Western Nankai Trough seismogenic zone: Results from a wide-angle ocean bottom seismic survey

The Nankai Trough, southwestern Japan, is recognized as a vigorous seismogenic zone with well-studied historic earthquakes. This paper presents results of a wide-angle ocean bottom seismographs (OBS) study at the western Nankai Trough seismogenic zone. The OBS data used were acquired on a profile (250 km long) across the presumed coseismic slip zone of the 1946 Nankaido earthquake (Ms=8.2). The main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake; The crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge. P wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities (V-p=5.0-5.6 km/s and 6.6-6.8 km/s in oceanic layers 2 and 3, respectively). The maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with V-p=3.4-4.6 km/s in the deeper part. The subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward: 3.2 degrees and 7.2 degrees at 0-50 km and 50-100 km from the trough axis, respectively. The oceanic crust is smooth to the hypocenter zone, down to 40 km depth beneath Shikoku Island. Our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust-island are crust contact zone. Even though there is large uncertainty about the seaward limit of the coseismic slip zone, the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism.