Seismic reflection profiles reveal steeply landward-dipping splay faults in the rupture
area of the magnitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These
splay faults branch upward from the plate-boundary interface (that is, the subduction
zone) at a depth of approximately 10 kilometers, approximately 50 to 55 kilometers
landward of the trough axis, breaking through the upper crustal plate. Slip on the
active splay fault may be an important mechanism that accommodates the elastic strain
caused by relative plate motion.