Three-Dimensional Geometry of the Reelfoot Blind Thrust: Implications for Moment Release and Earthquake Magnitude in the New Madrid Seismic Zone

Mapping of the Reelfoot blind thrust using portable array for numerical data acquisition (PANDA) seismicity suggests that it is a complex fault that changes its geometry along strike. The thrust appears to be bounded to the north by an east-trending strike-slip fault. The southern end of the thrust is defined by seismicity and does not terminate at a known transverse fault. The northern portion of the thrust steepens at shallow levels, forming a listric (concave upward) shape in cross section. The southern segment of the thrust is interpreted to flatten near the top of the Precambrian basement. Although some segmentation of the blind thrust is observed from the mapping of 3-km-wide strips of seismicity oriented perpendicular to the fault, it does not appear to be significant enough to prevent rupture along its entire length. The area of the blind thrust (1301 km 2 ), coupled with several estimates of displacement in the 7 February 1812 event, is used to determine the moment released during the event. The values of M 0 range from 6.8 × 10 26 to 1.4 × 10 27 dyne cm, with preferred values between 6.8 × 10 26 and 8.7 × 10 26 dyne cm. Similar calculations for an earthquake in A.D. 1450 yield a moment release of 1.0 × 10 27 dyne cm. Computed moment magnitude for the 7 February 1812 event ranges from M w 7.2 to 7.4, with preferred values between M w 7.2 and 7.3. The moment magnitude for the A.D. 1450 event is computed as M w 7.3. These values for the magnitude of the 1812 earthquake are lower than previous estimates based on historical records of shaking. However, this does not imply a lowered seismic risk in the New Madrid seismic zone, as site effects in the Mississippi embayment may significantly amplify the ground motion caused by earthquakes of a given magnitude.