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Abstract. We outline a procedure for the estimation of frequency-dependent source and propagation amplitude corrections for regional seismic discriminants (source path amplitude correction (SPAC)). For a given station and phase a number of well-recorded earthquakes are inverted for source and path corrections. The method assumes a simple Brune [1970] earthquake-source model and a simple propagation model consisting of a frequency-independent geometrical spreading and frequency-dependent power law Q. The inverted low-frequency levels are then regressed against rnb to derive a set of corrections that are a function of rnb and distance. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. The resulting discriminants are normally distributed and amenable to multivariate feature selection, classification, and outlier techniques. To date, most discrimination studies have removed distance corrections once a particular amplitude ratio is formed (distance corrected ratio (DCR)). DCR generally works well for phase ratios taken in a particular frequency band. However, when different frequency bands are combined (for phase spectral ratios or cross spectral ratios), significant source-scaling effects (e.g., corner-frequency scaling) can remain, causing the discriminants to vary as a function of event size and to be nonnormally distributed. It is then often necessary to construct nonphysical transformations in an attempt to make the discriminants multivariate normal. The SPAC technique can be used to construct discriminants that are multivariate normal by using simple physical seismic source and propagation models. Moreover, phase amplitude residuals as a function of frequency can be spatially averaged and used as additional path-specific corrections to correct for additional propagation effects such as phase blockages.

[1] We image regional seismic phase attenuation (Q) by extending amplitude ratio techniques, which are widely used to determine average Q within a sampled region, to obtain laterally varying Q structure using tomographic techniques. The method assumes that source radiation is isotropic and that the amplitude ratios can be modeled by a two-dimensional Q map and relative site terms. Application to 2286, 1-Hz L g amplitudes from 816 events recorded at 35 central and east Asia stations yields a misfit of 0.09 log 10 units, a variance reduction of 73%, relative to the uniform Q model that best fits the data, and laterally varying Q from 100 to 1000, with patterns closely following regional geology. The image is resolved, on average, to 2.5°, with resolution peaking at 1.5°in the best covered areas, which is remarkable given the sparse station geometry employed. The high resolution results from the dense distribution of earthquake sources, and is aided by the increased confidence in source locations provided by merging catalogs from many different agencies. Citation: Phillips, W. S., H. E. Hartse, and J. T. Rutledge (2005), Amplitude ratio tomography for regional phase Q, Geophys. Res. Lett., 32, L21301,

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Abstract. We have applied tomographic techniques to a data set of over 1700, regional distance, L a amplitudes from 12 stations for paths inside a 30 ø by 40 ø region of Central Asia. Our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination, as well as to study La propa-