New Madrid strain and postseismic transients



A crucial issue for the assessment of earthquake hazard in the New Madrid Seismic Zone (NMSZ) of the central United States is whether the small motions inferred from geodetic measurements are actually the result of strain accumulation that will eventually be released in damaging earthquakes. The interpretation of these measurements has led to an ongoing debate over the associated seismic risk and hazard assessment in the NMSZ [Zoback, 1999; Schweig et al., 1999; Newman et al., 1999a, 1999b; Stein et al., 2003]. The gist of the debate is whether or not models of high seismic hazard in this region are supported by the geodetic data and historic earthquake data.

A recent report by Smalley et al. [2005] on GPS measurements across the Reelfoot fault suggested a relatively high strain rate, of the order of 10−7 per year, comparable to that normally associated with convergence at plate boundaries. To some, these measurements seemingly ended the debate since they were taken to be the result of rapid strain accumulation that could unleash a large, devastating earthquake, which in turn prompted a general public warning from a top U.S. Federal Emergency Management Agency (FEMA) official [Brown, 2005]. Others, however, believe that the debate is not yet settled [Calais et al., 2005]; it has been argued that these GPS measurements show no statistically significant motion and instead reveal a puzzling offset in one of the GPS time series [Calais et al., 2006].