HR: 0800h
AN: S11B-1019    [Abstracts]
TI: Differential Travel-time Residual Analysis for Improved Site Correction and Catalog Quality, and Insights into Laterally Varying Earth Structure in China / East Asia
AU: * Rowe, C A
EM: char@lanl.gov
AF: Los Alamos National Laboratory, Geophysics, EES-11 M.S. D-408, Los Alamos, NM 87545 United States
AU: Phillips, W S
EM: wsp@lanl.gov
AF: Los Alamos National Laboratory, Geophysics, EES-11 M.S. D-408, Los Alamos, NM 87545 United States
AU: Steck, L K
EM: lsteck@lanl.gov
AF: Los Alamos National Laboratory, Geophysics, EES-11 M.S. D-408, Los Alamos, NM 87545 United States
AB: We are exploring the use of relative travel-time residuals to better constrain models of the earth for use in event location, and to usefully augment station correction information, using residual estimates from events which were previously thought to have little utility due to poor ground truth (GT) parameters. The residual differences also prove to be valuable in identifying poor arrival time picks. For an event recorded at two stations, the travel-time residual difference is insensitive to the event location for certain geometries. The residual difference represents variations in properties of the earth for paths to the two stations, which have not been addressed by the velocity model. Because of the relative insensitivity to event location, relative constraints on travel-time residuals may be helpful in developing relative station corrections for source regions having little in the way of high-quality (ground truth) sources, thus improving the overall confidence of derived correction surfaces. By combining all relative residual information and performing an inversion for consistent differential patterns, we derive a 2D function that maps laterally-varying residual slowness deviations from the background model, as well as to obtain site terms based on the residual differentials. We apply the technique to events from the combined global and zABCE catalog, relocated using IASP91. We have selected event / station geometries such that a 20 km change in source location results in less than 0.2 second change in the travel-time residual differences, based on synthetic sensitivity studies. For this study we restict our test to source-receiver distances of 2 - 20 degrees. Application of the inversion time terms reduces differential residuals by 50%. We observe regional slowness patterns similar to those obtained by other authors using single ray (Pn tomography) techniques. These show fast mantle velocities in central and western China, in particular under the Tarim, Jungar, Tsaidam and Sichuan basins, and slower velocities beneath eastern China, including the continental shelf and Shanxi graben regions. Site terms are negative throughout eastern China, reflecting thinner crust, and are positive for western China, especially Tibet. Results will be used to test improvement in location based on the collection of ground truth events in the area. We will also perform relocation of the entire catalog, using the differential results both to provide catalog pick quality control as well as station correction terms. This will provide an improved basis for finer analysis of seismicity, including high-precision relative relocation where event clustering is observed.
DE: 9320 Asia
DE: 7294 Instruments and techniques
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting