This paper provides some sensitivity analyses of probability of eartrthquake occurrence to some representative design parameters. Due to the drift to performance based design method, appropriate harmonization could come to be discussed on safety, on economy and on the others in design codes for different types of geotechnical structures in a region, country, and world. Meanwhile periodicity of earthquake occurrences at each active fault system have been recognized by some statistical researchers or esearch groups such as Sugai et al. (1999). Accordingly, the probabilities of earthquake occurrences in a 30-year time from active fault system distribute in a wide range. The probabilities are estimated to be less than 10~(-20) from some active fault systems that have just generated a big earthquake, and estimated to be more than 10% from some active fault systems that have not generated a big earthquake for long time. The present paper proviced results of some sensitivity analyses of probability of earthquake occurrence to some design parameters based on simple probability models of design parameters of theirs. Consequently, it shows that structure's resistance to guarantee a constant reliability of safety against design loads remarkably increase as the probability of earthquake occurrence in a certain period increase after the probability of earthquake occurrence goes greater than a critical value. The critical value depends on the design parameters such as earthquake loads, accuracy of estimating the load etc. The value larger, the earthquake load larger, or the other design load smaller. The value larger too, the accuracy of estimating earthquake load lower, or of estimating the other design load higher. The value mainly distributes between 10~(-3) and 10~(-4) for a set of the typical design parameters used in the present paper, It is found, however, that the value is closely related to the magnitude of reliability of safety. The paper also shows that the probability of earthquake occurrence is more than 10~(-3) ～ 10~(-4) at approximately 50 ～ 75% of the major active fault systems in the Japanese archipelago. Namely, the probability of earthquake occurrence should be noted near the 50 ～ 75% of the major active fault systems for the typical design cases.
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