Introduction
Displacements derived from the accelerogram recordings of the 1999 Chi-Chi, Taiwan
earthquake at stations TCU078 and TCU129 show drifts when only a simple baseline
derived from the pre-event portion of the record is removed from the records. The
appearance of the velocity and displacement records suggests that changes in the zero-level
of the acceleration are responsible for these drifts. The source of the shifts in zero-level are
unknown, but might include tilts in the instruments or the response of the instruments to
strong shaking. This note illustrates the effect on the velocity, displacement, and response
spectra of several schemes for accounting for these baseline shifts. The most important
conclusion for earthquake engineering purposes is that the response spectra for periods
less than about 20 sec are unaffected by the baseline correction. The results suggest,
however, that staticdisplac ements estimated from the instruments should be used with
caution. Although limited to the analysis of only two recordings, the results may have
more general significance both for the many other recordings of this earthquake and for
data that will be obtained in the future from similar high-quality accelerograph networks
now being installed or soon to be installed in many parts of the world.
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First posted May 27, 2003
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