Firstly, the kinds of natural disasters in cities are various and the frequency is higher and higher. All kinds of natural disasters will affect the cities and cause losses. According to figures, now our country has 156 cities located in high earthquake-intensity area of 7 and over 7 degrees. More than 70% of the cities, over half of the population and more than 75% industry and agriculture production value lie in the areas affected seriously by weather, ocean, flood and earthquakes, etc.
Secondly, with the scale of cities growing rapidly, the more prosperous of urban social economic and more concentration of prosperity, the tighter people rely on cities and the more dangerous cities will be due to the greater fragility and vulnerability. Cities are the center of politics, economic and culture of particular areas which centralize large amount of material wealth. On the one hand, it contributes to economic development and became an important driving factors of economic development, on the other hand, centralization of material wealth increases the fragility and vulnerability of cities. To provide good conditions and environment of dwelling, living and working and to accommodate to the social economic development of modern cities, the number of city buildings and structures accelerated and continuously extended to the air and underground. A mass of intensive high-rise buildings and underground mall, railway and tunnels appeared. Once water and electricity supply as well as energy and communication which human live on have been damaged by disasters, it will cause great loss to cities. For instance, the 7.2-magnitude earthquake happened in Osaka, Japan in January 17th,1955 destroyed nearly 100 thousand buildings and damaged another 100 thousand buildings using only 10 seconds. 6400 people died and 300 thousand people became homeless. All the water, electricity, gas, transportation and communication in the city have been cut off entirely with a direct loss of 100 billion dollars.
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