Deep seismic reflection profiling across active reverse faults in the Kinki Triangle, central Japan

Abstract

The Kinki triangle, central Japan, is marked by a dense distribution of active faults. For regional characterization of the crust in the Kinki metropolitan area to be used for seismic hazard mitigation, seismic reflection profiling was carried out using vibroseis trucks and explosive sources along a 135 km-long seismic line from Metropolitan Osaka to the Ise basin across several active reverse faults. The seismic reflection section portrays the west-dipping upper surface of the down-going Philippine Sea plate at 35 to 40 km depth, lower crustal reflectors at 27 km and mid-crustal reflectors at 15 to 16 km depth. Dipping reflections are dominant in the seismogenic upper crust and some of them can be traced as deeper extensions of active faults. A 60-km-wide uplifted zone between Osaka and Ise basins were formed as a pop-up structure bounded by double-vergent reverse fault systems. The mid-crustal reflectors beneath the uplifted zone form a slight convex shape and the lower limit of the hypocentral distribution of crustal earthquakes lies a few km above the reflectors. The dipping reflectors in the upper crust merged to the sub-horizontal, mid-crustal reflectors and are interpreted as a possible mid-crustal detachment. The hanging wall of active faults in the Osaka basin show back-tilting associated with reverse faults, suggesting a curved fault geometry and shallower detachment in the upper crust.