Is the zone along the Median Tectonic Line hot?

Abstract

The seismogenic layer in the zone on the southern side of the Median Tectonic Line that runs in the east-west direction crossing the Kii Peninsula and Shikoku is abnormally shallow. It is abnormal because the shallowness of the depth is not concordant with the generally observed feature that the higher the altitude of the area, the shallower the seismogenic layer. The depth of the seismogenic layer is as shallow as that in the mountainous zone along the Volcanic Front in northeastern Japan, nevertheless such low altitude areas as Wakayama and Tokushima plains exist in the zone on the southern side of the Median Tectonic Line. If it is postulated that the depth of the seismogenic layer is regulated by the brittle-ductile transition temperature of the crustal materials, the shallowness of the seismogenic layer means that the temperature in the crust is relatively high. Then, why is the temperature in the zone along the Median Tectonic Line high? We think it is not unreasonable to consider that thermal fluids are rising from the subducted slab there, if it is reminded that hot waters are estimated to exist beneath the Wakayama swarm region that belongs to the zone. Key words: The Median Tectonic Line, seismogenic layer, Wakayama swarm activity, thermal structure of the crust, thermal fluids from the subducted slab