Reflection and Transmission Imaging of the Upper Crust Using Local Earthquake Seismograms

We utilize the characteristic features of primary P- and SH-wave coda, and Sp waveforms from local microearthquake data and perform pre-stack migration in an attempt to image prominent reflectors in the upper crust of the New Madrid Seismic Zone (NMSZ. Our case study uses data from a broadband station, PARM, of the Cooperative New Madrid seismic Network (CNMSN). Near-by exploration well log data of the Wilson 2-14 and Dow Chemical/Wilson #1 wells are used to constrain the upper 4 km of the velocity model in the region. A constant velocity of Vp 2.50 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 Paleozoic sedimentary rocks can represent the structure beneath PARM. Despite polarity differences among P-, SH-, and Sp waveforms, seismic images show consistent reflectors common among the three wave types. There are excellent correlations associated with the base of the upper Cretaceous/Holocene Mississippi Embayment Supergroup and the base of Knox group, which were also reported in a study of near-by seismic-reflection profiles by Hamilton and Zoback (1982). The Bonneterre Formation seems to be a prominent seismic stratigraphic marker associated with an interface displayed in the profiling. We find that earthquake event S-P times must be ~3 sec or more to resolve reflectors at about 4 km depth. The basement occurs at about 4 km and appears on the reflected P- and SH-wave images.