Resolving S-Wave Velocity Structure from Weak-Motion S-Wave HVSR

Date: 4/26/2019

Time: 08:45 AM

Room: Pine

Quantifying the response of sediments to incident waves is needed for predicting the effects of strong earthquake shaking, which requires determination of the S-wave velocity structure. The importance of accounting for site response increases when the impedance contrast between the sediment overburden and underlying bedrock is strong, which can result in seismic waves being trapped, yielding amplification in excess of that due to the sediment-bedrock impedance contrast alone. The horizontal-to-vertical spectral ratio (HVSR) of weak-motion S-waves can estimate empirical site transfer functions (TF) for frequencies up to a site-dependent frequency, f_max. As determined from several deep-soil borehole sites, f_max is approximately five times the fundamental frequency of vertically propagating SH-waves. Amplification of vertical motions reduces the spectral ratio with respect to TF for frequencies greater than f_max.

We evaluated inversion of S-wave HVSR for S-wave structure at both deep-soil vertical seismic arrays in the northern Mississippi Embayment, USA—VSAP (100 m) and CUSSO (585 m)—at a nearby shallow-soil site, and at a deep-soil KiK-net borehole station. We used a guided Monte Carlo algorithm to optimize the fit between mean S-wave HVSRs and the theoretical responses from Thompson-Haskell propagator matrices for incident SH-waves, for frequencies below f_max, to obtain the best-fitting family of models. We found that (1) a reasonable initial sediment-column thickness is needed, (2) the ensemble median structures from multiple trials are consistent with published velocity models at the deep-soil sites, (3) for these sites, structures are resolvable from simple initial models consisting of uniform layers over a half-space, and (4) in contrast, best-fitting models at the shallow-soil site are inconsistent with the published structure, in part due to the absence of resonance modes above the fundamental mode. Therefore, S-wave HVSR is useful for constraining or resolving shear-wave models of thick, unlithified sediments overlying stiff bedrock.

Presenting Author: Seth Carpenter

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