Local Earthquake Wave Propagation through Mississippi Embayment Sediments, Part II: Influence of Local Site Velocity Structure on Qp-Qs Determinations

Elastic, near-surface structure and receiver depth effects are seen to control body-wave amplitude spectra in the band from 2 to 50 Hz for local earthquake observations in the New Madrid Seismic Zone. P- and S-wave spectral peaks are created by resonance in shallow low-velocity zones, and spectral nulls are created by interference of direct and free-surface reflections due to receiver depth. These wave propagation effects dominate Sp/S spectral ratios and mimic anelastic attenuation effects by making it appear that S waves are deficient in higher frequencies relative to Sp waves. There is no evidence from Sp/S spectral ratios that unconsolidated sediments of the Mississippi embayment are highly attenuating. Dominance of P- and S-wave resonance suggests that the unconsolidated sediments amplify rather than attenuate seismic waves at high frequency. This has important implications for estimating hazards from strong ground motions in the area. In the absence of any identifiable anelastic attenuation effect in the spectral ratio, the only allowable Qp-Qs relationship for embayment sediments is that Qs = 3Qp, based on consideration of wave travel times.