Local earthquake wave propagation through Mississippi embayment sediments, Part II: Influence of local site velocity structure on Qp-Qs determinations

Abstract P and S body waves from microearthquakes in the New Madrid Seismic Zone (NMSZ) are investigated at selected sites in an effort to understand wave prop-agation from future large earthquakes. Earthquake body waveforms display distinc-tive features that constrain the nature of P- and S-wave local site responses and wave propagation within the unconsolidated Mississippi embayment sediments. Modeling of the waveforms demonstrates that a near-surface low-velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large P-and S-wave resonance effects that can be used to infer near-site conditions. Site resonance effects change because of velocity heterogeneity between individual re-ceivers but imply that embayment sediments will substantially amplify ground mo-tions at high frequencies. Site resonance affects P- and S-wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters. Travel times of observed body-wave phases such as P, PpPhp (the first P-wave reverberation within the entire sedimentary column), Ps, Sp, S, and SsPhp can be used to estimate the average wave slownesses and Poisson’s ratio within the embayment sediments; an average Poisson’s ratio of 0.44 is obtained for the central NMSZ under station PEBM. Detailed S-wave velocities are derived for a Nafe–Drake sediment model using acoustic well logs and the travel-time constraints of observed seismic phases. Vp/Vs ratios vary from 5.5 near the surface to approximately 2.4 at the base of the sediments. Use of the well-log data in wave calculations also explains much of the nature of P- and S-wave coda within the waveforms and shows that 1D heterogeneity is a first-order influence on seismic-wave propagation within the Mississippi embay-ment