Effect of baseline corrections on displacements and response spectra for several recordings of the 1999 Chi-Chi, Taiwan, earthquake

Displacements derived from many of the accelerogram recordings of the 1999 Chi-Chi, Taiwan, earthquake 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 i n the zero level of the acceleration are responsible for these drifts. The source of the shifts in zero level are unknown, but in at least one case it is almost certainly due to tilting of the ground. This article illustrates the effect on the ground velocity, ground displacement, and response spect ra of several schemes for accounting for these baseline shifts. A wide rang e of final displacements can be obtained for various choices of baseline co rrection, and comparison with nearby GPS stations (none of which are coloca ted with the accelerograph stations) do not help in choosing the appropriat e baseline correction. The results suggest that final displacements estimat ed from the records should be used with caution. The most important conclus ion for earthquake engineering purposes, however, is that the response spec tra for periods less than about 20 see are usually unaffected by the baseli ne correction. Although limited to the analysis of only a small number of r ecordings, 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 insta lled or soon to be installed in many parts of the world.