INTRAPLATE FAULTS REVEALED IN CRYSTALLINE BEDROCK IN THE 1983 GOODNOW AND 1985 ARDSLEY EPICENTRAL AREAS, NEW-YORK

Bedrock geologic studies in the epicentral areas of the 1983 Goodnow (m(b) = 5.1) and 1985 Ardsley (m(L) = 4.0) earthquakes in New York State suggest that seismogenic intraplate faults have subtle, but clearly recognizable, expressions in crystalline bedrock along surface extrapolations of these well-defined earthquake ruptures. Both ruptures are small and totally confined to the subsurface. The 1983 Goodnow rupture and the 1985 Ardsley rupture are correlated with the Catlin Lake fault zone and the Dobbs Ferry fault zone, respectively. Although both structures had been recognized by other authors as brittle structures prior to the recent earthquakes, they had not been mapped as faults because of their small accumulated offsets. Our interpretation of these features as the surface expressions of seismogenic structures is based on: (1) the spatial correlation of the rupture plane, as defined by aftershock hypocenters, with a prominent fracture-controlled topographic lineament, and (2) observations of mesoscopic brittle structures along the lineaments that reflect the larger scale structure and, more importantly, are consistent with both the rupture orientation and sense of slip determined from seismic data. In both study areas mesoscopic structures indicate that fault formation at pre-existing joints was important in the overall evolution of the fault zones. The mesoscopic-scale observations are in agreement with studies of fault zone formation and growth in crystalline rock, and provide information on the geometry, segmentation, kinematics, and evolution of the fault zones. Fault zone segmentation seems to be controlled by wavelengths of folds in the pre-existing ductile structure.