Abstract
The particle motion of local earthquake seismograms is affected strongly by the fine structure details of upper crust. The angle of incidence gets frequency dependent, shear wave splitting occurs and strong P-SV conversions contaminate the P-coda. Contrary to teleseism, it is not possible any more to detect S-onsets by conformance tests between data and simple models. Instead, we must derive polarization images in the time-frequency plane that display particle motion without any assumptions. By suitable scaling, these images neutralize all high frequency effects and allow for onset recognition by simple patterns. The method was applied to the 800 events of 1989 evaluated by the Bochum University Germany (BUG) observatory. We determined a 67% success rate with 13% wrong and 20% rejected because of unstable phase energy. For two source regions, the automated results are shown to be more reliable than interactive routine evaluation by man.
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