Unexpected Values of Qs in the Unconsolidated Sediments of the Mississippi Embayment

Abstract

We studied the attenuation of shear waves at three sites in the Mississippi embayment using data recorded in boreholes drilled to depths of up to 60 m. The source was a highly repeatable compressed-air-driven hammer. To estimate attenuation we used a spectral ratio technique for fixed depth and variable frequency. The best-fit line for each depth z gives a measure of the cumulative attenuation, indicated by ␣(z). Then we fit a straight line to ␣(z) for a range of values of z. The slope of this line gives an estimate of the average attenuation per distance and was used to determine an average Q s. For one of the sites (Newport, northeastern Arkansas), Q s ranges between 34 (1.5 m Յ z Յ 44.2 m) and 44 (1.5 m Յ z Յ 51.8 m). These values are significantly higher than the more typical value of about 10 determined for unconsolidated sediments by other authors. In addition, these high values correspond to sediments with low average shear-wave velocity (about 300 m/sec). In contrast, average Q s and velocity for sediments in Shelby Forest (near Memphis, Tennessee) are 22 and 348 m/sec (22.6 m Յ z Յ 60.1 m), respectively. Therefore, these results go against the conventional wisdom that low velocity implies low Q. For the third site (Marked Tree, northeastern Arkansas), average Q s and velocity are 18 and 251 m/sec (9.8 m Յ z Յ 33.6 m), respectively. This site is about 75 km from Newport, and the differences in attenuation appear related to differences in lithology.

Site Amplification and Attenuation via Downhole Array Seismogram Inversion: A Comparative Study of the 2003 Miyagi-Oki Aftershock Sequence

“…4) results in an inversely proportional relationship between the two quantities, which in fact contradicts typically reported data of increasing Q s with increasing soil stiffness. Our results, corroborated by recently reported data by Pujol et al (2002) for the Mississippi embayment sediments, lead to the conclusion that the site lithology, stratification, and randomness significantly affect attenuation estimates in the near surface. As a result, velocity-attenuation correlations obtained by means of simplified models simulating the complex multitude physical mechanisms of energy absorption and redistribution in the upper few hundreds of meters of the crust may not be employed to extrapolate attenuation profiles in adjacent sites due to the highly heterogeneous nature of soil materials at these depths.…”

The 1997–1998 Umbria‐Marche sequence (central Italy): Source, path, and site effects estimated from strong motion data recorded in the epicentral area

“…Since Q ( f ) is independent of frequency above 8 Hz, the kappa ( k ) model can be used to describe the high frequency attenuation [ Anderson and Hough , 1984]. The parameter k is a standard parameter for stochastic prediction of ground motion [ Boore and Atkinson , 1987; Beresnev and Atkinson , 1997] and it can be considered a more stable and reliable indicator of attenuation at shallow depth than Q [ Anderson and Hough , 1984; Hough et al , 1988; Pujol et al , 2002]. The parameter k is computed from the high frequency decay of the semi‐logarithm acceleration spectra.…”

Local Earthquake Wave Propagation through Mississippi Embayment Sediments, Part I: Body-Wave Phases and Local Site Responses

“…These include earthquake body-wave studies by Al-Shukri et al (1988), Al-Shukri andMitchell (1990), Liu et al (1994), Cong et al (2000), and Chen et al (1994), a reflection wavefield study by Kang and McMechan (1994), a refracted wave study by Wang et al (1994), and an analysis of VSP S-wave data by Pujol et al (2002). In general, those studies that employed high-frequency P and S waves from local microearthquakes inferred very low values of Q p and Q s (10-30).…”