EVIDENCE FOR CONTEMPORARY VERTICAL FAULT DISPLACEMENT FROM PRECISE LEVELING DATA NEAR THE N.pdf

Bulletin of the Seismological Society of America, Vol. 71, No. 6, pp. 1933-1942, December 1981 EVIDENCE FOR CONTEMPORARY VERTICAL FAULT DISPLACEMENT FROM PRECISE LEVELING DATA NEAR THE NEW MADRID SEISMIC ZONE, WESTERN KENTUCKY BY F. STEVE SCHILT AND ROBERT E. REILINGER ABSTRACT Relative vertical displacements of bench marks in extreme western Kentucky have been determined by comparison of successive leveling surveys in 1947 and 1968. The resulting pattern of apparent surface deformation shows a steep offset which can be closely modeled by a normal fault buried in an elastic half- space. The offset is located near the northern boundary of the Mississippi Embayment and the New Madrid seismic zone, an area where faults have previously been inferred on the basis of both geological and geophysical evi- dence. If the apparent movement is due to slip along a fault, several lines of evidence (regional structure, earthquake data, and lineations) suggest that the postulated fault trends NNE. Thirteen earthquakes were recorded in this area between the times of leveling; focal mechanisms exist for three of these. The nearest of these three focal mechanisms to the leveling offset implies normal faulting. The magnitude of the earthquake, however, appears to be too small to account for the amount of slip required by the fault model. Thus the apparent deformation may have accumulated with several undetected small earthquakes, or gradually as aseismic creep. INTRODUCTION The purpose of this study is to examine the possibility that anomalous surface deformation near the northern end of the Mississippi Embayment, as measured by repeated precise leveling surveys, was caused by movement along a buried fault. The area studied is between Wickliffe and Paducah, Kentucky, and the leveling data, when supplemented with other geological and geophysical information, suggest dip-slip displacement along an unmapped, concealed fault. Brown and Oliver (1976) have compar