MODELING FULL SEISMIC WAVEFORMS IN A COMPLEX ENVIRONMENT

We model seismic waveforms from a series of buried explosions embedded in granite as measured along a set of radially-deployed seismometers at the Nevada National Security Site (NNSS; formerly Nevada Test Site). The primary goal is to improve understanding of the origin of S waves from an explosion. Modeling requires an accurate 3D velocity model. The site and surrounding region includes alluvium, carbonates, volcanics, and intrusive granites. It has been extensively characterized using data from geophysical measurements, mapping, and borehole measurements. We create 3D models of the site using two different techniques: 1) a geology-based model developed using the characterization efforts and 2) a model based on ambient noise interferometry. Using a 3D finite-difference code, we generate synthetic waveforms from both models and compare with observed data. The geology model predicts the boundaries of units well, but velocities, especially at shallow depths, are uncertain. The interferometric model resolves the shallow velocities but lacks the spatial resolution with the existing instrumentation to define boundaries. In general, the interferometric model reproduces the observed waveforms better than the geology model. We are working on ways to merge the two models in a robust fashion and to characterize uncertainties. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344