Annual Meeting - SCEC.org

Poster Abstracts | Group 1 – GMP
motions can potentially occur during strong ground shaking as the result of the soil being much
softer than the building foundation. The vertical motion is characterized by spectral peaks at 0.71
Hz, 0.82 Hz, 1.1 Hz, and higher. These are larger than the first few modes of horizontal vibrations.
Computing the spectral ratios between the top floor and basement for evidence of soil-structure
interactions is a topic for further research. We also analyze the peak rocking motion as a function of
earthquake distance and magnitude.
1-051
"RUPTURE-TO-RAFTERS" SYNTHETIC GROUND MOTIONS AND THE ROLE OF
NONLINEAR SITE RESPONSE PREDICTIONS Asimaki D, and Li W
Overarching goal of our research is to develop quantitative criteria that will allow efficient
integration of site response models in broadband ground motion simulations. For this purpose, we
combine downhole array observations and broadband ground motion synthetics and study the
prediction sensitivity of ground surface motion and nonlinear structural performance due to the
bias and uncertainty in nonlinear site response simulation. This work focuses on three downhole
arrays in Southern California installed at medium to soft soil sites, where we have evaluated
broadband synthetics based on the regional geology and fault systems for finite-source dynamic
rupture scenarios of weak, medium and large magnitude events (M = 3.5~7.5), on a surface station
grid of epicentral distances 2km~75km. For each site, we conduct elastic and nonlinear analyses
using multiple soil models, and first estimate the modeling ground motion variability by means of
the COV (coefficient of variation) of site amplification. For each model, we then assess the
parametric uncertainty of ground motion predictions by systematically randomizing selected input
parameters. Based on our results, we develop a frequency index, which, combined with the ground
motion intensity, is used as a quantitative measure to describe the site and ground motion
combinations where the nonlinear models show large prediction COV, namely where incremental
nonlinear analyses significantly deviate from empirical methodologies. We finally illustrate the role
of nonlinear soil response simulation in physics-based seismic hazard predictions by subjecting a
series of inelastic SDOF (single-degree-of-freedom) to the ensemble of ground motion predictions
obtained via the alternative site response methodologies. The bias and uncertainty in the structural
response predictions is also evaluated as a function of the frequency-intensity criteria proposed in
this work, to quantify the propagation of site response modeling variability to the assessment of
structural performance measures in rupture-to-rafters simulations. Our results show that large
sensitivity in the selection of site response methodology yields high bias and uncertainty in the
assessment of the inelastic displacement ratio for nonlinear structural response predictions,
indicating the efficiency of the proposed criteria for the optimal selection of site response model.
1-052
COMPARISON OF MEASURED VS30 VALUES AGAINST VS30 PREDICTIONS BASED
ON TOPOGRAPHIC SLOPE Pancha A, Louie JN, Yong A, Thompson M, and Dhar M
Independently, geology and topographic slope have been used as predictors of shallow shear wave
velocity (Vs30 or the average shear-wave velocity in the upper 30 meters) for seismic hazard
assessment. In the former approach, dominant grain size within site-response units have been
observed to correlate to variations in shallow Vs30 (e.g., Borcherdt 1970; Tinsley & Fumal, 1985;
Fumal & Tinsley, 1985; Wills et al., 2000; Wills & Clahan, 2006). In the latter approach, topographic
slope has been introduced as a correlative to Vs30, which can be easily computed from readily
available DEMs to provide first-order maps of site conditions (Allen & Wald, 2007; Wald & Allen,
2007). These maps predict shallow shear-wave velocity on the basis of Vs30 and slope correlations.
We compare results from 509 sites having observed Vs30 measurements acquired through the
refraction microtremor surface-wave dispersion analysis method, against the predicted Vs30 from
96 | Southern California Earthquake Center