Unexpected values of Qs in the unconsolidated sediments of the Mississippi embayment


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 α(z). Then we fit a straight line to α(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 Qs. For one of the sites (Newport, northeastern Arkansas), Qs ranges between 34 (1.5 m ≤ z ≤ 44.2 m) and 44 (1.5 m ≤ z ≤ 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 Qs and velocity for sediments in Shelby Forest (near Memphis, Tennessee) are 22 and 348 m/sec (22.6 m ≤ z ≤ 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 Qs and velocity are 18 and 251 m/sec (9.8 m ≤ z ≤ 33.6 m), respectively. This site is about 75 km from Newport, and the differences in attenuation appear related to differences in lithology.

Publication Title

Bulletin of the Seismological Society of America