Site amplification within the Mississippi embayment of the central United States: Investigation of possible differences among various phases of seismic waves and presence of basin waves

Abstract

The impact of unconsolidated sedimentary basins on ground motion amplification is of particular interest for earthquake engineers and seismologists. The Mississippi embayment (ME) of the New Madrid seismic zone, located in the central United States, is covered with a thick layer of unconsolidated soil deposits. Thus, the estimation of site response in this region is vital to simulate site-specific ground motions and to conduct site-specific probabilistic seismic hazard analysis. We evaluated site amplification at 11 stations within the ME, employing the horizontal-to-vertical spectral ratio (HVSR) technique.Regarding the results obtained from this study, weak ground motions recorded by stations on the unconsolidated ME sediments are amplified 3–7 times for frequencies less than 5 Hz compared to stations located on bedrock. The fundamental resonant frequencies vary from 0.2 to 0.4 Hz within the ME. We investigated differences between the HVSRs obtained from P-waves, S-waves, coda, and pre-event noise. All fundamental frequencies obtained from different seismic phases are in good agreement with a less than 10% difference. The fundamental frequencies of the P-wave and S-wave are relatively higher due to higher velocity of the P-wave and S-wave compared to other phases since the velocity of seismic waves and fundamental frequencies are proportional. There is a good correlation between the HVSR of the S-wave, coda, and pre-event noise portions for frequencies more than 4 Hz. For the frequencies less than 4 Hz, the HVSR of the S-wave is higher than the HVSR of coda by a factor of 3. Reflections of S-waves from the edges of the unconsolidated sedimentary basin of the ME produce surface waves. The presence of basin-induced surface waves in the coda portion for frequencies less than 3 Hz results in increased amplitude of coda, and as a result, slower decay rate with time implying higher Q values. These basin-induced surface waves have a period of 0.5–4.0 s.