STREAM PROFILES OF SOUTH ANNA RIVER TRIBUTARIES TO INVESTIGATE POSSIBLE LONG-LIVED SEISMICALLY INDUCED UPLIFT IN THE CENTRAL VIRGINIA SEISMIC ZONE

Significant intraplate seismic events in the eastern United States are infrequent and not fully understood. The most recent major event in the Central Virginia Seismic Zone was the 2011 Mw 5.8 Mineral earthquake, which was the most felt seismic event in US history (Carter et al., 2012). The associated reverse fault plane (as defined by USGS focal mechanism and the aftershock swarm) has a surface projection that trends perpendicular to the southeast-flowing South Anna River. Stream profiles of nine tributaries of the South Anna River were created using 1 meter resolution LiDAR data to examine the possibility of topographic changes caused by previous seismic activity in the area. Data processing and extraction was done using ESRI ArcMap 10.1, plotting of stream profiles was completed with R 3.1.2 software package.

Analysis of knickpoints in the stream profiles as well as sinuosity, relief, and slope calculations were performed in order to determine if previous uplift related to prolonged local seismicity has caused significant topographic changes between tributaries in the footwall and the hanging wall. Data analysis revealed a prominent knickpoint population at ~85-89 meters (280-290 feet) above sea level in tributaries in both the hanging wall and footwall of the fault that ruptured in 2011. Importantly, the knickpoint set is not controlled by the underlying bedrock composition. The consistency of equal elevation knickpoints suggests that no major difference in uplift has been recorded across the fault after the formation of the knickpoint set.

Our study reveals higher relief and slope values of tributaries in the hanging wall compared to those in the footwall. This could indicate disparate processes between the hanging wall and footwall, but must be interpreted while considering that only two tributaries in the footwall were analyzed due to the LiDAR coverage area. Until more tributaries in the footwall are examined, the slope and relief metrics in this study can only hint at the possible steepening of topography in the hanging wall of the reverse fault active in 2011.