3) terraces are sub-parallel with the modern profile. Correlations (1) and (2) are further modeled assuming a tri-shear geometry, constrained by the aftershock data. The results of this study are an integral part of the effort to characterize the historic seismicity in central Virginia and elsewhere on the U.S. East coast.