岷县漳县6.6级地震黄土场地放大效应地脉动测试研究

Microtremor Measurement Research of Amplification Effect in the Loess Site of the MinXian-ZhangXian M_S6.6 Earthquake

2013年7月22日甘肃岷县漳县MS6.6级地震在极震区出现了黄土场地放大效应,为了从理论上解释黄土场地放大效应的原理,本文利用地脉动测试了极震区2个典型的黄土场地进行研究。结果发现:在黄土覆盖地区场地卓越频率特征显著,即在同一区域内山顶卓越频率较低,集中在1～2Hz,山沟卓越频率较高,集中在3～6Hz;同时,黄土场地随着高程的增加表现为厚黄土分布特征,其场地卓越频率在1.3～1.8Hz,与地震观测波的卓越频率1.0Hz接近,呈现出较明显的场地放大效应。结果与实际受灾情况相符。

An Ms6.6 earthquake occurred on July 22, 2013, in Minxian--Zhangxian, Gansu prov- ince, and the amplification effect of the loess site appeared in meizoseismical area. To explain the principle of amplification effect of loess site in theory, we measured and researched two typical lo- ess sites by using microtremor measurement. The results show that, the damage of adobe house is more serious on the summit than on the gully. On the summit the damage of adobe house is se- rious and there are even entire collapses, but on the gully there are cracks and not collapse of ado- be house. The intensity difference between the summit and gully is intensity Ⅰ-Ⅱ , which shows obvious amplification effect of the massif. In the paper, we have made contrastive study of the predominant frequency of microtremor measurement by choosing two typical areas of the ob- vious amplification effect of loess site at the meizoseismal area （intensity 8）, which are Yongxing village and Daleng village, Majiagou village and Xinglin village in Meichuan town, Minxian Coun- ty. At Yongxing village and Majiagou village, the damage of adobe house is serious and there areentire collapses. However, at Daleng village and Xinglin village, there are cracks and not collapse of the same adobe house. The earth pulsation has rich geophysics information, which is aggrega tion of body wave and surface wave. In the paper, the principle, test method and causes of ground microtremors are introduced. According to engineering examples, the determination for the predominant period of a site is described,moreover, its application in evaluation for the cate gory of the site and soil and the effect of different site conditions on the characteristics of frequen cy spectrum are analyzed. The results show that the thick soil and thin soil structure obviously affect the frequency spectra of the ground microtremors, and there are corresponding relation- ships between them. The research results show： the characteristic of predominant frequency of the site is obvious in the covered loess area, in which the predominant frequency is lower on the summit than on the gully, the range of which is 1~2 Hz and a^6 Hz respectively;Meanwhile, with the increase of the altitude of the loess site, which shows the distribution characteristic of thick loess, the range of the site predominant frequency is 1.3~1.8 Hz, which is similar to the predominant frequency 1 Hz of earthquake observation wave and shows obvious amplification effect of the site, which is the same with the actual damage fact.