Abstract

Data from the Portable Array for Numerical Data Acquisition (PANDA) deployment in the central part of the New Madrid Seismic Zone have been used to study the attenuation of body waves. The spectral decay technique is used to analyze acceleration amplitude spectra of both P and S waves in a frequency range of 5 to 25 Hz in order to obtain the high frequency attenuation parameter κ. A multiple, weighted least-squares method is applied to obtain the site attenuation factor κ0 at each station and to get dκ/dr, which were interpreted in terms of a two-layer Q structure consisting of a sedimentary layer overlying the upper crust. The κ0 values for S waves are stable and show a positive correlation with sediment thickness. The κ0 values for P waves in the sediments show significant scatter, which may be due to greater sensitivity of P-wave attenuation to water content of the soils than that of S waves, which gave more stable κ0 values.

Consistency checks on the assumptions made in obtaining the κP and κS values are made by studying the source spectrum of several events and the differential κSp/S from the spectral ratio of the converted Sp to S phases. The three sets of values are found to be internally consistent, but our results are limited to deep sediment sites, since the complete vertical distribution of Q with sediment depth is not known.

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