[S-CG44] Science of slow-to-fast earthquakes

[SCG44-28] 南海トラフのプレート境界断層に沿って推定された間隙水圧と浅部超低周波地震

*于 凡¹、Jamali Hondori Ehsan¹、朴 進午¹ (1.東京大学 大気海洋研究所)

キーワード：南海トラフ、間隙水圧、マルチチャンネル地震探査、浅部超低周波地震

At the Nankai Trough margin, the Philippine Sea Plate subducts beneath the Eurasian Plate to the northwest at a convergence rate of ~4 cm/yr. Historically, great megathrust earthquakes with a recurrence interval of ~100-200 years have generated strong motion and large tsunamis along the Nankai Trough subduction zone. The plate-boundary fault (i.e., decollement) is a source fault of the devastating Nankai megathrust earthquakes. The composition of sediments in the Shikoku Basin has been investigated using Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP) drilling samples, showing that turbidite sediments are widely distributed in the Shikoku Basin and also underthrust beneath the decollement. While there were lots of seismic reflection studies across the Nankai Trough margin to reveal the seismic structure of the Nankai Trough, only a few studies on pore-fluid pressure estimation were conducted. In order to figure out detailed crustal structure of the Nankai Trough subduction zone off the Kii Peninsula, southwest Japan, we carried out 2D pre-stack depth migration (PSDM) imaging by using multi-channel seismic (MCS) reflection data that were acquired by JAMSTEC in 2011.The PSDM seismic profile on line KI01 off Cape Shiono of the Kii Peninsula showed that the turbidite sediments underthrust beneath the decollement, in contrast with line 215 off Cape Muroto of the Shikoku Island. Moreover, we estimated physical properties for the Nankai Trough decollement between 0 km (trench axis) to 30 km landward of the trench axis using the PSDM velocities. We found that the vertical effective stress is sufficiently lower than the expected effective stress. The decollement development region can be divided into two different zones: Zone 1 (0 to ~13 km) and Zone 2 (~13 to 30 km). In Zone 1, the physical properties in this zone are constrained by high permeability of turbidite sediments. In Zone 2, the physical properties may be influenced by either permeability of turbidite or potentially abundant fluid supply by clay mineral dehydration. Very few shallow very low frequency earthquakes (SVLFEs) are observed on line KI01 off Cape Shiono, while SVLFEs occur more frequently around line 215 off Cape Muroto. It is consistent with relatively higher shear stress off Cape Shiono. Compared to offshore Cape Muroto, relatively higher vertical effective stress off Cape Shiono suggests that the decollement is more coupled, which is consistent with slip-deficit rate distribution and shallow VLFE activity.