Site amplification effect of coda waves in West Tunnan Experimental Site

The site effect of coda waves for four types of basement under 6 stations was studied by using the local earthquakes from digital seismograph network of West Yunnan Experimental Site. Results from the band 0.75 to 24 Hz showed that the site effect was abviously interrelated with not only the frequency but also the site condition of the station. For those stations which sited on the hard-rock, such as Jianchuan (limestone), Shuanglang (limestone), Qiaohou (sandstone) and Eryuan (metamorghic rock), the site amplification as a function of frequency was similar in shape and the changes in amplification were flat. A typical behavior of sediment was measured at Qina station. The site amplification remained high at low frequency, but turned to a low level as frequency increased. However, in contrast to Qina, the variations of site amplification at Yangbi station were quite different. Such different behavior between the two sites could be caused by the difference in sediment thickness under valley basins. It could be seen that the site amplification pattern of horizontal motion approached approximately to the vertical despite of some differences. The Fourier spectra of small earthquakes also showed the character of site. It can be expected that the site response has an important effect on the shape of the spectra and that the size of the magnitude is not likely responsible for the spectra recorded at the same site. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, Supp., 1993.