Natural open-spaces are essential not only for providing natural amenities, but also for evacuation in the case of earthquake disasters in urban area. These spaces are particularly important as many urban areas are vulnerable to natural disasters. In addition, environmental planning for earthquake disaster mitigation is important because the construction of many disaster prevention facilities can be problematic, particularly when the return period is long. In this paper, regional diagnoses for earthquake disaster mitigation is carried out, focusing on evacuation activities in the Hokusetsu region (Osaka Prefecture). This region is assumed big damage by three earthquakes. The topographic and land use characteristics of this region resemble those of Kobe city, where big damage occurred as a result of the Hanshin-Awaji earthquake. Firstly, regional analysis is done to make where these spaces should be create for reduction of earthquake disaster risk clear. This analysis is composed of necessity of evacuation action and simulation model regarding this action. The former is evaluated by the possibility of building collapses and fire spread. The latter is developed for evaluation of danger of evacuation action in earthquake disaster considering with possibility of district isolation and the location of neighboring chou-chou-moku. This simulation model involves eight plausible assumptions with respect to the planning methodology. This model is made possible to derive important indexes in order to evaluate earthquake disaster risk. As a result, it is making clear that districts of southern part of Suita city are most dangerous. Secondly, this study proposes new approach which is water side creation using sewage treatment water as an alternative for earthquake disaster mitigation. The mitigation effect of this alternative is shown by the analysis using simulation model.