UCLA

We are working on the development of an earthquake ground-motion model (GMM) for effective amplitude spectra (EAS, the smoothed quadratic mean of two horizontal-component Fourier amplitude spectra) as part of the NGA-West3 project. κ0 is a model parameter that controls the shape of the high frequency portion of the EAS spectrum by determining how far it deviates from an ω^{-2} source model. Here, we focus on development of κ0 models within the GMM, with increasing levels of complexity: regional constants, regional relationships with VS30, and a modular model based on the depth to the 1 km/s shear wave velocity isosurface (z1). We estimate average values of κ0 range from 0.023 – 0.065 s. Mexico, southern California, and Japan have lower than average κ0 (less attenuation) whereas Taiwan and northern California have higher than average values (more attenuation). We find that sites with low time-averaged shear wave velocity in the upper 30 m (VS30) have larger κ0, whereas VS30 has no control on κ0 for stiff sites (VS30 ≥ 300 m/s). Variations in depth to the 1 km/s isosurface (z1) have a stronger control on κ0, up to a factor of three in some regions; we parameterize a model using z1 such that it can be combined with our κ0(V_{S30}) model.