A Two-Dimensional Numerical Model for Current Seismicity in the New Madrid Seismic Zone

Abstract

In order to understand the mechanics of intraplate earthquakes better, a simple 2D numerical model was developed to try to explain current seismicity in the New Madrid seismic zone, using a distinct element method. The model comprises a block geometry representing the structural framework of the New Madrid seismic zone, consisting of intersecting faults with elastic properties corresponding to the known geology. The blocks were subjected to tectonic loading for four days along the direction of the maximum horizontal stress field and the resulting patterns of stress and strain distributions were studied. The results of the modeling showed that shear stresses were higher within the Reelfoot Rift than outside it. In this 2D model the shear stresses on the horizontal plane gave a sense of rotation of the modeled blocks, and an implied sense of movement on the faults. They duplicated the right-lateral strike-slip movement along the Blytheville Fault zone and New Madrid North Fault, and left-lateral strike-slip movement along the Reelfoot Fault. Due to the two-dimensional nature, however, results of modeling do not show the observed reverse motion along the Reelfoot Fault. The observed seismicity pattern was consistent with the amplitudes and signs of the maximum shear stresses along the major faults located within the Reelfoot Rift. A linear extrapolation of model results gave an annual strain rate consistent with geodetic observations. The results of modeling support the idea that in a localized volume of pre-existing weak crust, fault intersections act as stress concentrators and cause anomalous stress build-up in their vicinity, resulting in observed seismicity.

Finding Faults in the Charleston Area, South Carolina: 2. Complementary Data

“…In Figure 14A we note that Woodstock lies near the intersection of the northeast extension of the WF(S) with the southeast extension of the Lincolnville fault. Fault intersections are known to be stress concentrators (see, e.g., Gangopadhyay et al 2004). So the observed vertical movements at Woodstock together with an extensive belt of craterlets west of Woodstock along a S80°W oriented ridge could be manifestations of the release of stress building at the intersection of these faults that resulted in vertical movements along the Lincolnville fault and southwesterly horizontal motion along the WF(S).…”

Relocation of Earthquakes in the New Madrid Seismic Zone: Estimation of 1D Velocity Structure and Geometry of a Seismogenic Fault

“…The seismic activity of the NMSZ is strongly associated with the intrusion of plutonic complexes (Hildenbrand and Hendricks, 1995). To explain the mechanism of intraplate earthquakes in the central United States, several studies based on the stress field in the NMSZ have been proposed (Sbar and Sykes, 1973;Sykes, 1978;Zoback and Zoback, 1981;Braile et al, 1982Braile et al, , 1984Gomberg, 1993;Gangopadhyay et al, 2004;Li et al, 2005;Johnson et al, 2014). Earthquakes within the embayment indicate a strikeslip fault system dominated by an approximate east-west compression due to asthenospheric viscous drag and/or ridge push (Zoback and Zoback, 1980).…”

A possible mechanism for the spatial distribution of seismicity in northern Gulf of Mexico

“…In our models, we follow Tuncay & Ortoleva (2001) and use these representative values for the salt bodies. For the sediments surrounding the salt bodies, we compute the bulk modulus using generic formulae described in detail in Gangopadhyay et al (2004), wherein we incorporate the density, and Poisson's ratio. In an in situ evaluation of the response of seafloor sediments to passive dynamic loads at a site in the GOM (∼27.3 • N, 94.3 • W) near the epicentral region of the 1978 July 24 earthquake, Huerta-Lopez et al (2003) obtained an average shear modulus of 13.23 GPa, density of 1781 kg m -3 , and Poisson's ratio of 0.32 for the sediments.…”

“…In an in situ evaluation of the response of seafloor sediments to passive dynamic loads at a site in the GOM (∼27.3 • N, 94.3 • W) near the epicentral region of the 1978 July 24 earthquake, Huerta-Lopez et al (2003) obtained an average shear modulus of 13.23 GPa, density of 1781 kg m -3 , and Poisson's ratio of 0.32 for the sediments. Using generic formulae described in Gangopadhyay et al (2004), we obtain the bulk modulus for the sediments as 33.65 GPa. We use these values of bulk and shear moduli and density for the surrounding sediments in our models.…”

“…To our knowledge site-specific mechanical properties for these structures are unavailable. Therefore, for these we use normal and shear stiffnesses (101 and 76 GPa m -1 , respectively) based on laboratory studies of typical rock samples following Rosso (1976), as detailed in Gangopadhyay et al (2004). We also assign a friction value of 0.5 to these structures, adapted from experimental results on typical rock types reported by Barton (1976), and consider them to be cohesionless.…”