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Three-dimensional attenuation structure in and around the New Madrid seismic zone

DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES SAINT LOUIS UNIVERSITY, SAINT LOUIS, MISSOURI 63103

Abstract

Digital seismograms from the Central Mississippi Valley seismic network have been used to study the three-dimensional attenuation structure of the New Madrid seismic zone and its surroundings. Attenuation time (t^*) values were calculated for each of 252 seismograms using a spectral decay technique assuming that their values are independent of frequency. The observed t^* values were inverted using a stochastic least-squares inversion scheme to determine a three-dimensional image of the attenuation structure of the upper crust. Homogeneous and heterogeneous starting models of the upper crust produce similar images of that structure. The resulting models include a substantial increase of attenuation within and close to the zones that contain the most active faults of the New Madrid seismic zone. This region was previously found (Al-Shukri and Mitchell, 1988) to be characterized by lower velocities than surrounding regions. The reduced velocities and increased attenuation values in the active portion of the New Madrid seismic zone are explained as being caused by fluid-filled cracks in the upper crust. The presence or absence of fluid-filled cracks may govern whether or not faults in this region are active or inactive.

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