Study of blind thrust faults underlying Tokyo and Osaka urban areas using a combination of high-resolution seismic reflection profiling and continuous coring

Abstract

We acquired high-resolution seismic reflection profiles and continuously cored boreholes to evaluate active flexures\rproduced by major blind thrust fault systems within two densely populated Neogene-Quaternary sedimentary\rbasins in Japan: the Fukaya Fault System near Tokyo in the Kanto Basin and the Uemachi Fault System in\rthe Osaka Basin. The high-resolution seismic reflection survey made clear the length, geometry and growth history\rof fault-related folds, or flexures formed above the two blind thrusts. Continuously cored boreholes linked\rwith high-resolution seismic profiles enabled us to estimate the uplift rate as defined by shallow stratigraphic\rhorizons and constrain the age of the most recent growth of the flexures during earthquakes on the Fukaya and\rUemachi fault systems. Even with the high quality of the data we collected, it is still not possible to exactly constrain\rthe age of the most recent blind thrust earthquake recorded by flexure of these fault-related folds. Data\rpresented in this paper form the basis for future efforts aimed at mechanical and kinematic models for fault\rgrowth to evaluate the activity of blind thrusts underlying urban areas.