09:15 〜 09:30
[SSS04-02] Geometrical Dependence on the Stress and Slip Tendency Acting on the Subduction Megathrust of Nankai Seismogenic Zone Off Kumano
キーワード：南海トラフ地震発生帯掘削、Slip tendency、3D地震探査
Slip tendency of a pre-existing fault, a measure for its closeness to failure, is governed not only by the regional stress and fault strength, but by its geometry (dip/strike) and overburden as well. We attempt to estimate the slip tendency near the updip edge of Nankai seismogenic zone megathrust, from the reprocessed 3D seismic volume with an improved velocity model and from IODP NanTroSEIZE drilling data off the coast of Kii Peninsula, central Japan.
Plate boundary fault surface is manually picked from 3D seismic volume. The fault surface, with its depth ranging 3500 to 6500 m below sea floor, is divided into 3 groups; low dip (10-15º) trending NW~N on the NE portion, intermediate dip (~25º) trending north on the western portion, and high dip (30-40º) trending NW on the SE portion. We then calculate the overburden (Sv) by converting 3D velocity to bulk density. Sv ranges from 100 MPa near SW edge to 160 MPa on the NE corner. The slip tendency (Ts) can be calculated from the dip angle and dip azimuth of the fault surface. Ts is low (~0.1) in low-angle dip region, whereas it is high (>0.2) in high-angle dip region. Ts in the latter region roughly coincides with its maximum, suggesting that the high-angle fault is optimally oriented under this condition. Low Ts in the low-angle dip region would correspond to a weaker portion due to excess pore fluid pressure, assuming that the fault surface should slip simultaneously.
Drilling into the fault zone at Site C0002 will provide a ground-truth evidence on the stress and strength at the fault and we can make an important step toward a better understanding of the slip likelihood of the Nankai seismogenic zone megathrust.
Plate boundary fault surface is manually picked from 3D seismic volume. The fault surface, with its depth ranging 3500 to 6500 m below sea floor, is divided into 3 groups; low dip (10-15º) trending NW~N on the NE portion, intermediate dip (~25º) trending north on the western portion, and high dip (30-40º) trending NW on the SE portion. We then calculate the overburden (Sv) by converting 3D velocity to bulk density. Sv ranges from 100 MPa near SW edge to 160 MPa on the NE corner. The slip tendency (Ts) can be calculated from the dip angle and dip azimuth of the fault surface. Ts is low (~0.1) in low-angle dip region, whereas it is high (>0.2) in high-angle dip region. Ts in the latter region roughly coincides with its maximum, suggesting that the high-angle fault is optimally oriented under this condition. Low Ts in the low-angle dip region would correspond to a weaker portion due to excess pore fluid pressure, assuming that the fault surface should slip simultaneously.
Drilling into the fault zone at Site C0002 will provide a ground-truth evidence on the stress and strength at the fault and we can make an important step toward a better understanding of the slip likelihood of the Nankai seismogenic zone megathrust.