We present evidence for significant deviations of the true instrument gain from that reported for several modern broad-band permanent seismograph stations. Our result derives from a systematic comparison of observed and synthetic long-period seismograms for approximately 600 large earthquakes. Seismograms were collected from globally distributed stations and analyzed using the centroid-moment-tensor (CMT) algorithm for estimating earthquake parameters. Following the source inversion, synthetic seismograms corresponding to the final earthquake parameters were compared with the observed seismograms and an optimal amplitude-scaling coefficient for each seismogram was determined. Scaling coefficients for earthquakes occurring in a given calendar year were then averaged to investigate...

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