Seismic Reflection Surveys Across the Median Tectonic Line Active Fault Zone in Niihama City, Ehime Prefecture and Awa City, Tokushima Prefecture

The Median Tectonic Line active fault zone is a 350-km-long, right-lateral strike slip fault zone with slip rates of up to 10mm/yr. This fault zone is composed of two types of active fault: faults following the terrane boundary between the Sambagawa and Ryoke metamorphic rocks and faults located several kilometers to the north of the terrane boundary. To reveal the geometric relations between the terrane boundary fault and active faults to the north, we conducted seismic reflection surveys across the Median Tectonic Line active fault zone at Niihama City, Ehime Prefecture in 2002 and Awa City, Tokushima Prefecture in 2003. A migrated depth section of the Niihama line clearly illustrates that the Ishizuchi fault, which reactivates the terrane boundary fault, dips 25°N. The Okamura fault lying 1.5km to the north of the Ishizuchi fault, however, is not well imaged on this section. The terrane boundary fault on a migrated depth section of the Awa line also dips to the north at 20-25°, thrusting the Izumi Group of Cretaceous age over the Dochu Formation of Plio-Pleistocene age. The subsurface configuration of the Chichio fault north of the terrane boundary fault is not clearly imaged