Local earthquake wave propagation through Mississippi embayment sediments, part I: Body-wave phases and local site responses


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 propagation from future large earthquakes. Earthquake body waveforms display distinctive 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 receivers but imply that embayment sediments will substantially amplify ground motions 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 embayment.

Publication Title

Bulletin of the Seismological Society of America