HR: 1340h
AN: S23B-0309    [Abstracts]
TI: Application of Coda Wave Techniques to Regional Explosion Monitoring
AU: * Phillips, S
EM: wsp@lanl.gov
AF: Los Alamos National Laboratory, MS D408, Los Alamos, NM 87545 United States
AU: Rautian, T
EM: trautian@aol.com
AF: Lamont Doherty Earth Observatory, P.O. Box 1000 61 Rt 9W, Palisades, NY 10964 United States
AU: Khalturin, V
EM: vkhaltur@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, P.O. Box 1000 61 Rt 9W, Palisades, NY 10964 United States
AU: Mayeda, K
EM: kmayeda@llnl.gov
AF: Lawrence Livermore National Laboratory, 700 East Ave., Livermore, CA 94550 United States
AB: The stability of local (within 100 km) earthquake coda has been well established through observations that have shown coda amplitudes to be independent of distance, path details and azimuth with respect to the focal mechanism (e.g. Aki, JGR, 1969). This behavior is consistent with homogeneous scattering models of coda and allows accurate and high precision measurement of relative amplitudes. Absolute source spectra can be obtained from the relative measurements by applying empirical Green's function (EGF) analysis followed by ties to independently determined moments (Chouet et al., BSSA, 1978; Rautian and Khalturin, BSSA, 1978) for purposes of studying variations in source scaling in different regions. Recently, Mayeda et al. (BSSA, 2003) extended coda techniques from local to regional scales by employing empirical models of coda shape to describe distance effects. We further extend regional techniques: 1) to account for lateral variations in attenuation, and 2) to include empirically derived coda shapes for event clusters. The attenuation maps, as well as relative site terms, are obtained by applying tomographic techniques to amplitude ratio data. Following Aki and Richards (1980; equation 12.166), ratios are taken between stations that record each event, thus removing source effects from the problem. Application of the method to data from over 40 stations and 2250 multiply recorded events covering China and central Asia reduces ratio residuals (RMS) to under 0.1 log10 amplitude units for bands between 0.03 and 6 Hz. For 0.7 to 1 Hz data, variance reduction is 86% relative to the best-fit uniform attenuation model. Attenuation patterns closely follow regional geology, with low Q in Tibet, the East China Basin, the western Tien Shan, and the Baikal rift, and high Q throughout platform regions to the north and in southeast China, as well as the Sichuan, Tarim, Tsiadam and Ordos basins. Our extension to empirically derived coda shape is accomplished by inverting for the amplitude at each elapsed time increment using many events recorded along a common path. No coda shape assumptions are required and the resulting "typecurve" can be compared to individual coda. This gives the highest precision relative amplitudes between events in clusters, but reverts to normal levels of precision between or outside clusters. This technique is especially applicable to test site explosions because saturation of older digital data forces the use of late time segments for many events, and effects of elapsed time, commonly observed for model fits to coda data, is avoided. After applying site and path corrections to amplitude data from over 6500 events that are obtained using a combination of the curve fitting and typecurve methods, and converting to absolute amplitudes as described above, we can directly compare spectra from a wide distribution of sources to study variations in source scaling. We observe apparent stress drop that varies regionally, with lower stress events occurring in limited areas of Tibet, and high stress events in perhaps more brittle regions to the north. These patterns depend on how well path effects are removed and we seek to confirm these results using EGF techniques. Results for Asia explosions rely on data from the Borovoye archive and on ChISS (Soviet, pre-digital era, band pass recording) data from Talgar, and show marked spectral peaks that scale with source size.
DE: 7219 Nuclear explosion seismology
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting