A Study On Near-fault Strong Ground Motions In The Beijing Area

Near-fault strong ground motions have become a very attractive subject of research in seismology and earthquake engineering in the last decade. In the end of the last century, many destructive earthquakes have caused great damages and shown some common characteristics in near-fault zones, which impel researchers to study the basic characteristics and the prediction method of near-fault strong ground motions. Based on these studies, people can predict the spatialdistribution and the time history of near fault strong ground motions for city planning, seismicstructural design and earthquake damage prediction. This thesis is the sub-topic of the seismic hazard evaluation which belongs to the project of the active fault detection in cities. This study will be helpful for the seismic structural design and city planning in the Beijing area, and to provide the technological storage for seismic hazard evaluation in other big cities of China.The research area of this work is about 7800 square kilometers, which includes themid-eastern Beijing and small part of Hebei Province. Mountains are dominant in the north and northeast part of the study area, and plains which are covered with unconsolidated Holocenesediments are located in other part of the study area. In the research area, there are a lot of earthquakes and active faults which seriously affect people's life safety and economical construction. The major regional faults are divided into two groups of NE and NW direction in the study area, and the NE Xiadian fault that is the seismogenic fault of the 1679 Sanhe-pinggu M=8 earthquake is the objective of this research.The study process of near-fault strong ground motions is composed of establishment of the earthquake source models and transmitting medium models, calculation of models and analysis of the calculated result.First is establishment of the seismic source models. The 1679 Sanhe-pinggu M=8 earthquake is the biggest event in the Beijing-Tianjin-Tangshan region with detailed historical records. Seismologists have carried out a great deal of work on this earthquake. In this thesis, reasonable earthquake source models are established and the corresponding parameters are given according to the following four data.1) Historical records of the 1679 Sanhe-Pinggu M=8 earthquake and the isoseismal map of historical earthquakes which is compiled on the basis of historical records and "The Seismic Intensity Table of China".2) The field survey parameters of the more than 10 kilometer long earthquake surface rupture zone and the data of the deep and superficial seismic prospecting in research areas.3) The parameters of the 1976 Tangshan M=7.8 earthquake for reference, because the two earthquakes happened in North China block and are similar in the regional tectonic stress field and of the earthquake rupture mode.4) The empirical relations and formulas between the earthquake source parameters in the North China block and world.Utilizing the above mentioned data, I established 6 different earthquake source models for the finite-difference method(FDM) and 1 earthquake source model for the finite-element method(FEM). Source parameters of the FEM are given in Table 1. Meanwhile, by using the regional geological data, thirteen profiles and the contour map of Quaternary and Tertiary deposits, interface G in the upper crust and interface C in the middle crust in study area, I established every transmission medium model and endowed with corresponding parameters by utilizing the FDM and FEM.Secondly, using the FDM established on previous work and a three-dimensional finite-difference computing program of near-fault strong ground motions, the peak ground velocity(PGV) caused by the destructive earthquake is simulated. In this simulation, a total of 2.28×10~6 grids are generated with the minimum grid spacing of 0.15km, and one computation took about 4 hours by workstation. In addition, applying a three-dimensional finite-element program and parallel computation technique developed by the Institute of Engineering Mechanics, China Earthquake Administration, the PGV and the velocity history for 60 seconds duration time of 32,400 observation points on the ground are calculated. In the simulation of the FEM, the earthquake source is divided into 4928 0.5km×0.5km grids, and the transmission medium model which comprises 36 different character parts is divided into 2.187×10~7 elements with the grid spacing of 0.08km.Finally, some contour maps of the PGV and velocity histories of observation points are drawn according to the simulation results. The analysis of some PGV contour maps of the FDM reveals, some basic characteristics of near-fault strong ground motion, for example:1) Concentration effect. Near-fault strong ground motions with high amplitude are located in a narrow belt area along the projection of the fault on the ground;2) Hanging wall effect. For a dip-slip fault, the ground motion in the hanging wall is stronger than that in the foot wall, and the attenuation of the ground motion in the hanging wall is slower than that in the foot wall;3) Rupture directivity effect. If rupture starts from one end of the fault and sweeps to another, the PGV in front of the rupture will be larger and the frequency will be higher and the duration will be shorter;4) Basin effect. Because of the amplification effect of the loose sediments in the basin and the interaction of the seismic wave with basin margins, the ground motion and duration time in the basin are more obvious than that in the rim of the basin.Then, I selected and analyzed 24 velocity histories of observation points which were along two beelines and a contour map of horizontal PGV in order to verify the simulation result of the FEM. Utilizing the first wave arrival and the elastic rebound theory, the correctness of 24 velocity histories is validated. Meanwhile, he basin effect and rupture directivity on 24 velocity histories of observation points are observed. In addition, it is found that the contour map of horizontal PGV is well in accordance with the isoseismal map of the 1679 Sanhe-Pinggu M=8 earthquake. The results from the simulation and analysis suggest that the source parameters and transmit medium parameters in the models are suitable, and the staggered-grid FDM and the explicit and decoupled FEM are applicable to estimate the distribution of strong ground motions in the study region.