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TELEDYNE GEOTECH, 314 MONTGOMERY STREET, ALEXANDRIA, VIRGINIA 22314
Abstract
Discrimination of small events begins with detection, association, and location. Recent advances in detection include use of updated measures of the noise variance to control the false alarm rate; and updating the noise amplitude spectrum, N, for application of the optimum detection filter S/N** 2 where S is the signal spectrum. Postdetection processes of interest include determination of signal azimuth and emergence angle from linear three-component processing. Association programs have recently been improved by implementation of techniques taking advantage of information provided by arrays.
Location advances include modification of the a priori travel-time residual covariance matrix to account for the fact that travel-time residuals are correlated at stations near to each other, simultaneous determination of location and phase velocity when using regional phases, and direct use of phases other than P, e.g., Pg, Lg, S, pP, and PKPBC.
According to most source theories, small earthquakes and explosions have the same, flat, displacement spectrum for P waves at teleseismic frequencies. Aside from location, discrimination therefore rests on focal mechanism differences and generation of shear waves by earthquakes. The Ms:mb discriminant rests on the shear wave generation; but there is overlap of explosions and earthquakes at low magnitudes for dip-slip earthquakes. Low frequency (0.3 Hz) P waves are weak from explosions due to the surface pP reflections; P/pP amplitude ratios for earthquakes vary over the focal sphere in a way impossible for a pair of explosions. Regional discrimination shows promise in the ratio of maximum amplitudes before and after Sn, and in the observed fact that shear phases from explosions detonated near high impedance contrasts are of lower frequency than those from earthquakes in the same region, possibly due to the generation of the explosion shear phases by P to S scattering.
Recent experimental analyses show that there is almost no decoupling at high frequencies, in agreement with earlier theories. These frequencies may be observable at regional distances.
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