Spatial variations in the onset times of active faultings in Japan

A spatial and temporal evolution of active faulting can provide geological constraints on estimating a potential area of future faulting and landform development. Especially, a started age of present faulting pattern is meaningful to suggest the beginning of neotectonic regime such as evolution stages of mountains. We are carrying out a collection and analysis of existing knowledge to organize the method and information on the started age of present faulting pattern. In this contribution, the onset times of faulting in the Japanese Islands are estimated using data of previous study. We used a calculation method which is the total displacement subdivided by the slip rate. The assumption involved in this method is that the slip rate has been constant since the onset time of faulting. We also used information of the strata indicating the appearance of the faulting, such as the deformed or thickened strata by the faulting. It is thought that the calculated onset time includes the error of several hundreds of thousands of years. The main results are shown as follows: 1: In northeast Japan, reverse faulting in a N-S direction started since about 3 million years ago. Especially, the onset times of reverse faulting are estimated in the rear-arc side between 3.0 and 1.7 Ma, and in the fore-arc side between 1.7 and 0.7 Ma, corresponding to the onset time of uplift of the Dewa Hills and the Ou Backbone Range. 2: The Niigata-Kobe Tectonic Zone (NKTZ) along the Japan Sea coast undergoes contraction in the WNW-ESE direction, which is a few times larger than in the surrounding regions. There appear to be many active faults in the NKTZ which started since about 1 million years ago. 3: In southwest Japan, the onset times of faulting are estimated in the Kinki district and the northern part of the Izu peninsula, which are younger than those in other regions. A part of this study was carried out under the contract with the Agency for Natural Resources and Energy, Ministry of Economy, Trade and Industry in 2010.