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

摘要： 分析了921台湾集集地震近断层强震记录基线偏移的原因,指出强震仪的倾斜是导致近断层强震记录基线偏移的主要原因,并提出了相应的基线校正方法,比较好地消除了近断层强震记录的基线偏移.根据作者的研究,原始记录经基线初始化之后,基线偏移对反应谱在工程界最感兴趣的0.1～6s周期范围内的影响是很小的,对PGA的影响也很小,但对PGV,PGD和永久位移(D-last)的影响则相当大.因此,对近断层强震记录进行基线校正具有重要的意义.文中所提出的基线校正方法,可应用于对1999年土耳其地震及921台湾集集地震余震产生的近断层强震记录和其他近断层强震记录的基线校正.

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