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

1999 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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 static displacements 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. The digital accelerograph installed and maintained by Central Weather Bureau in Taiwan produced a rich set of records from the September 20, 1999 M = 7.6 Chi-Chi, Taiwan earthquake and its aftershocks (Shin et al., 2000). This earthquake has produced the most complete set of strong-motion recordings ever obtained. The data were obtained from Teledyne Geotech Model A900 digital accelerographs, with ±2 g gain and 16-bit resolution, recorded at 200 samples per second. The transducers are flat to acceleration from dc to 50 Hz. The system uses a trigger algorithm and a buffer so that the pre-trigger part of the ground motion is preserved. At this time data from only two stations have been released (Lee, 1999), but the features found in the analysis of these records are probably not restricted 1 to those records. The stations for which data have been released are TCU078 and TCU129. These stations are within …