Crustal construction along arc-backarc transition zone in the Japan Sea and implications for seismogenic processes

Abstract:

The Japan Sea, which is a backarc basin between Japanese island arc and the Asian continent, has a unique setting in terms of a formation process as well as a seismogenic process. The opening of the Japan Sea was initiated by crustal rifting and the separation of Japan Island Arcs from the Asian continent in the early Oligocene (~ 32 Ma), with subsequent ocean floor spreading in the late Oligocene (~ 28 Ma). Then, the opening stopped, between 10 and 3.5 Ma, and at 3.5 Ma, the crustal shortening occurred under a strong compressional stress regime in the eastern margin of the Japan Sea. Several seismic surveys had been conducted in this region since the last more than two decades, however, a conclusive discussion concerning a crustal construction in the arc-backarc transition zone had not been made, due to lack of resolution of structural models and sparse distribution of profiles. Moreover, magnitude-7 class earthquakes repeatedly occurred along this margin, such as, the 1964 Niigata earthquake (M7.5), 1983 Nihonkai–Chubu earthquake (M7.7), 1993 Hokkaido Nansei-oki earthquake (M7.8), and those events are recognized to have compressional fault mechanisms by reflecting a present-day stress regime. However, structural factor controlling the distribution and mechanism of those compressional events has not been well studied. In order to understand a crustal formation process in this margin and its relation to the seismogenic process at the present, we have been carrying our series of active-source seismic survey to cover the eastern margin of the central to northern Japan Sea. Results from those surveys successfully mapped a distribution of the arc crust, the oceanic crust and the amorously thick oceanic crust in the transition zone. From a comparison the rupture zones of the magnitude-7 class earthquake with the detailed crustal structure, we conclude that the large compressional events, more than M>7.5, occurred in a seismogenic zone fault which used to be formed a structural boundary between the arc crust and the thick oceanic crust. This structural boundary (i.e., seismogenic fault at the present) is clearly recognized in seismic sections as a landward dipping reflector.