| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 91 === In this paper, the accelerograms recorded by the network of Taiwan Strong Motion Instrumentation Program of the Central Weather Bureau are used to analyze the strong motion characteristics in west-central Taiwan. The data consists of 9 earthquakes with ML>= 5.3 occurred in Taiwan area between 1995 and 1999. Firstly, the station sites are classified into three types by their spectral ratios of horizontal to vertical components(H/V) and geological data, from hardness to softness, called type A, B and C respectively. Then, the strong motion characteristics in regard to source magnitude, depth, hypo-distance and site type are analyzed. In order to highlight the near-fault effect during Chi-Chi mainshock, several kinds of comparisons have been made. The results are summarized below:
The H/V spectral ratios at the specific site calculated from different events are very similar, implying that the H/V ratios are more independent of source magnitude, depth and path at a specific site. Also, the classification results obtained by H/V method are compatible with geological data. For any event, the dominant frequency of H/V spectrum decrease gradually with the site type from A to B to C, but the dominant amplification factor is increased. Of the Chi-Chi mainshock, its dominant frequency and amplification factor are the lowest among the 9 events.
Within the same event, the response spectra for 3 types of sites are very consistent in short period range, however, those have much differences in long period range. The spectral shape has no significant change as the hypo-distance increased from near to far data group is lower than 30 km. Among the different events, however, the spectral shape has much more differences even for the same type of site as well as the same hypo-distance. Therefore, we can conclude that the source effect has more influence on response spectra than the site effect has.
The total means of normalized response spectra calculated from all of 8 events are similar to other study, those from the Chi-Chi mainshock are amplified in long period range. The current seismic design spectrum, anchored at a PGA value of 0.33g, appears to be much adequate for three types of site experiencing the 8 events, and seems to be underestimated, during Chi-Chi mainshock, for site type of A below about 1.2 second.
The ground motion parameters, v/a and d/v, have significant variations among the different events. We obtain the larger values of these two parameters from events which have the greater magnitude, shallower depth, and farther hypo-distance. The Chi-Chi mainshock has the largest values of parameters among the 9 events, its maximum ground velocity and displacement, for a unit ground acceleration(1g), are twice to 3 times of the values given by other studies. However, the parameter ad/v2 has no significant trends among the 9 events, they almost lie between 2 and 4.
For farther hypo-distance, the softer sites of type C not only have the larger values of PGA but also the larger ratios of vertical to horizontal components(V/H). On the viewpoint of total mean of all station sites, the PGA V/H ratios from events with greater magnitude are larger than those with smaller magnitude, but the PGV is not obvious. Of the Chi-Chi mainshock, its V/H ratios for both PGA and PGV are the largest among the 9 events, these ratios also exceed 2/3 which is commonly used in seismic zone A.
The amplification factors of response spectrum related to peak ground motion are almost between 2 and 3, and no exceptions for Chi-Chi mainshock.
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