HR: 0800h
AN: S11B-1017    [Abstracts]
TI: Regional Seismic Travel-Time Prediction, Uncertainty, and Location Improvement in Western Eurasia
AU: * Flanagan, M P
EM: flanagan5@llnl.gov
AF: Lawrence Livermore National Laboratory, Earth Sciences Division PO Box 808, L-205, Livemore, CA 94550
AU: Myers, S C
EM: myers30@llnl.gov
AF: Lawrence Livermore National Laboratory, Earth Sciences Division PO Box 808, L-205, Livemore, CA 94550
AB: We investigate our ability to improve regional travel-time prediction and seismic event location using an {\it a priori,} three-dimensional velocity model of Western Eurasia and North Africa: WENA1.0 [{\it Pasyanos et al.,} 2004]. Our objective is to improve the accuracy of seismic location estimates and calculate representative location uncertainty estimates. As we focus on the geographic region of Western Eurasia, the Middle East, and North Africa, we develop, test, and validate 3D model-based travel-time prediction models for 30 stations in the study region. Three principal results are presented. First, the 3D WENA1.0 velocity model improves travel-time prediction over the {\it iasp91} model, as measured by variance reduction, for regional {\it Pg, Pn}, and {\it P} phases recorded at the 30 stations. Second, a distance-dependent uncertainty model is developed and tested for the WENA1.0 model. Third, an end-to-end validation test based on 500 event relocations demonstrates improved location performance over the 1-dimensional {\it iasp91} model. Validation of the 3D model is based on a comparison of approximately 11,000 {\it Pg, Pn, }and {\it P} travel-time predictions and empirical observations from ground truth (GT) events. Ray coverage for the validation dataset is chosen to provide representative, regional-distance sampling across Eurasia and North Africa. The WENA1.0 model markedly improves travel-time predictions for most stations with an average variance reduction of 25% for all ray paths. We find that improvement is station dependent, with some stations benefiting greatly from WENA1.0 predictions (52% at APA, 33% at BKR, and 32% at NIL), some stations showing moderate improvement (12% at KEV, 14% at BOM, and 12% at TAM), some benefiting only slightly (6% at MOX, and 4% at SVE), and some are degraded (-6% at MLR and -18% at QUE). We further test WENA1.0 by comparing location accuracy with results obtained using the {\it iasp91} model. Again, relocation of these events is dependent on ray paths that evenly sample WENA1.0 and therefore provide an unbiased assessment of location performance. A statistically significant sample is achieved by generating 500 location realizations based on 5 events with location accuracy between 1 km and 5 km. Each realization is a randomly selected event with location determined by randomly selecting 5 stations from the available network. In 340 cases (68% of the instances), locations are improved, and average mislocation is reduced from 31 km to 26 km. Preliminary test of uncertainty estimates suggest that our uncertainty model produces location uncertainty ellipses that are representative of location accuracy. These results highlight the importance of accurate GT datasets in assessing regional travel-time models and demonstrate that an a priori 3D model can markedly improve our ability to locate small magnitude events in a regional monitoring context. {\it This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, Contribution UCRL-CONF-206386. }
DE: 9335 Europe
DE: 7219 Nuclear explosion seismology
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting