REGIONAL-SCALE DIFFERENTIAL TIME METHODS : DEVELOPMENT AND APPLICATION TO THE SIBERIA DATA SET

The University of Wisconsin-Madison (UW-Madison) group is developing improved methodologies for regionalscale three-dimensional (3D) seismic tomography, and is working collaboratively with Los Alamos National Laboratory (LANL) and Michigan State University (MSU) on applications of these methods to the Siberia data set assembled by MSU. The tomographic study proposed here will emphasize the accretionary regions to the south and east of the Siberian craton, providing the first detailed 3D look at the seismic structure of continental eastern Russia. There are four main tasks in this project: (1) an extension of our development of double-difference (DD) seismic tomography to the use of station-pair residual differences, including incorporation of a new method for resolution matrix calculation; (2) testing, refinement, and adaptation of a method for spherical-earth finite-difference (SEFD) travel time calculations for use in DD tomography; (3) an extension of our Cartesian adaptive-grid DD tomography algorithm to spherical coordinates; and (4) collaborative work among the UW-Madison, LANL, and MSU groups to apply these analysis tools to the Siberia data set. Our work under Task 1 has so far focused on the incorporation and testing of the new resolution matrix calculation method in an existing DD tomography code. The PROPACK package developed by Larsen (1998) is able to efficiently and accurately estimate singular values and vectors for large matrices based on the Lanczos bidiagonalization with partial reorthogonalization (BPRO). We have made substantial progress towards incorporating the PROPACK package into the double-difference seismic tomography code tomoDD, allowing its use to estimate the model resolution matrix for large seismic tomography problems. Compared to previous LSQRbased methods for estimating the model resolution matrix, the PROPACK-based method accurately calculates the full resolution matrix and thus gives a complete description of how well the model is resolved. Task 2 involves the testing, refinement, and adaptation of a new method for SEFD travel time calculations developed by S. Roecker. The basic concept is the extension of a standard Cartesian FD travel time algorithm to the spherical case by (1) developing a mesh in radius, co-latitude, and longitude; (2) expression of the FD derivatives in a form appropriate to the spherical mesh; and (3) the construction of "stencils" to calculate extrapolated travel times. S. Roecker has developed this code, and we have begun to test it against another existing SEFD code (Flanagan et al., 2000, 2006). We will then integrate it into our DD tomography algorithms. Our other work planned for Year 1 includes: acquire and enter additional seismic phase data from eastern Siberia into the MSU Siberia data base (MSU); parse newly acquired datasets into NNSA Schema tables (LANL); load and integrate new data from LANL and MSU into the LANL research knowledge base (LANL); advance and apply interstation travel-time distance inversion method for catalog pick quality control to Siberia data set (LANL); develop P and S datasets from LANL research knowledge base for use in DD tomography and participate in the application of DD tomography (LANL); apply new DD tomography algorithms to the Siberia data delivered by MSU and LANL (3 subregions) (UW-Madison). 28th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies