Search

Models for estimating earthquake ground motions are a key component in seismic hazard analysis. In data-rich regions, these models are mostly empirical, relying on the ever-increasing ground-motion databases. However, in areas in which strong-motion data are scarce, other approaches for ground-motion estimates are sought, including, but not limited to, the use of simulations to replace empirical data. In Israel, despite a clear seismic hazard posed by the active plate boundary on its eastern border, the instrumental record is sparse and poor, leading to the use of global models for hazard estimation in the building code and all other engineering applications. In this study, we develop a suite of alternative ground-motion models for Israel, based on an empirical database from Israel as well as on four data-calibrated synthetic databases. Two host models are used to constrain model behavior, such that the epistemic uncertainty is captured and characterized. Despite the lack of empirical data at large magnitudes and short distances, constraints based on the host models or on the physical grounds provided by simulations ensure these models are appropriate for engineering applications. The models presented herein are cast in terms of the Fourier amplitude spectra, which is a linear, physical representation of ground motions. The models are suitable for shallow crustal earthquakes; they include an estimate of the median and the aleatory variability, and are applicable in the magnitude range of 3-8 and distance range of 1-300 km. [ABSTRACT FROM AUTHOR]

Velocity reversals are layers within the soil/rock profile that have a velocity lower than that of the overlying strata. When performing site-response analysis, these reversals are often avoided because of the complexity they introduce. In this study, we explore the effect of a velocity reversal on the fundamental mode of the 1D linear elastic response for the case of two layers over a half-space. We define a criterion for tolerable velocity reversals, in which the amplitude change imposed by the reversal is relatively insignificant. We do so by modeling three-layer velocity profiles with different combinations of thickness and velocities. We observe that the amplitude is affected by both the impedance ratio and the thickness ratio between the upper two layers. We find that the amplitude of the linear transfer function at the fundamental frequency is, on a first order, a function of the effective impedance ratio--the impedance ratio between the half-space and a layer having the combined thickness of the upper two and their time-averaged shear-wave velocity. We suggest a model to compute the minimum allowable velocity in the reversal layer while maintaining an amplitude change smaller than 10% with respect to a nonreversal velocity profile. The reversal velocity in our model is a function of the overlaying velocity and the thickness ratio. [ABSTRACT FROM AUTHOR]

We estimate the variance in ground motions related to repeated large earthquakes occurring on the same fault segment with similar magnitudes. We find eight earthquake pairs for which suitable strong-motion records exist. Two are crustal strike-slip earthquakes from California and six are subduction zone earthquakes from Japan. We consider only large earthquakes and deal with frequencies greater than the earthquake corner frequency, so the variability that is considered here is related to smaller scale differences in the rupture process, particularly on the part of the fault nearest the station. We find that the variance of the 5% damped spectral accelerations of these pairs, termed βF2, averages to about 45% and 80% of βr2 for the crustal and subduction zone earthquakes, respectively, in which βr2 is the contribution of source variability to the total variability of ground motion estimated by some recent groundmotion prediction equations. We suggest that tf is lower than βr2, for the frequencies at which βF2 is estimated, because it depends primarily on only local physical properties of a fault that are the same in repeated earthquakes. We therefore suggest that at sites where the hazard is controlled by a single rerupturing source, one could potentially use a between-event variance that is smaller than βr2 in seismic-hazard calculations. Thus, these results may help to resolve the inconsistencies that are now present between the national hazard maps and some precariously balanced rocks in southern California. [ABSTRACT FROM AUTHOR]

Understanding and reducing uncertainties in ground-motion prediction are high priorities for seismic-hazard analysis. This article examines Φs2s, the variability in synthetic ground motions at rock sites caused by the variability in randomly generated velocity profiles of the geological column from 5 km depth to the surface. Only sites with VS30 of 500 m/s or higher are considered, and linearity is assumed. These synthetic estimates of the mean value of Φs2s are a complicated but understandable function of magnitude, period, and VS30. The distribution of modeled residual response spectral amplitudes at several oscillator periods is not lognormal, but the deviations are in the central part of the distribution, in which the effect on probabilistic seismic-hazard analysis may not be very large. Adding another constraint to the velocity profile, namely that the shear-wave velocity of the uppermost layer should be at least 70% of VS30, greatly reduces the uncertainty at high frequencies. We tentatively identify sites with this property as nonresonant rock, because it excludes sites with a strong resonance in a thin shallow layer. The reduction in uncertainty that this allows might reduce or eliminate the contradiction between the U.S. National Seismic Hazard Map and precarious rocks in southern California. Furthermore, for nonresonant rock sites, the residual impact of the ergodic assumption might be reduced to more tolerable levels. [ABSTRACT FROM AUTHOR]