iPlex Lunch spring-2016

Seismic Amplitudes from Ambient Noise Wavefront Tracking: Maps from USArray and Long Beach, and the Connection to Engineering Hazards

Tracking the wavefronts of Rayleigh waves through ambient noise has been demonstrated to reasonably recover site amplification terms across a very dense array in Long Beach, CA (0.5-1.5 sec periods) as well as the USArray (12-32 sec periods). The maps provide empirical observations of the relative strengths of Rayleigh waves, for a given period, laterally across both city and country scales. The methodology accounts for 2D propagation effects including attenuation, focusing and defocusing of the energy, and requires no assumptions about the structure at depth. Results correlate well with velocity models, and this provides an opportunity to test and understand shortcomings of both the models and method. We particularly focus on the tectonic and geologic structures that give rise to the observations, and the extent to which further 3D complexities such as topography and basin shape are represented in the amplifications. These measurements of surface waves are, however, fundamentally different from the site response terms traditionally estimated by vertically propagating waves in a 1D layered medium, and the relation between the two types of measurements is explored through comparisons of various datasets and simulations.