| Two methods, reflection and transmission imaging and waveform inversion modeling, have been developed and applied to analyze broadband waveforms from local earthquakes to explore shallow crust velocity structure in the upper Mississippi embayment. Pseudo-reflection profile images obtained from the primary, reflected, and transmitted P- and S-coda, and from S to P converting wave are interpreted. The upper 4 km crustal velocity structure beneath a seismic station PARM can be represented by a Vp=2.165 km/sec and Vs=0.70 km/sec for the unconsolidated sediments and Vp=6.0 km/sec and Vs=3.2 km/sec for the underlying Paleozoic and older sedimentary rocks. Beneath station PENM the velocity values are Vp=2.15 km/sec and Vs=0.615 km/sec for the unconsolidated sediments and Vp=6.0 km/sec and Vs=3.4 km/sec for the Paleozoic and older sedimentary rocks. This study indicates that earthquake event S-P time must be ≥3 sec to resolve reflectors at about 4 km depth. A possible basement interface at about 4.0 and 4.5 km appears on the reflected P-wave image for PARM and the S-wave image for PENM, respectively.;High frequency reverberations whose amplitudes decay with time characterize the P and S waveforms from local microearthquakes. Incorporating geotechnical data such as VSP and SCPT field experiments at nearby sites, a gradient velocity structure for the near-surface soils beneath PARM can be defined. A good starting model is attained using the grid testing approach. Four types of joint P and S wave inversions are performed to fit the timing and amplitude of the observed P and S displacement seismograms. The inversions show that much of the site response can be explained by a near-surface gradient velocity structure represented by 7 thin layers with thicknesses of 1 m. This demonstrates that the high-frequency reverberations observed on P and S waveforms can be used to gain insight into near-surface soil structure through wave propagation effects. Both methods are suitable for determining high-resolution structural information for the near-surface soils and the deep sedimentary basin. |
