Through the observation and analysis of the ground mierotremors of Tianshui city,it is shown that there are many factors which influence the amplitude of microtremors.The amplitude of the microtremors changs wi-th the topographical and geomorphoiogical conditions.The horizontal mic-rotremors evidently appear to have a predominate direction,which is con-strained by the surrounding topography and geomorphdogy as a whole,and the effects Of the local site conditions on it is secondary.
Some variations of the spectrum characteristic of the ground microtre-mors appear only in the large area,and the spectrum character is cons-trained by the situation of the area geology and new tectonic motion.The resulting spectrum characteristic of the microtremors are the superposi-tion of the effects of the local site conditions and the influences of the area background.
The relationship between the characteristics of the ground microtre-mors and the strong earthquake ground motions is an important research theme on the study of field earthquake ground motion in future.
According to the predominate period and aspect of the spectra of gro-und microtremors,We have divided Tianshui city into some microzones.