A Global 3D P-Velocity Model of the Earth's Crust and Mantle for Improved Event Location

• Main Authors: Begnaud, Michael L, Ballard, Sanford, Young, Christopher J, Hipp, James R, Chang, Marcus C, Encarnacao, Andre V, Rowe, Charlotte A, Phillips, W S, Steck, Lee K
• Format: Report
• Language: English
• Subjects: EARTH CRUST; EARTH MANTLE; EARTH MODELS; Geodesy; POSITION(LOCATION); SEISMOLOGY; SURFACE TRUTH; TOMOGRAPHY; VELOCITY

To test the hypothesis that high quality 3D Earth models will produce seismic event locations that are more accurate and more precise than currently used 1D and 2/2.5D models, we are developing a global 3D P wave velocity model of the Earth's crust and mantle using seismic tomography. In this paper, we present the most recent version of our model, SALSA3D (SAndia LoS Alamos 3D) version 1.7, and demonstrate its ability to reduce mislocations for a large set of realizations derived from a carefully chosen set of globally-distributed ground truth events, compared to existing models and/or systems. Our model is derived from the latest version of the Ground Truth (GT) catalog of P and Pn travel time picks assembled by Los Alamos National Laboratory. To prevent over-weighting due to ray path redundancy and to reduce the computational burden, we cluster rays to produce representative rays. Reduction in the total number of ray paths is 50%. The model is represented using the triangular tessellation system described by Ballard et al. (2009), which incorporates variable resolution in both the geographic and radial dimensions. For our starting model, we use a simplified layer crustal model derived from the NNSA Unified model in Eurasia and Crust 2.0 model everywhere else, over a uniform ak135 mantle. Sufficient damping is used to reduce velocity adjustments so that ray path changes between iterations are small. We obtain proper model smoothness by using progressive grid refinement, refining the grid only in areas where the data warrant it. In previous versions of SALSA3D, we based this refinement on velocity changes from previous model iterations. For version 1.7, we utilize the diagonal of the model resolution matrix to control where grid refinement occurs, resulting in more consistent and continuous areas of refinement than before. Published in the Proceedings of the 2011 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies, 13-15 September 2011, Tucson, AZ. Volume I. Sponsored by the Air Force Research Laboratory (AFRL) and the National Nuclear Security Administration (NNSA). U.S. Government or Federal Rights License