GEOMORPHIC EVIDENCE FOR PERSISTENT FAULTING CONSISTENT WITH THE 23 AUGUST, 2011 LOUISA COUNTY, VA EARTHQUAKE

The recent Virginia seismic sequence, originated by the M 5.8 Louisa County, VA earthquake in August 2011, offers a singular opportunity, to our knowledge, to integrate seismicity with a long term record of deformed geomorphic markers in the intraplate setting of the eastern U.S. passive margin. Over 340 recorded aftershocks define a clear NE-SW striking SE dipping fault plane extending 7-9 km along strike at depths from 1 to 7 km.

Preliminary geomorphic field work along the South Anna River in Louisa County has mapped evidence for repeated faulting and surface deformation surrounding the illuminated fault plane, as warped (middle-late Pleistocene?) straths and terrace deposits. The South Anna River has several large knickpoints in its long profile. We suspect that these have been created by both far-field base level fall and local faulting. In the area affected by the recent earthquake, the top of a large knickpoint at Byrd Mill (VA Rt 649) is accordant with a low-gradient strath terrace that projects downstream across the epicenter of the recent earthquake, in the Yanceyville area. Continuing downstream for several kilometers beyond Yanceyville, the strath terrace and its thin alluvial cover climbs about 6 m in elevation, ultimately showing a clear downstream divergence from the South Anna channel. This observation is best explained by a history of local uplift of the bedrock consistent with the reverse fault focal mechanism of the recent earthquake. This interpretation is further supported by a higher, older terrace that shows a similar downstream divergence, and by channel form, that changes from low-gradient, low sinuosity in the subsiding footwall to steeper, higher sinuosity in the uplifting hanging wall.