MONITORING RESULT AND ANALYSIS OF SLOPE SEISMIC RESPONSE DURING THE JIUZHAIGOU M_S7.0 EARTHQUAKE

The research group, just in the next day after the "8·8" Jiuzhaigou earthquake, installed microseismic monitoring instruments at the bottom(1^# monitoring point) and the top(2^# monitoring point) of the Jubao Mountain in the Jiuzhaigou county. They collected a good amount of seismic data during aftershocks. The paper seeks to research the seismic response characteristics through analysis of the monitoring data of 1^# monitoring point at the elevation of 1414 m and 2^# monitoring point at the elevatoion of 1551 m above mean sea level. The earthquake monitoring data show the follow features. (1)Compared with the low elevation monitoring point, the peak acceleration of the ground motion in all directions of the high elevation monitoring point show an increasing trend. The horizontal acceleration amplitude of each monitoring point is generally larger than the vertical direction. And the topographic amplification effect is remarkable. (2)During the aftershocks, near SN trending ridge of the Jubao Mountain quaked more fiercely along the east-west direction, which swung violently along the East and West sides of ridge, formed seismic fractures that developed along the ridge. The study of the aftershock monitoring data confirms the existence of the directional effect of the slope ground motion. The peak acceleration amplification effect of 2^# monitoring station in horizontal East-West direction is obviously stronger than that in other directions. Seismic wave energy significantly magnified at the horizontal east-west direction. So, the top buildings were more easily damaged along this direction. These results proved that the local terrain had a controlling effect on slope seismic response. (3)The vertical main frequency of the amplitude spectrum of 2^# monitoring point focused on the range between 6 Hz and 12 Hz. The horizontal East-West frequency focused on the lower frequency range between 5 Hz and 8 Hz. And the main frequency of horizontal north-south direction was mainly 5~10 Hz. Compared with the 1^# monitoring point, the main frequency in all directions of the 2^# monitoring point had obvious attenuation. That is, with the increase of the elevation, the main frequency of the seismic vibration showed a decreasing trend. And the seismic wave in the upper part of the slope was dominated by medium and low frequency. Further calculation and analysis showed that the topographic amplification effect of the thin ridge, the strip-type mountain and multiple free surfaces mountain were closely related to the half wave length of the seismic wave. Under the coupling effect of local topographic dimensions and the rich wavelength components of seismic waves, the topographic amplification effect was remarkable, and the seismic damage of the mountain was developed.