Abstract

Broadband velocity seismographs installed at the Ohtsu water level observatory and Monsizu stations of the Warning Information System of Earthquake (WISE) network recorded long-period pulses produced by tilt motions resulting from the 2003 Tokachi-Oki earthquake (M 8.3) of 25 September 2003. The long-period pulse signal is represented using a low-pass-filtered expansion composed of intrinsic mode functions. By eliminating the leading phases, such as a direct P phase that appears before the onset of tilt motion from the expansion, the transient signal produced by the tilt motion is approximately obtained. A reverse filter, which is defined by the inverse of the frequency characteristics of the broadband velocity seismometer for the acceleration input, is applied to the transient signal to estimate the tilt motion. The estimated permanent acceleration of tilt motion at the two sites ranges from 0.6 to 6.5 cm/sec2. The river embankment adjacent to the Ohtsu water level observatory site collapsed due to liquefaction at the bottom of the embankment. The Monsizu site was constructed at a depth of 2 m in soft soil floodplain deposits. These two broadband velocity seismograms involving the tilt motion reveal that the tilt motion is mainly generated under the influence of the collapsed soil structure or of the deformation of the soil deposits. These observations indicate that the broadband velocity seismograph is more sensitive to tilt motion than acceleration-type strong-motion seismographs.

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