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tens or more than a hundred accelerograms recorded at each event, and mostly recorded by digital
accelerographs. These instruments possess a wide dynamic range, broad frequency band, high
sampling rate and pre-event memory. The low frequency errors in the records of these instruments are
mainly due to instrument noise and site background noise. The total noise level could be estimated by
comparing the records with the pre-event recordings. A result based on limited data showed that the
longest period of the effective frequency band could reach a dozen seconds (Xie Lili etal., 2000). By
checking if the long period drift could be removed completely in the displacement time history by a
double integration of the acceleration record, a high pass cut-off frequency was selected as
0.067Hz~0.083Hz.
In this paper, 132 free field records from the 1999 Chi-Chi earthquake are used for comparing
systematically the Fourier spectra of the pre-event recordings (i.e. noise) with those of the strong
ground motion records. In general, the Fourier spectrum amplitudes of noise show a smooth
distribution hi the band of most engineering interest, while they are getting larger at periods of about
10 seconds. The spectrum amplitudes of strong ground motion are generally much higher than those of
the noise, but descend gradually at very short and long periods, becoming similar to those of the noise.
All results from the 132 records show that the signal to noise spectral ratio is greater than 90 at period
of 10.24 second. It is higher than the
ratio at high frequency range of more
than 30 Hz. The facts show that the
digital records of strong ground motion
are reliable at least up to 10 second. The
seismic effect coefficient at periods in Mtf
the 6.0-10.0 second range could be
stipulated in the seismic design code 1 icf
from these records. An example of a
Fourier spectra comparison of noise and
strong ground motion recorded by the
same accelerograph at the same position in
two time periods with just tens seconds in
between, is shown in figure 2.
; ia 2 ia 3 ia 2 ia 1 icP io 1 10 2
Lgf(mHz)
Fig.2 Comparison of Fourier spectra of noise and
ground motion
As mentioned above, the amplitude of the design spectrum is depressed obviously by the improvement
in the new code. However, it is still conservative if the band of the spectrum extends up to 10 seconds.
In the seismic design codes of many earthquake countries (Euro code 8, 1993), the descending rates
are given as T" 1 and T" 2 in moderate and long period ranges respectively. The reliability and safety
redundancy of a design spectrum descending with T 1 at long period range are validated on the basis of
760 strong ground motion records. The numbers of records in groups of magnitudes (M) and distances
(R) are listed in the following table.
Table 2
Strong ground motion data in groups
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