Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee

We measured site resonance periods throughout metropolitan Memphis, Tennessee. The site-resonance was derived from recordings of ambient ‘microtremor’ and is a direct measurement of seismic ground shaking. In the Memphis metropolitan area, the period, T0 (in s) is predicted by the empirical relation T0=−0.39899+7.8922×10−3H−3.0634×10−6H2, [350<H<1100 m], where H is the thickness (in m) of the unconsolidated sediments that overly hard Paleozoic rocks. Assuming that a quarter-wavelength relationship governs the relationship between resonant period, sediment thickness, and average shear wave speed, we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses: Vs=521.15+0.37459H [350<H<1100 m]. The scatter in the observations, and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work.