Q(P)-Q(S) RELATIONS IN THE SEDIMENTARY BASIN OF THE UPPER MISSISSIPPI EMBAYMENT USING CONVERTED PHASES

Three-component digital seismograms recorded by the 40 PANDA (Portable Array for Numerical Data Acquisition) stations in the New Madrid seismic zone were analyzed to study seismic wave attenuation in the sedimentary basin using the spectral ratio method. A prominent S-to-P (S(p)) converted phase was generated at the boundary between the uppermost sedimentary basin and the underlying Paleozoic rocks. The direct S wave on the horizontal component is characterized by a lower-frequency content than that of the converted S(p) wave on the vertical component. The differences in frequency content between the direct S and the S(p) converted waves can be attributed to the different attenuation effects between P and S waves in the unconsolidated sediments. The spectral ratio between the low-frequency S wave and the high-frequency converted S(p) wave from the bottom of the sediments can be used to yield a relationship between Q(p) and Q(s) in the sediments. Results from PANDA stations with well-constrained spectral ratios in the frequency range from 2 to 25 Hz give the Q(p) value ranging from 25 to 60 and Q(s) from 25 to 30 for the sedimentary basin in the upper Mississippi Embayment.