Strong Ground Motion Site Effects in the Central United States: Issues and Alternatives

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Ground-motion site effects are of concern in the central United States, particularly for communities sitting on thick sediments in the Mississippi and Ohio River Valleys. Site effects were found to be the cause of damage in Maysville, Ky., from the 1980 Sharpsburg earthquake, for example. Since the early 1990s, we have collected data necessary for quantifying site effects, including shear-wave velocities, borehole logs, and recordings of ambient noise and local earthquakes; these data have enabled us to conduct thorough studies on site effects for various locations in the region. Our results indicate that the key parameters influencing site effects are shear-wave velocity structure, unlithified soil-column thickness, damping, nonlinear response of sediments, and bedrock shear-wave velocity. We also found no clear relationship between V_s30 (average shear-wave velocity for the top 30 m) and the site characteristics (i.e., transfer function), which are determined by those key parameters and the impedance contrast between bedrock and the sediment overburden. Thus, V_s30 is not an appropriate parameter for quantifying site effects in in the central United States.

We have found that horizontal-to-vertical spectral ratios (HVSR) of weak-motion S-waves from earthquakes approximates site transfer functions at frequencies up to the fifth resonant mode, and can be used as an alternative for predicting site response in the central United States. This low-cost and accurate technique is a means to quantify the response of the entire unlithified soil column, even at deep-soil (> 100 m) sites. We also found that the S-wave HVSR demonstrates characteristics of nonlinear response: Resonant frequencies decrease with increasing ground motions at strong-motion stations in China and Japan. Thus, the S-wave HVSR can be used as an empirical transfer function.

Presenting Author: Zhenming Wang

Authors