关键词: 近断层强震记录, 基线校正, 永久位移, 921台湾集集地震

Abstract: The Chi-Chi earthquake firstly supplied us unprecedented near fault recordings and made it possible to study deeply the characteristics of near fault ground motion. However, there are considerable offsets of baseline of the near fault accelerograms obtained in this event. The reasons of the offset of baseline are more complex than those occurred in other near field recordings with farther fault distance. Therefore, general baseline correction schemes for near field recordings, such as the methods of BAP (Converse, 1992) and Iwan (Iwan et al., 1985), could not be used directly in baseline correction of this event, as pointed out by Boore (1999). As far as our information goes, no robust procedure for the correction of baseline offsets that can be applied to near fault digital recording of this event has been proposed. This has hampered to a considerable degree the further studies on the near fault ground motion of this event. To develop such a procedure, the specific sources of the baseline offsets must be understood. Basing on the result of our systematical analyses, we propose that the tilt of instruments due to differential settlement of the materials in the vicinity of the instrument should primarily be responsible for the large baseline-offset occurred in the near fault recordings of this event. A baseline correction scheme for the offsets caused by the tilt of accelerograph is put forward in this paper, which could eliminate the offsets of baseline effectively. It is noted that a low-cut filter can eliminate the effects of the baseline offsets, but it may cause the loss of information about the permanent displacement. The method given by the authors in this paper does not involve any filter, so that we can get the permanent displacements from the double-integrated accelerograms, which coincide well with the co-seismic displacements measured by GPS instruments. According to this study, the offsets of baseline have fairly small effects on PGA and response spectra within the period of 0.1 to 6 second, which is of great interest to engineering circle. However, PGV, PGD and D-last are strongly affected by the problem of baseline offset. Therefore, the baseline correction of near fault ground motion recordings is of great importance. The method of baseline correction proposed in this paper can be applied to the near fault ground motion recordings of the 1999 Turkey and Chi-chi earthquakes.

Key words: Chi-chi earthquake, near fault ground motion recordings, baseline correction, permanent displacement