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The Influence of Impedance-Ratio Distributions on 1D Linear Site Response Proxies

Description: 

Ground-motion site response is inherently site specific since it depends on in situ dynamic parameters, particularly shear-wave velocity, Vs. Thus, identifying the best proxies to predict site-response remains a challenge. Using Vs profiles developed at 193 central and eastern U.S. sites with underlying lithologies ranging from bedrock to deep (>100 m) soils, we investigated the influence of impedance ratio, IR, distributions on 1D linear site responses at the first peak frequencies, f₀. We categorized each site as “gradient”, “single-layer”, or “multilayered”, depending on the number of Vs-profile interfaces with IR≥3.0, where gradient sites lack any such interface. We then evaluated correlations between various Vs-profile attributes with f₀ and corresponding amplifications, A₀, both calculated from 1D linear site-response analyses. Depth-based attributes, Z_1.0, Z_2.5, and Z_IRmax, the depths to velocities of 1.0 km/s, 2.5 km/s, and the maximum IR, IR_max, respectively, correlate well with f₀ at single- and multilayered sites, whereas Vs30, the average Vs in the upper 30 m, is the only strong predictor of f₀ at gradient sites. Vs30 also correlates with f₀ at other site types but is unreliable at low frequencies (≤~3 Hz) and slow Vs (≤~300 m/s). IR_max correlates with A₀ for all site categories with the strongest correlation at single-layer sites and the weakest at gradient sites. Average Vs to Z_1.0 correlates marginally (r<0.6) with A₀ at single-layer sites as does Vs30 at gradient sites. These results suggest: 1) the most appropriate site response proxies depend on the distribution of strong IRs beneath a site, 2) no single proxy has skill to predict both f₀ and A₀, 3) IR_max and its depth are the most versatile and useful attribute pair, and 4) Vs-profile characteristics may contribute to variability in ground-motion model residuals.

Session: Understanding and Modeling the Uncertainties in Earthquake Ground Motions

Type: Oral

Date: 4/20/2023

Presentation Time: 10:15 AM (local time)

Presenting Author: Seth Carpenter

Student Presenter: No

Invited Presentation: 

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