Kc. Chen et al., SHEAR-WAVE VELOCITY OF THE SEDIMENTARY BASIN IN THE UPPER MISSISSIPPIEMBAYMENT USING S-TO-P CONVERTED WAVES, Bulletin of the Seismological Society of America, 86(3), 1996, pp. 848-856
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.