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2.2 Low frequency errors correction method of accelerograms recorded by digital
accelerographs
After comparing the results of different methods, the correction method of low frequency errors
of accelerograms recorded by digital accelerographs is proposed, as follows:
1. Adjust zero baseline of original accelerogram by subtracting the average value of preevent
records, and subtracting, the subtract average value of whole length of accelerogram or use
in the least squares method if there is no pre-event records can be available.
2. Filtering the accelerograms by Butterworth digital filter, and the cut-off frequency can be
determined by Fourier spectrum analyses for seismic records and pre-event noises records. If
there is no sufficient long pre-event records can be used, we can select one or several cut-off
frequency according to the experiences to trial, and a suitable cut-off frequency can be
determined according to if the drift of zero baseline of displacement curve is eliminated or not.
3. Calculating acceleration response spectrum, velocity response spectrum and displacement
response spectrum of filtered records. Integrating the velocity and displacement time histories,
adjusting the zero baseline of displacement time history using least squares method in order to
eliminate linear trend.
2.3 Features of Long-Period Spectrum of SMART-1 Array Records
2.3.1 Record and disposal of digital strong motion Accelerograph
236 accelerograms recorded by SMART-1 array in 8 earthquakes with magnitudes above 6 are
selected for analyzing the long period component characteristics of seismic ground motion on the
same category of site in a small region. Table2 list the data of the 8 earthquake.
All of the epicentral distances in Table2 are the epicentral distances of central point (COO), the
maximum distance between various stations is only 4km, so the epicentral distances of each
station can be considered as same in these earthquakes, except No.7 earthquake. All records are
processed using the method described above.
2.3.2 Response spectra characteristics of accelerograms obtained in an earthquake
Figure 11 and Figure 12 show the dynamic amplification factor /3 curves of accelerograms
recorded at different stations during two earthquakes (No,4 and No.8) respectively (damping
ratio is 0.05). In the Figures, the symbols "o" and "*" represent average values of /3 and a
standard deviation respectively. It can be seen that in short period below 4s, although /j curves
are similar for most of stations, some f$ curves are very different and the standard deviations are
large. For example, in case of No.4 earthquake, the corresponding periods for peak values of p
curves are Is or so for most stations, and are 0.3~0.7s for other stations (Figure6). The
epicentral distances of all stations are almost same in the two earthquakes. So, It shows that the
acceleration response spectra, in short period part are quite different even if the accalerograms
are recorded at same category of sites in an earthquake. However, for the accelerograms obtained