Regional Stress Orientations and Slip Compatibility of Earthquake Focal Planes in the New Madrid Seismic Zone

The New Madrid seismic zone (NMSZ) in the central United States is one of the most active regions of intraplate seismicity in North America and site of the devastating 1811–1812 earthquake sequence (Nuttli, 1973; Johnston and Schweig, 1996). The NMSZ lies within the NE–SW trending Reelfoot rift (Fig. 1), which represents a failed rift arm that developed during the Late Proterozoic and Early Cambrian opening of the Iapetus Ocean on the southeast margin of early North America (Ervin and McGinnis, 1975). Fault offsets inferred from seismic reflection and trench data suggest that current seismicity levels likely initiated during the Holocene (Pratt, 1994; Schweig and Ellis, 1994; Van Arsdale, 2000) when optimally-oriented Proterozoic and Cambrian faults were reactivated in the contemporary stress field (Zoback et al., 1980; Braile et al., 1986; Dart and Swolfs, 1998). The recurrence of large and potentially damaging earthquakes in late Holocene time (Tuttle et al., 2005) may be a response to Pleistocene deglaciation (Grollimund and Zoback, 2001; Calais et al., 2010). Contemporary seismicity illuminates a complex network of fault trends and deformation styles within the NMSZ (Stauder et al., 1976; Andrews et al., 1985; Himes et al., 1988; Chiu et al., 1992; Liu, 1997; Pujol et al., 1997; Mueller and Pujol, 2001; Dunn et al., 2010), although the seismic zone is characterized by three primary faults; the southern, NEstriking Axial fault, the central, NNW-striking Reelfoot fault, and the northern, NE-striking NewMadrid North fault (Johnston and Schweig, 1996; Baldwin et al., 2005; Csontos and Van Arsdale, 2008). The Axial and New Madrid North faults exhibit strike-slip motion on near-vertical fault planes while thrust motion characterizes the southwest-dipping Reelfoot fault (Liu, 1997; Saint Louis University Earthquake Center). The fault patterns and senses of offset, which have been interpreted as a dextral strike-slip fault system with a left-stepping crossover (Russ, 1982), are geometrically consistent with those expected for active faults in an ENE-WSW compressive stress field (Zoback and Zoback, 1981). What remains to be examined in detail, however, is whether or not the faults and senses of offset are frictionally consistent with slip in the current stress field. In this paper, we investigate whether or not local stress sources or anomalous fault strengths are required to explain active NMSZ faulting. Twelve well-constrained individual earthquake focal plane mechanisms augment previously available stress information and allow us to examine the frictional consistency of NMSZ fault planes. Following the methodology described in Zoback (1992), the new focal plane mechanisms are used to update the regional stress map, evaluate the consistency of maximum horizontal compressive stress (SHmax) orientations, and investigate fault stability using the Mohr–Coulomb failure criterion.