SITE EFFECTS OF SLOPE PASSED BY FAULT ON PGA,A CASE STUDY AT MIANZHU JIULONG TOWN

There are many landslides passed by faults. They are results from Wenchuan Ms 8.0 earthquake in 2008. Besides the movement of the seismogenic fault, its site effect can also influence the slope dynamic response and increase the slope instability. In this paper, the microtremors and seismic observations are performed along the front slope surface in Mianzhu Jiulong town. It uses the spectral ratio of horizontal to vertical components(H/V) of microtremor data and PGA amplification factors of acceleration records of two typical aftershocks, and obtains the spectral characteristic curves. By comparative analysis of the curves, it is found that: 1)with the growth of elevation, the values of PGA amplification factors decrease at first, and then increase, presenting aconcave shape. The concave-shape curves appear at the monitoring point around the fault zone. The values of PGA amplification factors can just reach to 0.4~1.0,while the values of PGA amplification factors at the monitoring point around the upper slope can even get to be 1.0~2.0. As the turning points appear around the fault zone, it shows the site effects of the fault from the geological view. 2)The NS/UD spectral ratio is generally greater than the EW/UD spectral ratio. The predominant frequency around the fault zone is near to the low frequency of 1Hz, smaller than any other monitoring point. It has the biggest spectral ratio of 3.0~4.0. It tells that the soil and rock of the surface rupture is loose and broken after the Wenchuan main shock. So the predominant frequency of the fault zone is low. The high frequencies of near-field aftershocks are filtered by the fault materials. Then the earthquake acceleration amplification coefficients are reduced as well. This study is helpful to enhance the understanding of the impact of the fault site effect on slope dynamic response.