Investigating the role of the Itoigawa-Shizuoka tectonic line towards the evolution of the Northern Fossa Magna rift basin

AbstractThe Itoigawa-Shizuoka tectonic line (ISTL) fault system is considered to have one of the highest probabilities for a major inland earthquake occurrence in the whole of Japan. It is a complex fault system with the dip directions of the local fault segments changing from north to south between an east-dipping low-angle thrust fault, a strike slip fault and a west-dipping thrust fault. The tectonic relations between the different parts of the fault system and the surrounding geological units are yet to be fully explained. This study aims to reveal the juncture of the northern and central parts of the ISTL and investigate its contribution towards the shaping of the Northern Fossa Magna rift basin. We conducted 3 deployments of 1 or 2 linear arrays of seismic stations across the central and northern ISTL regions and observed local micro-earthquakes for a period of 3 years. Each deployment recorded continuous waveform data for approximately 3 months. Using arrival times of 1193 local earthquakes, we jointly determined earthquake locations and a 3D velocity model, applying the tomography method. We were able to image the regional crustal structures from the surface to a depth of 20km with a spatial resolution of 5km. Subsequently, we used the obtained 3D velocity model to relocate the background local seismicity from 2003 to 2009. The juncture of the northern and central parts of the ISTL was well constrained by our results. The depth extension of the northern parts of the ISTL fault segments follows the bottom of the Miocene Northern Fossa Magna rift basin (NFM) and forms an east-dipping low-angle fault. In contrast, the central parts of the ISTL fault segments are estimated to lie along the eastern boundary of the Matsumoto basin forming an oblique strike slip fault (Fig. 1)

プレート沈み込み帯における地震波低速度層―地震発生への影響と探査法

low-velocity zonesubduction zoneearthquake rupture processtrapped wavedownhole seismic observationSubduction zone drilling into a seismogenic zone has recently been planned to understand the mechanics of large thrust earthquakes. We review studies on fault zone structures in land areas and fault mechanics studies related to the fault zones. We also point out possibilities of estimating the parameters of slip-weakening and/or rate and state dependent friction laws from a parameter of thickness of fault gouges or of fault damaged zones observed in the borehole. Two types of low velocity zone, the fault and the oceanic crust low-velocity zones (FLVZ and CLVZ), can be expected along the subduction zone plate-boundary. Delineations of FLVZ and CLVZ clarify the frictional properties of the sunduction zone plate-boundary and detailed structures of the subducting oceanic crust at deeper parts. The trapped wave observation in the subduction zone borehole is a useful tool for delineating FLVZ and CLVZ in the upper parts of the subducting slab