ALBERT

Description: Fisk and Phillips (2013) motivated the need to constrain trade-offs among correction parameters for regional phase amplitudes to improve seismic discrimination and magnitude (yield) estimation. Using an empirical Green’s function approach to cancel path and site effects, relative spectra of direct regional phases and coda were fit for many thousands of nearby, similar earthquake pairs of different moments, to estimate reliable source corner frequencies and relative moments. Detailed comparisons demonstrated the benefit of using independent measurements of coda and direct phases to provide a large set of corroborated source terms for earthquakes throughout Eurasia. The spectra were subsequently corrected for source terms to estimate more reliable Q , geometric spreading rates, and site effects. Regression analysis was used to estimate geometric spreading rates and to establish a very consistent set of absolute moments. Here, frequency-dependent Q and site terms are examined for , Sn , , and Pn spectra. Comparisons to independent Q estimates from amplitude tomography exhibit good agreement for many paths. Large discrepancies are shown for higher frequencies, in low Q regions, and/or at the edges of the tomography grid, which significantly impact P / S discrimination. Our frequency-dependent site terms are compared with independent estimates from coda tomography, showing good agreement (even for and Pn , at many stations) but also some substantial differences.

Description: Reliable use of regional seismic phases for discrimination and magnitude estimation requires accurate corrections. Procedures that simultaneously invert for source, attenuation ( Q ), spreading, and site parameters have trade-offs that result in large errors for source and distance corrections. This motivates our efforts to improve corrections by constraining trade-offs. Using an empirical Green’s function approach, relative spectra of regional phases are computed for nearby, similar earthquake pairs of different moments, to cancel path, site, and focal mechanism effects, giving reliable estimates of source corner frequencies and relative moments. Many such pairs are available for this analysis throughout Eurasia. A large dataset of three-component regional seismograms from Incorporated Research Institutions for Seismology (IRIS) is assembled and processed for events listed in the preliminary determination of epicenters from 1989 to 2009. A relative Brune (1970) source model is fit to network-median relative spectra for over 46,000 pairs, corresponding to about 9400 unique events. Pseudorelative spectra are also computed from coda envelopes in 16 frequency bands. Coda is less sensitive to focal mechanism, event separation, and station coverage ( Mayeda et al. , 2007 ) but more prone to data quality issues. Results are presented with good corroboration of the moments and corner frequencies from coda and direct phases. Detailed case studies are shown to indicate the level of agreement, interstation variability, comparisons to published results based on local networks, and causes of various discrepancies between coda and direct phases. The spectra subsequently are corrected for source terms to estimate more reliable Q , geometric spreading rates, and site effects. Examples are compared to amplitude tomography results. Our estimated spreading rates are consistent with published studies, except for long distance Pn and mantle P . As important, the spreading analysis also provides a very consistent set of absolute scalar moments. Further details and comparisons to independent Q and frequency-dependent site terms are presented in a companion paper ( Fisk and Phillips, 2013 ).

Description: The task of monitoring the Earth for nuclear explosions relies heavily on seismic data to detect, locate, and characterize suspected nuclear tests. Motivated by the need to locate suspected explosions as accurately and precisely as possible, we developed a tomographic model of the compressional wave slowness in the Earth’s mantle with primary focus on the accuracy and precision of travel-time predictions for P and Pn ray paths through the model. Path-dependent travel-time prediction uncertainties are obtained by computing the full 3D model covariance matrix and then integrating slowness variance and covariance along ray paths from source to receiver. Path-dependent travel-time prediction uncertainties reflect the amount of seismic data that was used in tomography with very low values for paths represented by abundant data in the tomographic data set and very high values for paths through portions of the model that were poorly sampled by the tomography data set. The pattern of travel-time prediction uncertainty is a direct result of the off-diagonal terms of the model covariance matrix and underscores the importance of incorporating the full model covariance matrix in the determination of travel-time prediction uncertainty. The computed pattern of uncertainty differs significantly from that of 1D distance-dependent travel-time uncertainties computed using traditional methods, which are only appropriate for use with travel times computed through 1D velocity models.