Treatise on Seismicity and Velocity Structure of the Northern Mississippi Embayment

Seismicity of the New Madrid seismic zone, velocity structure of the northern Mississippi Embayment and extent of lithospheric discontinuities have been investigated in this work. Spatial variations of seismic b-value in the NMSZ have been determined. A region of high b-value (b~1.8) in the northern segment of the Reelfoot fault has been found which reflects higher frequency in the occurrence rate of small earthquakes. This is attributed to fault creep in a medium with high silica content, high pore fluid pressure and extensive velocity heterogeneity. In another study, power spectral ratios of horizontal to vertical (H/V) and vertical to horizontal (V/H) components of teleseismic P-waves recorded by broadband seismic stations inside the Mississippi embayment have been examined to produce a 3-D average velocity map of the sediments. Fundamental resonance frequencies for S-wave reverberations in the northern Mississippi Embayment are about 0.2 to 0.4 Hz. Detailed linear gradient velocity models for Vp and Vs have also been calculated using teleseismic vertical and radial transfer functions at 60 broadband stations inside the Mississippi embayment and simultaneously inverting for velocity parameters. This led to a detailed 3-D linear gradient Vp and Vs model of the sediments for the first time. P-wave velocity starts at about 1.0 km/s near the surface and increases with depth to 3.5 to 4 km/s in deeper parts of the embayment. S-wave velocities vary from 0.3km/s to 1.6 km/s in deeper sections to the southwest. This detailed velocity structure of the unconsolidated sediments is developed so that it could be used to downward continue the transfer functions to the depth of 5 km to remove the effects of observed amplification, reverberation and large P-S conversions that occurs at the sediment-bedrock interface. The resulting waveforms are depth migrated to image the lateral and vertical changes of major reflectors with depth. The Moho boundary is observed with high resolution at depths of 45 to 55 km in the study area