ACTIVE FAULT ZONES IN THE BLUE RIDGE OF WESTERN NORTH CAROLINA

At least two prominent topographic lineaments correspond to zones of seismicity in the Blue Ridge of western North Carolina. The Laurel Creek lineament trends east-west and runs from Spruce Pine, NC to Hot Springs, NC. There have been four earthquakes with magnitudes greater than 2.5 located on or near this lineament in the past 1.5 years as well as earthquakes with larger magnitudes in the past. The Swannanoa lineament trends WSW-ENE and can be traced from east of Asheville, NC (and maybe as far east as Hickory, NC) to Fontana Dam near the Tennessee state line to the west. The largest historic earthquake in North Carolina occurred on or near this lineament (1916, magnitude 5.2) and more recent, smaller-magnitude quakes have occurred along this lineament. An important question is whether the lineaments represent the surface traces of the seismogenic faults or whether they are related to earlier episodes of deformation. To address this question we have collected and analyzed fracture data, minor fault populations, and fracture-controlled stream orientations. We have also mapped key geologic contacts across both lineaments to estimate slip. These data indicate that the lineaments correspond to zones of intense fracturing, with fracture orientations that are parallel or subparallel to the trends of the lineaments (roughly east-west). Fractures outside of the lineaments trend primarily NW-SE and NE-SW. The presence of extensional fractures along the lineaments combined with the lack of strike-slip separation of contacts across the lineaments suggests the lineaments correspond to extensional faults. The focal plane solution for the August 25, 2005 Hot Springs, NC earthquake (M=3.8) is consistent with a normal fault that strikes parallel to the dominant fracture set within the lineament. It appears that the lineaments are the surface expressions of low-seismicity, but perhaps long-lived, active fault zones.