Ground-Based Nuclear Explosion Monitoring R&D Validation of Regional Seismic Travel Time (RSTT) Predictions and Use in Event Location Stephen C. Myers 1 Michael L. Begnaud 2 Sanford Ballard 3 Abelardo L. Ramirez 1 , Michael E. Pasyanos 1 , W. Scott Phillips 2 + RSTT Team (LLNL, LANL, SNL) 1 Lawrence Livermore National Laboratory, 2 Los Alamos National Laboratory, 3 Sandia National Laboratories This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Security, LLC, Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. The views expressed here do not necessarily reflect the views of the United States Government, the United States Department of Energy, or the Lawrence Livermore National Laboratory Sponsored by: US Department of State LLNL-PRES-638493
Introduction Ground-Based Nuclear Explosion Monitoring R&D • Goal of the RSTT project: – Improve travel time prediction accuracy with respect to global base models for the regional phases Pn, Pg, Sn and Lg using a real-time calculation. • Real-time computation must be achieved with commonly available computers, as exist at any CTBT National Data Center (NDC). • Real-time computation enables application to flexible networks and data centers who may wish to combine IMS and other networks to achieve optimal location accuracy. Pg Lg Sn Pn LLNL-PRES-638493
IASPEI Definitions for Regional Phases Ground-Based Nuclear Explosion Monitoring R&D • Computes travel times for Pn, Pg, Sn, Lg phases commonly used for routine event location. IASPEI definitions for phases computed by RSTT RSTT follows the convention used by explosion monitoring data centers, whereby Pg is not distinguished from Pb and Lg is not distinguished from Sb . Pg Lg Figures from Stochak et al., 2003 Sn Pn IASPEI is the International Association of Seismology and Physics of the Earth’s Interior LLNL-PRES-638493
Regional Phases are a Significant Percentage of REB Data for Small Events Ground-Based Nuclear Explosion Monitoring R&D Time-defining phases in REB 2009-2010. Distances 0 ° to 95 ° 0.6% Local 16% Regional 83% teleseismic Percentage Total Teleseismic Teleseismic Regional Regional Local Local S. Myers, LLNL REB is the Reviewed Event Bulletin LLNL-PRES-638493
Regional Data Can Degrade Location Accuracy When Using a Simple Radially Symmetric Model Ground-Based Nuclear Explosion Monitoring R&D Degradation in location error when 1 Pn is added • Location of low-magnitude (RSTT Validation Data, ak135 (1D) model) events may rely on 1 or more regional stations. Average number of phases used in REB • When using a simple 1D 51 Median Error Degradation (km) (2008 to present) to locate events of velocity model, adding even 1 varying magnitude regional (Pn) arrival time tends mb=3.5 to degrade location accuracy. Need a regional phase velocity • mb=4.0 model that will not degrade locations when regional phases are combined with teleseismic. mb=4.5 Global data used to evaluate location accuracy mb=5. 0 S. Myers, LLNL Number of P phases LLNL-PRES-638493
Regional Phase Travel Times Vary More Than Travel Times at Other Distances Ground-Based Nuclear Explosion Monitoring R&D Average Pn Travel Time Error Area of distance ranges Regional Teleseismic Modified from Rodi and Myers (2013) 95 Local LLNL-PRES-638493
Lateral Variation in Wave Speed is the Cause of Poor Regional Travel Time Prediction Ground-Based Nuclear Explosion Monitoring R&D Pn P LLNL-PRES-638493 S. Myers, LLNL
RSTT Accounts for Lateral Variability in Seismic Velocity to Improve Prediction Accuracy Ground-Based Nuclear Explosion Monitoring R&D From Previous Presentation (T1-06) M. Begnaud, LANL LLNL-PRES-638493
Travel Time Validation, Pn Ground-Based Nuclear Explosion Monitoring R&D Procedure 1) Randomly sample 10% of the tomography data set for validation 2) Repeat tomographic inversion, withholding the validation data 3) Difference observed and predicted travel times � Account for unknown origin time by removing the median residual for each event 77K paths 6K stations 19K events S. Myers, LLNL 355K summary paths 11K stations 25K events LLNL-PRES-638493
RSTT Reduces Residual Trends (Biases), Pn Ground-Based Nuclear Explosion Monitoring R&D AK135 RSTT Curves include measurement errors LLNL-PRES-638493
Travel Time Validation, Pg Ground-Based Nuclear Explosion Monitoring R&D Procedure 1) Randomly sample 10% of the tomography data set for validation 2) Repeat tomographic inversion, withholding the validation data 3) Difference observed and predicted travel times � Account for unknown origin time by removing the median residual for each event 7.8K paths 2.3K stations 4.5K events 32K summary paths 6K stations 8K events LLNL-PRES-638493
Travel Time Validation, Pg Ground-Based Nuclear Explosion Monitoring R&D AK135 RSTT LLNL-PRES-638493
Travel Time Validation, Sn Ground-Based Nuclear Explosion Monitoring R&D Procedure 1) Randomly sample 10% of the tomography data set for validation 2) Repeat tomographic inversion, withholding the validation data 3) Difference observed and predicted travel times � Account for unknown origin time by removing the median residual for each event 15K paths 2.8K stations 7.9K events 84K summary paths 6K stations 17K events LLNL-PRES-638493
Travel Time Validation, Sn Ground-Based Nuclear Explosion Monitoring R&D AK135 RSTT LLNL-PRES-638493
Travel Time Validation, Lg Ground-Based Nuclear Explosion Monitoring R&D Procedure 1) Randomly sample 10% of the tomography data set for validation 2) Repeat tomographic inversion, withholding the validation data 3) Difference observed and predicted travel times � Account for unknown origin time by removing the median residual for each event 2.8K paths 0.8K stations 1.9K events 16.0K summary paths 3.0K stations 5.0K events LLNL-PRES-638493
Travel Time Validation, Lg Ground-Based Nuclear Explosion Monitoring R&D AK135 RSTT LLNL-PRES-638493
Location Validation Events Ground-Based Nuclear Explosion Monitoring R&D Validation events used to test location accuracy These events not used in tomography to create RSTT validation model GT2(2) GT0 (1) GT5(69) GT1(14) LLNL-PRES-638493
Network Sampling Ground-Based Nuclear Explosion Monitoring R&D 10 random samples of network stations Event and network Bolivia Chile Argentina S. Myers, LLNL S. Myers, LLNL 14 of 23 stations chosen LLNL-PRES-638493
Examples of Network Realizations Ground-Based Nuclear Explosion Monitoring R&D Network coverage (azimuthal gap) can differ significantly between realizations AzGap=117 ° AzGap=91 ° AzGap=33 ° S. Myers, LLNL LLNL-PRES-638493
Realizations of Locations Ground-Based Nuclear Explosion Monitoring R&D Reference (GT5) ak135 (51 km median err) RSTT (10 km median err) 14 Pn arrivals Chile Argentina S. Myers, LLNL LLNL-PRES-638493
Average Location Accuracy Ground-Based Nuclear Explosion Monitoring R&D Results based on averaging � 10 network realizations ak135 � for each number of picks � for 58 global test events RSTT � Over 10,000 locations � Using RSTT Pn travel times results in more accuracy event locations on average. � Improvement in epicenter accuracy is consistent regardless of the number of phases used. � The plateau in location accuracy for ~20 phases and more is thought to be pick- error related. LLNL-PRES-638493
IMS Locations Typically Utilize Both Regional and Teleseismic Data Ground-Based Nuclear Explosion Monitoring R&D Dashed lines: Location accuracy using varying numbers of Pn ak135 teleseismic data as in previous slide. + regional Solid line: Location accuracy using ak135 regional ak135 travel times and a mixture RSTT regional of regional (3 Pn) and varying numbers of teleseismic P data. Recall number of P for average REB event Median Error Degradation (km) mb=3.5 mb=4.0 mb=4.5 mb=5.0 P Number of Picks Pn Number of P phases LLNL-PRES-638493
RSTT Enables the Use of a Mixed Regional and Teleseismic Data Set Ground-Based Nuclear Explosion Monitoring R&D Using RSTT for the 3 regional phases calculations mb=3.5, 27% regional � Improves location accuracy for small events for which regional mb=4.0, 20 % data make up a larger mb=4.5, 10% percentage of the data set. � Allows NDCs to add their mb=5.0, 5% regional data to further improve location accuracy. ak135 teleseismic+ regional ak135 teleseismic+ RSTT regional ak135 regional Number of Picks P RSTT regional Pn LLNL-PRES-638493
Ground-Based Nuclear Explosion Monitoring R&D Efforts to Improve the RSTT Global Model LLNL-PRES-638493
Pn Travel Time Residuals Geographically Ground-Based Nuclear Explosion Monitoring R&D Median Residuals Eurasia North America RSTT -> 0.71 (s) RSTT -> 0.64 (s) Ak135-> 0.84 (s) Ak135-> 0.95 (s) Latin America RSTT -> 0.86 (s) Ak135-> 0.98(s) S. Myers, LLNL LLNL-PRES-638493
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