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Abstract As one of the most serious secondary hazards induced by earthquake，the rock collapse aggravates the seismic damage and restricts the reconstruction greatly. However，there are less effective measures that can be used to diagnosis and treat it danger rocks just for its mechanism is not clear at present. Thus from the true structure of rock，based on the rock fracture mechanics，and taking energy method as a main way，the mechanism of input and dissipation of seismic energy on danger rocks is studied. Then by analyzing the condition and process of crack propagation，the instability mechanism of dangerous rock caused by seismic wave is studied clearly. Furthermore，the calculation formula for critical acceleration and propagation length are also suggested. At last，an example is given to prove the theory. The result shows the cracks on danger rock can propagate and develop under enough big seismic action，which result in the failure of danger rock and the collapse occasion at last.
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Received: 08 January 2009
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