Using the downhole strong ground motion array to study the nonlinearity of soft soil layers
Speaker: Yi-Wei Cheng
Abstract
Both of Taiwan and Japan have high s eismic a ctivity because of they are located on the colli sion boundary between plates. There is not only direct destruction by the seismic wave but also occur others effects like liquefaction. Liquefaction is one of phenomenon caused by the non-linear soil response in strong ground motion. In recently, the non-linear soil response is being more and more importan t because of the high-quality digital seismic recording from major earthquake become available. A main obstacle to study the non-linear soil response is that seismic recordings are contaminated by source, path and site contributions. That's why we should remove the source and path effects first if we want to identify the local site effect. The method is to transform the seismic recording from time domain to frequency domain and take the ratio with a nearby re ference station. The ratio is an approximation of the frequency-dependent site amplification function. In the theory, we usually choose a station which is installed on a hard rock outcrop as the reference station. Because of the station is relatively unaffected by site. But the near reference station is difficult to found. So, in this study, we use the data recorded by the vertical array of seismometers to estimate the site effect. The distance between the seismometers is negligibly short compared with the distance to the hypocenter . Even if the borehole depth of the vertical array do not reach the basement, it is useful to study the local site effect where the nonlinear effects are expected to occur, in the soft soil layers closer to the surface. In this study, the data we use are recorded by three vertical seismic arrays. There are two arrays which is dispose d at Lotung, Taiwan called LSST array and Hualien, Taiwan called SMART2 array. Another array is dispose d at the Port Island, Kobe, Japan. We find clear non-linear response at the LSST array and the array is disposed at Japan after comparing the spectral ratios of surface to downhole horizontal accelerations on weak and strong motion. And the response is more significant at the near-surface layer. The behaviours include a shift of fundamental frequency to lower frequency area and the deamplification of the strong motion occurred in the frequency range from approximately 1 to 10 Hz. But there are no similar non-linear response is found on the SMART2 array. We make a guess at the stiffer soil conditions and weaker accelerations achieved at the SMART2 site. The results indicate that the non-linear response can be found at the near-surface soft soil layers above a threshold acceleration level.
Reference
Wen, K. L., Beresnev, I. A. and Yeh, Y. T. (1995). Investigation of non-linear site amplication at two downhole strong ground motion arrays in Taiwan. Earth. Eng. Structure. Dyn ., 24 , 313-324.
Aguirre, J. and Irikura, K. (1997). Nonlinearity, Liquefaction, and velocity variation of soft soil layers in Port Island, Kobe, during the Hyogo-ken Nanbu earthquake. Bull, Seism. Soc. Am. 87, 1244-1258.