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Automated Detection, Extraction, and Measurement of Regional Surface Waves



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Pure appl geophys 158 2001 1531 1545 0033 4553 01 081531 15 1 50 0 20 0 Birkha user Verlag Basel 2001 Pure and Applied Geophysics Automated Detection Extraction and Measurement of Regional Surface Waves A L LEVSHIN1 and M H RITZWOLLER1 Abstract Our goal is to develop and test an e ective method to detect identify extract and quantify surface wave signals for weak events observed at regional stations We describe an automated surface wave detector and extractor designed to work on weak surface wave signals across Eurasia at intermediate periods 8 s 40 s The method is based on phase matched lters de ned by the Rayleigh wave group travel time predictions from the broadband group velocity maps presented by RITZWOLLER and LEVSHIN 1998 and RITZWOLLER et al 1998 and proceeds in three steps Signal compression signal extraction or cleaning and measurement First the dispersed surface wave signals are compressed in time by applying an anti dispersion or phase matched lter de ned from the group velocity maps We refer to this as the compressed signal Second the surface wave is then extracted by ltering noise temporally isolated from the time compressed signal This ltered signal is then redispersed by applying the inverse of the phasematched lter Finally we adaptively estimate spectral amplitude as well as group and phase velocity on the ltered signal The method is naturally used as a detector by allowing origin time to slide along the time axis We describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the Chinese Lop Nor test site from 1992 through 1996 and one explosion on the Indian Rajasthan test site that occurred in May of 1998 Key words Surface waves Rayleigh waves matched lters group velocity nuclear monitoring Ms mb discriminant 1 Introduction The Ms mb discriminant and its regional variants are the most reliable transportable means of discriminating earthquakes from explosions To measure



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