A Study On Long-period Acceleration Response Spectrum Of Ground Motion Affected By Basin Structure Of Beijing

With the rapid growth of urban population and soaring land price, a lot of high-rise buildings have been built in the city, and the numbers continue to rise. On the other hand, many cities, such as Beijing, Yinchuan, Xian, Hohhot in China, are built in basins, and also have tectonic background prone to major earthquakes. Earthquake disaster characteristics of buildings in several major historical earthquakes indicate that basins can amplify the long-period ground motion significantly, and cause many high-rise buildings to be destroyed. So, the seismic fortification of high-rise buildings built in the basins becomes more and more important. However, in our current Code for Seismic Design of Buildings and Code for Seismic Safety Evaluation, basin amplification effect on ground motion is still not specified. It brings potential risk to city seismic disaster prevention and future planning.As our country’s political center, economic center and cultural center, Beijing city has a very important position. From view of science research, firstly, Beijing city is located in the basin, and has tectonic background prone to major earthquakes. Secondly, thousands of high-rise residential buildings over25stories and super high-rise commercial building over100meters have been or will be built. The natural periods of these buildings are usually longer than three seconds, and some of them are even10seconds. Because of the resonance effect with3-10-second ground motion, these buildings are prone to be ruined when major earthquake occurs. Besides, strong ground shaking can cause fire. These will not only cause a severe casualty and property loss, but also make earthquake emergency and rescue very hard. So, studying Beijing basin’s amplification effect on3-10-second ground motion acceleration response spectrum is very urgent, because it can help us to understand reasonable seismic fortification requirement of high-rise building in basins, and provide scientific reference for city future planning, seismic design for new high-rise buildings, seismic disaster prevention of high-rise building, earthquake emergency and rescue.In view of the shortage of strong ground motion records in Beijing basin, this paper uses parallel-computing finite difference method to simulate ground motion, and studied how3D basin structure of Beijing amplifies3-10-second ground motion acceleration response spectrum. The main work and conclusions include the followings:1. Basic principle and parallel-computing implementation plan of long-period ground motion simulation by finite difference method are explained in detail, and parallel-computing platform is introduced.2. On the basis of collecting and sorting of seismic exploration data, geological data, drill data, seismic velocity data, and other investigation data, we have modified the existing basin model of Beijing, and set up a new3D velocity structure model including topography. Through analyzing the historical earthquake with M>5and activity characteristics of active fault, we have modeled reasonable seismic sources which generally consist with seismic tectonics environment of Beijing basin, and provided right seismic inputs for simulating ground motion by finite difference method.3. Through simulating ground motions with seven seismic sources, basin structure’s amplification effect on acceleration response spectrum is analyzed. The research demonstrates that the amplification effect on3-10-second ground motion acceleration response spectrum is mainly controlled by basin structure, although different seismic sources may lead to the deviation to a certain extent. Obviously, it is connected with the thicknesses of both Quaternary and Tertiary, especially the thicknesses of Quaternary.4. The new conception called "equivalent sediment thickness" is proposed. The function relationship between equivalent sediment thickness and basin amplification factor of3-10-second ground motion acceleration response spectrum is given. The result tells us:Basin amplification factor is not a constant. It is in good correlation with equivalent sediment thickness, and increase with equivalent sediment thickness. A binomial formula can be satisfied very well with the function relationship.5. At a place where equivalent sediment thickness is larger than200meters, and in3-10second period range, basin amplification effect on3-second acceleration spectrum is strongest, and then decreases with period. When equivalent sediment thickness and period keep constant, the average basin amplification factor on horizontal acceleration spectrum is larger than that on vertical acceleration spectrum. Additionally, for the two horizontal components, the average basin amplification factor on fault-parallel acceleration spectrum is larger than that on fault-perpendicular acceleration spectrum.6. Basin amplification effect and surface soil amplification effect are compared. In the areas where equivalent sediment thicknesses are not larger than500meters, basin amplification coefficients on T>3-second acceleration response spectrum are basically equivalent to surface soil amplification coefficients. However, in other areas where equivalent sediment thicknesses are larger than500meters, basin amplification effect has larger values than surface soil amplification effect.7. Based on the functional relationship between average basin amplification coefficient and equivalent sediment thickness, zonation of basin amplification effect is made. It gives some scientific advice for seismic disaster prevention and future planning of Beijing city.8. Distribution, layer and height information of both high-rise residential buildings over25stories and super high-rise commercial buildings over100meters are statistically analyzed, and their natural periods are calculated. According to the zonation of basin amplification coefficient on acceleration response spectrum, we can do the following things:(1) The relationship between risk level of seismic disaster of existing high-rise buildings and basin structure is discussed. CBD (Central Business District), the eastern region to CBD, the south region between the West Second Ring Road and the West Third Ring Road, and the northern region to the North Fifth Ring should be key considerations in earthquake emergency preparedness and rescue, because severe seismic disaster perhaps happens in these regions.(2) The centers of Shunyi depression and Dachang depression are pointed out to be the places where basin amplification effect is strongest, and should be avoided to build the same kind of high-rise buildings in the future.(3) When building high-rise buildings in other regions where amplification factor are smaller, the seismic fortification criterion should be improved according to the natural period of the building structures and our research results.