SEISMIC RESPONSE OF XUEJIABA SLOPE DURING THE JIUZHAIGOU M_S3.2

In order to research the seismic response laws of a mountain at different depths of a slope, a seismic monitoring array was installed in the Xuejiaba slope of Jiuzhaigou county. Four monitoring instruments at the foot and top of the mountain were triggered by a M_S3.2 aftershock. The data reveal the following feature. The terrain of the hill, which is located at the protruding position on the surface of the slope, has obvious amplification effect. At the same time, the horizontal acceleration peak is greater than the vertical direction. Comparing with the another monitoring at the foot of the mountain, the PGA at the 2^#-1 monitoring point on the surface of slope is maximum. Its arias intensity enlarges more than five times. The largest amplification effect is obvious in the vertical direction. The acceleration is 2.48 times and the alias intensity is 5.24 times. The amplification effect of acceleration at each monitoring gradually attenuates with the deepening of the horizontal depth of the mountain. The value of PGA decreases from 2.48 to the inner of 2.03, and the Alias intensity is reduced from 5.84 to 3.92. The peak acceleration decreases greatly within the surface range of 0 to 25 m and at the inner slope falls at a slow rate. Fourier spectrum indications the following features. The frequency range get from the foot of mountains is 0-50 Hz. The main frequency value is about 23 Hz. The frequency range of the 2^#-1 monitoring point has not changed significantly compared with the data from the foot of the mountain. But the main frequency value is obviously reduced about 5 Hz. Spectral frequency components are complex, with the depth of the hole deeper in horizontal direction, the amplitude and frequency components of each monitoring point gradually decrease. Through the analysis of all of the data, it is confirmed that under the earthquake, with the increasing of the amplitude of acceleration, the energy of the ground vibration can increase geometrically, and the surface of the mountain can be the most powerful position. If accumulation of the vibration energy excesses the strength of the rock mass in short time, it is easy to trigger collapse and landslide, which have a great impact on the safety of the construction.