SUPERSONIC MODELLING OF EARTHQUAKE GROUND MOTION IN THE BEIJING DEPRESSION

The investigation of the earthquake damages during the Tangshan big earthquake of 1976 showed that the distribution of anomalous zones of shock calamities in Beijing City corresponds to the two wings of the Beijing depression.Taking a cross-section perpendicular to the long axis of the Beijing depression, and simplifying both the coverlayers and bedrocks into homogeneous bodies, a model is designed in the light of the requirements of simulation. It is presumed that the centrum lies in the southeastern part of the site and it is simulated by the transmitting transducer which has similar radiation characteristics with the centrum of nature earthquakes. At the same time receiving transducers with different receiving senstivity along different directions are used in order to pick up every component of seismic ground motion.It is observed in the experiment that on the general attenuation trend with increase of distance from the earthquake center, the simulated earthquake ground motion intensifies greatly near Tiantan, Qianmen and Xisi, and this coincides well with the earthquake damages. Simulation of earthquake ground motion shows that the shocks are in the manner of 'shaking after bumping'. The vertical component has a considerable intensity and the duration time of the most energetic part of the horizontal components is longer. The response spectrums of the horizontal components show the shapes of double or multiple peaks. In addtion at one time there are differences among the earthquake forces at different places, and this is one of the causes which produce the horizontal couple of the earthquake. The earthquake damages in Beijing City during the Tangshan big earthquake of 1976 are in good correspondence with the above results.Further analyses and investigations reveal the physical essence of the effects of underlying bedrock topography on earthquake motion, and the paper also inquire into the regularity of the effects of the varying of bedrock surface on seismic waves.Having considered the centrum, propagation paths and the geological structures of the site, the method of supersonic seismic model experiments is an ideal method for the synthetic investigation of the problems of earthquake ground motion. This paper is a priliminary attempt to introduce this method into the study of the problems about effects of the underlying bedrock topography on earthquake ground motion.