The Initial Study On Strong Ground Motion Near Fault

Recent years, Strong ground motion near fault has become a hot focus of seismology and earthquake engineering. In this paper, the general features of strong ground motion near fault are analyzed and summarized using simple source and simple medium model, then the accelerograms of Shidian main-shock (Ms=5.9) are synthesized using modified empirical Green's function. The mam researches in the thesis are summed as follows:1. The numerical method for dislocation point source in homogeneous elastic half space put forward by professors Yuanyifan and Liaozhengpen are summarized and be extended to homogeneous elastic layered half space in order to consider dynamic response of soil layer.2. The spatial distribution of strong ground motion generated by a vertical strike slip or thrust slip fault and by general fault with a dip angle in homogeneous elastic half space or in homogeneous elastic layered half space are studied using numerical method for dislocation source and source model , and some conclusions are as follows:For shallow vertical strike slip or thrust slip fault, amplitudes of strong ground motion are distributed in a narrow region along the project of the fault on the surface. For shallow dip slip or thrust slip fault, amplitudes of strong ground motion are still distributed in a narrow region, but the region deviate the project of the fault on the surface, the deviation distance is related to the dip angle of the fault. As the depth of the fault increases, the amplitude of the three component of strong ground motion descend, therefore the attenuation with epcientral distance become slow, the impacting region also become wide. For homogeneous elastic layered half space, the amplitudes of strong ground motion become large and the impacting region become wide because of the amplification of the soil. In the meantime, as the multi-reflection of the seismic wave, the distribution of strong ground motion become disorder, but the larger amplitudes of strong ground motion are still distributed in a narrow region. Some components of strong ground motion may show directivity effect because of the rupture process of the fault, however the high frequency components of strong ground motion hardly show directivity effect, hi some periods, amplitude peaks of strong ground motion are focused on the two endpoints of the fault. In the component perpendicular to the fault, it is easy to form big velocity pulse.3 The accelerograms of Shidian earthquake (Ms=5.9) are synthesized using modified empirical Green's function, and two defects are found in the process of synthesizing. The first is that the small earthquake as empirical Green's functioncan't contain all the response of the complicated propagated medium, because big earthquake record near a fault contain the nonlinear response of the site, but small earthquake record can't do. The second is that it is questionable that the empirical Green's function of sub-sources in the fault are obtained only using a small earthquake record near fault plus a modified geometrical and medium coefficient. Because near field strong ground motion decay rapidly with epicentral distance and the depth of fault, and the strong ground motion in various epicentral azimuth is very different. Based on the idea, this paper also presents a method of modified empirical Green's function near fault so as to simulate the main-shock record.