Earthquake Focal Mechanisms in the New Madrid Seismic Zone

Abstract

Online Material: Table of focal mechanisms; color versions of figures. The occurrence of intraplate earthquakes is largely an unexplained phenomenon. The New Madrid Seismic Zone (NMSZ) lies within the interior of the North American plate and is approximately 2000 km from the nearest active plate boundary. Yet, it has produced sequences of large earthquakes with widespread and coincident areas of paleoliquefaction features: A.D. 900±100, A.D. 1450±150, and 1811, 1812 (Tuttle et al. , 2002). Several theoretical driving mechanisms of NMSZ intraplate earthquakes have been proposed (e.g., Grana and Richardson, 1996; Liu and Zoback, 1997; Kenner and Segall, 2000; Grollimund and Zoback, 2001; Pollitz et al. , 2001; Forte et al. , 2007; Tavakoli et al. , 2010; Calais et al. , 2010). A short summary of these driving mechanisms is available in Powell and Horton (2009). Our aim is to contribute earthquake focal mechanism solutions for the study of seismotectonics in this region. The dominant trends in strike of the nodal planes are compared with the orientations of major structures in the region. We also estimate the variation in crustal stress via stress inversion in the northern Mississippi embayment. The focal mechanism results from this study (Johnson, 2008) show a possible rotation of the regional horizontal compressive stress. For future studies, these results may serve as a constraint to physical models of deformation. Focal mechanisms in this study are grouped into four areas, herein referred to as segments, which relate dominant trends of focal mechanism nodal plane strike to localized areas of seismicity (Fig. 1). Two of these segments strike northeast–southwest and lie at the northern and southern ends of the NMSZ, having nearly vertical dip. These will be referred to as the northeastern segment (northern end) and southwestern segment. They are characterized by mainly strike‐slip events and …