Site Effect Of Strong Ground Motion For Dislocation Basin And Fault Fracture Zone

Analysis and Simulation of ground motion field is one of the main aspects in engineering seismology. Besides source effect of causative fault and attenuation effect of ground motion, local site conditions are the important influencing factors of ground motion, which contains local site soil, local topography, and local tectonics. There is no common understanding about the effect of the local tectonics on ground motion.In the key region of earthquake monitoring and defending in southwestern China, such as the surrounding areas of the Xianshuihe fault zone and Anninghe fault zone, many strong motion observation stations were arranged in the 'tenth five-year plan'. Just two months later after these stations entered formal operation, numerous records were obtained from Wenchuan earthquake and its aftershocks, some of which are very useful to study the effect of the non-causative fault ion ground motion.In this study, by analyzing the peak ground acceleration (PGA) and spectral characteristics of the earthquake acceleration records from the main shock and aftershocks of Wenchuan Earthquake, the difference of the ground motion at both sides of the non-causative faults such as Xianshuihe fault zone, Anninghe fault zone and Weihe fault zone are discussed. By analyzing the difference of the local site conditions at the observation stations, such as local site soil, local topography, and local tectonics, pilot explaining of the discrepancy is attained, and numerical solution of 2-D model of those sites derived by the explicit finite element combining with local multi-transmitting artificial boundary method is used for interpreting the mechanism of difference of the ground motion. Results of numerical calculation indicate that, in spite of its simplicity, such an idealized model is also successful in reproducing many of the essential characters of ground motion and the mechanism of the difference of ground motion within the fault zones of Xianshuihe, Anninghe and Weihe.The character of dislocation basin under control of the Anninghe fault zone may be used for the interpretation of observed seismic data obtained in Wenchuan main shock and theoretical calculations of near fault responses in Xichang City. And the character of dislocation basin and fault scarp under control of the fault may be used for the interpretation of observed seismic data obtained in Wenchuan mainshock and theoretical calculations of near fault responses in Baoji City. By comparing the difference between the acceleration records in Shimian region obtained in Wenchuan main shock and its aftershocks and Shimian shock, and theoretical calculations of near fault responses in this area, anomalously large amplitude and extremely rich high frenquency in the ground motion are found near source side (right side of the fault in this case). The results indicate that the non-causative fault can obstruct the spread of seismic wave. And the isolation effect of the non-causative fault is found.Abundant research on the fault parameter influencing the site ground motion was done, and some valuable results are achieved. Numerous numerical simulation indicates that the parameters of low velocity fault zone (LVFZ), such as the width and wave velocity, influence the site ground motion prominently. The numerical simulation shows that transform waves can develop inside LVFZ when struck by external, vertically incident seismic radiation. And trapped waves can develop obviously inside the LVFZ while the wall rock's wave velocity is 3 times larger than LVFZ's. This result implies that the invariance of energy, struck by external incident seismic radiation, is congregated at the low velocity zone, and the convergence effect increases with the increase of the width of the LVFZ. The study of 2-D vertical LVFZ model struck by external oblique incidence seismic radiation shows that, vertical LVFZ can obstruct the spread of oblique incidence seismic wave, and the isolation effect decreases with the increscing incidence angle.The study about the seismic response of a variety of LVFZ models with different width and velocity struck by external vertically incident seismic radiation implies that, the amplification characteristics of ground motion in near fault zone site are frequency dependent. The site amplification characteristics are different under the earthquake input with different predominant frequency. The higher frequency components of earthquake input are amplified obviously by the fault fracture zone with narrow width and low medium velocity, rather than vice versa. It is because that the site amplification coefficient is frequency dependent, and the narrower width and lower velocity of fault fracture zone, the lower predominant frequency.The analysis of ground motion at the hidden fault site shows that the vertical incidence of earthquake input is obstructed by the overlying rock mass, and with the increase of thickness of the rock mass, the obstruction effect becomes stronger. Besides that, the effect of fault fracture zone on the ground motion becomes smaller with the depth increasing, and the effect can be neglected at some depth. The amplitude and high frequency components of ground motion at the whole site and the projection area of the fault fracture zone decrease with the increase of the depth of overburden, meanwhile, the influence scope of the surface becomes smaller and the intensity of ground motion decreases with the increase of the depth. With the decrease of overburden velocity, the ground motion becomes stronger; however, the effect of fault fracture zone on the ground motion becomes weaker. The analysis of ground motion at the dip-slip fault site shows that the ground motion at the upside area of the fault fracture zone is smaller than that at the underside, and the isolation effect exists when the vertical incidence of earthquake input spreads through the fault fracture zone.