A PROCEDURE FOR ESTIMATION OF SOURCE AND PROPAGATION AMPLITUDE CORRECTIONS FOR REGIONAL SEISMIC DISCRIMINANTS

We outline a procedure for the estimation of frequency-dependent sourc e and propagation amplitude corrections for regional seismic discrimin ants (source path amplitude correction (SPAC)). For a given station an d phase a number of well-recorded earthquakes are inverted for source and path corrections. The method assumes a simple Brune [1970] earthqu ake-source model and a simple propagation model consisting of a freque ncy-independent geometrical spreading and frequency-dependent power la w e. The inverted low-frequency levels are then regressed against mb t o derive a set of corrections that are a function of m(b) 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 discrimi nants are normally distributed and amenable to multivariate feature se lection, classification, and outlier techniques. To date, most discrim ination studies have removed distance corrections once a particular am plitude ratio is formed (distance corrected ratio (DCR)). DCR generall y works well for phase ratios taken in a particular frequency band. Ho wever, 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 b e spatially averaged and used as additional path-specific corrections to correct for additional propagation effects such as phase blockages.