Progress Towards Next Generation, Waveform Based Three-Dimensional Models and Metricsto Improve Nuclear Explosion Monitoring in the Middle East

摘要

Efforts to update current wave speed models of the Middle East require a thoroughly tested database of sources and recordings. Recordings of seismic waves traversing the region from Tibet to the Red Sea will be the principal metric in guiding improvements to the current wave speed model. Precise characterizations of the earthquakes, specifically depths and faulting mechanisms, are essential to avoid mapping source errors into the refined wave speed model. Errors associated with the source are manifested in amplitude and phase changes. Source depths and paths near nodal planes are particularly error prone as small changes may severely affect the resulting wavefield. Once sources are quantified, regions requiring refinement will be highlighted using adjoint tomography methods based on spectral element simulations (Komatitsch and Tromp, 1999). An initial database of 250 regional Middle Eastern events from 1990-2007, was inverted for depth and focal mechanism using teleseismic arrivals (Kikuchi and Kanamori, 1982) and regional surface and body waves (Zhao and Helmberger, 1994). From this initial database, we reinterpreted a large, well-recorded subset of 201 events through a direct comparison between data and synthetics based upon a centroid moment tensor inversion (Liu et al., 2004). Evaluation was done using both a ID reference model (Dziewonski and Anderson, 1981) at periods greater than 80 seconds and a 3D model (Kustowski et al., 2008) at periods of 25 seconds and longer. The final source reinterpretations will be within the 3D model, as this is the initial starting point for the adjoint tomography. Transitioning from a ID to 3D wave speed model shows dramatic improvements when comparisons are done at shorter periods, e.g., 25 s. Synthetics from the ID model were created through mode summations while those from the 3D simulations were created using the spectral element method. To further assess errors in source depth and focal mechanism, comparisons between the thr.