Murray State University

Gravity Modeling in the New Madrid Seismic Zone

Institution

Murray State University

Abstract

The objective of this study was to model the Bouguer gravity of the New Madrid Seismic Zone (NMSZ). This work was intended to investigate the relationship (if any) between modeled gravity and earthquake foci in the NMSZ. Previous gravity work in the NMSZ has been done by Langenheim (1995) and Hildenbrand (1985, 1977). Bouguer gravity within the NMSZ has been modeled by Keller, et al. for Kentucky (1978) and by Johnson and Stearns (1967) for Tennessee. Kellie and MacKay (2002) modeled the geoidellipsoid separation in western Kentucky. Structural investigations in the NMSZ are reported by Crone et al. (1985, 1982, 1979), Grohskopf (1955), and Rabat et al. (1987). Based on this work, it seemed appropriate to use an independent gravity data model and classical terrain corrections over the NMSZ in an attempt to investigate the relationship (if any) between earthquake foci and Bouguer gravity. Modeled gravity for this research was determined for a grid of 600 points using the Surface Gravity Prediction Model of the National Geodetic Survey. Bouguer gravity was then computed using standard correction formulae as provided by Sharma (1986) and Telford (1967). The location of earthquake foci in the NMSZ for M = 2 to M = 5 were obtained from the U.S. Geological Survey. Earthquake foci and Bouguer gravity then were plotted in Surfer (Golden Software, 2003) as a series of isoline maps and surface models. The results of this work show little, if any, correlation between Bouguer gravity, earthquake foci, or the location of the NMSZ as evident from foci plot. The location of a suspected pluton in the northwest quadrant of the NMSZ, as reported by Langenheim (1995) was confirmed, but no other structural features are apparent directly from Bouguer gravity. Work with second derivative analysis was inconclusive. The second derivative plot does correlate with the northwest pluton previously mentioned, but it is otherwise unremarkable. Additional work suggested by this research includes development of a denser gravity grid on which to base Bouguer anomalies. In addition, the second derivative analysis should be refined and apparent isotrophy in the plot examined.

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Gravity Modeling in the New Madrid Seismic Zone

The objective of this study was to model the Bouguer gravity of the New Madrid Seismic Zone (NMSZ). This work was intended to investigate the relationship (if any) between modeled gravity and earthquake foci in the NMSZ. Previous gravity work in the NMSZ has been done by Langenheim (1995) and Hildenbrand (1985, 1977). Bouguer gravity within the NMSZ has been modeled by Keller, et al. for Kentucky (1978) and by Johnson and Stearns (1967) for Tennessee. Kellie and MacKay (2002) modeled the geoidellipsoid separation in western Kentucky. Structural investigations in the NMSZ are reported by Crone et al. (1985, 1982, 1979), Grohskopf (1955), and Rabat et al. (1987). Based on this work, it seemed appropriate to use an independent gravity data model and classical terrain corrections over the NMSZ in an attempt to investigate the relationship (if any) between earthquake foci and Bouguer gravity. Modeled gravity for this research was determined for a grid of 600 points using the Surface Gravity Prediction Model of the National Geodetic Survey. Bouguer gravity was then computed using standard correction formulae as provided by Sharma (1986) and Telford (1967). The location of earthquake foci in the NMSZ for M = 2 to M = 5 were obtained from the U.S. Geological Survey. Earthquake foci and Bouguer gravity then were plotted in Surfer (Golden Software, 2003) as a series of isoline maps and surface models. The results of this work show little, if any, correlation between Bouguer gravity, earthquake foci, or the location of the NMSZ as evident from foci plot. The location of a suspected pluton in the northwest quadrant of the NMSZ, as reported by Langenheim (1995) was confirmed, but no other structural features are apparent directly from Bouguer gravity. Work with second derivative analysis was inconclusive. The second derivative plot does correlate with the northwest pluton previously mentioned, but it is otherwise unremarkable. Additional work suggested by this research includes development of a denser gravity grid on which to base Bouguer anomalies. In addition, the second derivative analysis should be refined and apparent isotrophy in the plot examined.