SHEAR-WAVE VELOCITY OF THE SEDIMENTARY BASIN IN THE UPPER MISSISSIPPIEMBAYMENT USING S-TO-P CONVERTED WAVES

From mid-October 1989 to August 1992, 40 three-component PANDA (Portab le Array for Numerical Data Acquisition) stations were deployed in the central New Madrid seismic zone. Three-component digital seismograms recorded by the PANDA stations in the region are characterized by (1) the very weak direct S arrivals on the vertical component, which can b e identified unambiguously from the two horizontal components, and (2) at least two prominent secondary arrivals between the direct P and S arrivals, one (S-p) dominant on the vertical component and another (P- s) with smaller amplitude on the two horizontal components. Travel-tim e differences between the S-p and S and between the P and P-s are the same for different earthquakes recorded at the same station but are di fferent at different stations even for the same event. Polarization an alyses of three-component seismograms and travel-time measurements con firm the interpretation that these two secondary arrivals are the P-to -S (P-s) and S-to-P (S-p) converted waves that occur at the bottom of the sedimentary basin beneath each station. Since abundant well-log da ta are available in the upper Mississippi embayment, the thickness of the sediments beneath each seismic station can be estimated. Travel-ti me differences between the direct and the converted waves can be used to calculate average shear-wave velocity for the sediments beneath eac h station, The estimated shear-wave velocities of the sediments beneat h PANDA stations vary from 0.45 to 0.67 km/sec. The higher shear-wave velocity associated with thicker sediments can be interpreted as a con sequence of increasing compaction of unconsolidated sediments due to i ncreasing overburden.