Probabilistic Assessment of Buildings Damage Considering Aftershocks of Earthquakes

Abstract eng:
In Japan, buildings are always exposed to the risk of earthquakes. Various researches dealing with earthquake risk have been undertaken very actively in Japan. Most of the research, however, focuses on the risk from the main shock while research treating aftershock risks has been seldom paid attention to so far. Although it is obvious that the major building damage comes from the main shock, it has been reported that some serious building damage occurred due to a series of aftershocks of the Chuetsu earthquake on Oct.23, 2004 in Niigata area. That is why it is necessary to demonstrate how much the seismic risk can increase when taking into consideration of aftershocks. It is assumed in this study that the relationship between the magnitude of aftershocks and the total number of aftershocks can be represented by the Gutenberg-Richter law, and the relationship between the elapsed time after the main shock and the occurrence rate of aftershock follows the modified Omori’s law. Using the ground motion attenuation relation together with the above two laws, the ground motion caused by aftershocks can be calculated and then an aftershock hazard can be estimated. In this paper, three damage states of buildings, corresponding to the three modes of an event-tree in the Markov chain, are taken: collapse, half collapse and no damage. Then the seismic risk of aftershocks along with the main-shocks can be evaluated theoretically by use of an event-tree and the Markov chain. The Markov chain is very appropriate because the occurrence rate of aftershocks varies depending upon the elapsed time, and the state of buildings might change due to consecutive occurrence of aftershocks. Some numerical examples show the important effect due to aftershocks.

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