Study On Near-field Characteristics Of Strong Ground Motion During The 1999 Chi-Chi, Taiwan, Earthquake

The digital accelerograms from the 1999 Chi-Chi, Taiwan, earthquake, with a Richter magnitude 7.3 (Mw 7.6), provided valuable research potentials for the studies on the long period properties of near-fault strong ground motions. Partly based on evidence from some correlative literatures and ground motion recordings, we understand that the acceleration attenuation should be differentiated between the east and west side of the causative fault (Cher-Lung-Pu fault). And on the other hand, a similar phenomenon seems to be found due to the non-causative fault (Chang-Hua fault). This lead to a research question, that is how non-causative fault affects the ground motion transmitting process.In addition, a lot of research works on the peak acceleration and peak velocity attenuation with the epicentral distance and the perpendicular distance. However, topics and paper is rare about the attenuation of the peak ground displacement. The photos of the destructive buildings, bridges, pipeline show that the main cause was the difference between the two sides of the Cher-Lung-Pu fault. By using the digital broad-band accelerograms, we could get plenty of pre-event and after-shock records. Then the goal of this paper is the research on the displacement properties of the earthquake ground motions on the two sides of the causative fault and non-causative fault. The main research works and results in this paper areI. Firstly, All sources of data are from digital records stored in a CD-ROM published by the Central Weather Bureau (CWB) at Taipei, Taiwan. Out of a total of 640 digital accelerograms deployed at the "free-field" sites contained in the CD-ROM, 441 digital strong-motion records were prepared and processed from 663 data files based on 441 accelerograms. In addition, 35 of 55 strong-motion arrays in operation in buildings and bridges recorded the main shock. In order to obtain the corresponding velocity and displacement time-histories of those accelerograms we adopt Wang's baseline correction method (Guo-Quan Wang ), which is suitable in dealing with data of near-fault strong ground motion. The velocity and displacement time-histories of the earthquake ground motions on the two sides of the causative fault andnon-causative fault are obtained by the baseline correction method and integration method.2. After getting the displacement time-histories of the earthquake ground motions near the causative fault, we find that perpendicular distance is adopted in preference to epicentral displacement because statistical results indicates that the latter falls short of the research's methodological assumptions as the appropriate displacement research parameter. In this paper, we choose two static parameters, peak ground displacement and permanent ground displacement to study the displacement properties of the ground motions near the causative fault. The difference between the two parameters is considered to describe the dynamic earthquake transmitting process. As the result, a linear relationship between the peak ground displacement and permanent displacement is presented. The results show the great difference between the upper wall and footwall.3. To analyze diversity between the upper wall and footwall of the causative fault, we deal with the some periods' acceleration spectrum and deduce their attenuation by the perpendicular distance.4. we also deduce what the existing non-causative fault acts in the strong ground motion attenuation. Although it is not very clear, because of the absence of the earthquake records, it can be sure the non-causative fault do something in the attenuationThe author hopes that the conclusions in the paper are useful in the future on the relative works.