THE AUGUST 25, 2005 HOT SPRINGS, NORTH CAROLINA EARTHQUAKE: EXTENSIONAL FAULTING IN THE DETACHED ALLEGHENIAN OVERTHRUST BELT OF THE SOUTHERN APPALACHIANS

CHAPMAN, M.C., Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, mcc@vt.edu.

A magnitude (Mw) 3.7 earthquake occurred in the Blue Ridge of western North Carolina at 03:09:42.0 UTC Aug. 25, 2005. The epicenter determined by CERI and reported by NEIC is 35.88N, 82.80W, depth 8 km. The author estimated the hypocenter at 35.887N, 82.803W, depth 6.0 km, 03:09:41.7 UTC, using a velocity model developed by Vlahovic and others. The nearest station was at 78 km, and the station gap was 138 deg. The error estimates are 1.1 km (seh max) and 1.6 km (sez). A focal mechanism based on polarities at 31 stations indicates predominately normal faulting on E-W trending nodal planes (NP1: strike N117E; dip 44 deg; rake -60 deg; NP2: strike N259E; dip 53 deg; rake -115 deg). The trend and plunge of the P and T axes are N109E, 69 deg; N7E, 5 deg, respectively. This mechanism is similar to that determined from an inversion of waveform data by Dr. Robert Herrmann at St. Louis University (NP1: strike N90E; dip 60 deg; rake -60 deg; NP2: strike N221E; dip 41 deg; rake -131 deg). Herrmann finds Mw = 3.65 and depth 8 km. Surface reflections pPmP, sPmP, pPn and sPn were identified on some recordings at ranges greater than 115 km. A least-squares fit to the arrival time differences indicates a depth of 5.5 km. The shallow depth and sub-horizontal T axis contrast with previous findings from the eastern Tennessee seismic zone (ETSZ) to the west. Shocks in the ETSZ (in the Valley and Ridge geologic province) occur in the depth range 5 to 22 km with E-NE trending, subhorizontal P axes (predominately strike-slip). The epicenter is 2 km east of Hot Springs, noted for its thermal springs. The town lies in the water gap cut by the French Broad River as it flows through the Blue Ridge from SE to NW. The surface geology is complex. The epicenter plots on an E-W trending fault, previously interpreted as part of a thrust window. The thickness of the detached Precambrian and Paleozoic rocks here is 8 km, inferred from seismic reflection profiling.