There is a dispute about the principal character of the ground microtremor but it is universally accepted that the microtremor is controlled by the condition of soil character of surface of the site. On the other hand it is necessary to have a further study on how deep the soil structure is reflected from the ground microtremor and whether it can be used to identify the type of the site soil and its combination type, In this paper on the basis of observations of the ground microtremor of the cover layer of 400 to 900 m thick in Xian and the data from deep drilling, the following results have been achieved:
1. The maid factor which makes an impact on the frequancy of the ground microtremor is the site soil layer structure and its combinational type;
2. The depth affecting the first predominant frequancy of spectrum of the ground microtremor would be 120-190m;
3. The different frequancy featurдs are associated with different sites and conbinational types of soil structure;
4. The surface layer plays the main part in t⁵e impact on the frequancy of microtremor.
In terms of experiments of explosions, it is seen that the soil structure can be damaged when the acceleration of the surface motion reaches. 1.0 g namely 10 degrees of intensity. The frequancy spectra of microtremor before and after explosions seems alike while the amplitude of microtremor after explosions descents and the peak value moves to the lower frequency end. Compared these results with the spectra of ground acceleration during the explosions, the three are extremely resemble. Similar features could be seen in the spectra of the strong ground motion recorded in Wenxian Station associated with a strong earthquake of M=7 in Songpan.