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A Search for Temporal Variations in Station Terms



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Bulletin of the Seismological Society of America Vol 98 No 5 pp 2118 2132 October 2008 doi 10 1785 0120070243 A Search for Temporal Variations in Station Terms in Southern California from 1984 to 2002 by Guoqing Lin Peter M Shearer and Egill Hauksson Abstract We use relative arrival times and locations for similar earthquake pairs that are found using a cross correlation method to analyze the time dependence of P and S station terms in southern California from 1984 to 2002 We examine 494 similar event clusters recorded by Southern California Seismic Network SCSN stations and compute absolute arrival time variations from the differential arrival time residuals obtained following event relocation We compute station terms from the robust means of the absolute arrival time residuals from all events recorded by each station at 3 month intervals We observe nine stations with abrupt offsets in timing of 20 70 msec which are likely caused by equipment changes during our study period Taking these changes into account could improve the relative location accuracy for some of the event clusters For other stations we generally do not see systematic temporal variations greater than about 10 msec Analysis of residuals along individual ray paths does not reveal any clear localized regions of apparent velocity changes at depth These results limit large scale long lasting temporal variations in P and S velocities across southern California during this time period to less than about 0 2 However there is an increased fraction of individual travel time residuals exceeding 20 msec immediately following major earthquakes from source regions near the mainshock rupture Introduction Recently waveform cross correlation has become an increasingly important tool for improving relative earthquake locations characterizing event similarity and studying earthquake source properties e g Nakamura 1978 Poupinet et al 1984 Got et al 1994 Dodge et al 1995 Nadeau et al 1995 Gillard et al 1996 Shearer



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