Abstract

Broadband velocity seismometers generate transient signals with long-period pulses for the tilt motion of the seismometer base; this signal is caused by the response of the feedback circuits incorporated into the seismometers. The reverse estimation of tilt motion is possible using such transient signals. The current study proposes a two step procedure for estimating the tilt motion using the transient signals from broadband seismograms. First, the transient signal is estimated using a median filtering technique. Second, the tilt-motion is estimated by applying a filter with the inverse response characteristics of the broadband seismometer, whose input is the derivative of ground acceleration, to the estimated transient signal and then integrating the output signal of the inverse filter. Thus, the translation motion and tilt motion of the seismometer’s base are distinguished and then identified. The current study reports an example of the tilt-motion estimation using the broadband seismogram recorded at the WJM (Wajima) site of F-net for the Noto-hanto earthquake (M 6.9) of 23 March 2007. The method is also applied to strong-motion accelerograms for the purpose of estimating tilt motion, thereby validating its practical usability.

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