Quantification of pore pressure in subduction zones and its implication for the slip behavior of the Plate Interface

Abstract

In this Ph.D. dissertation, pore pressure in the Nankai Trough, North Sumatra, and Northern Hikurangi subduction zones is investigated through multi approaches including uniaxial consolidation experiments, porosity-based prediction equations, and fluid flow models. In the Nankai Trough subduction zone, continuous excess pore pressure is revealed in the accreted and underthrust sediments, implying the Nankai décollement is not a fluid barrier. Pore pressures 84%-93% of the lithostatic stress along the décollement are inferred and lead to extremely low shear strength at the deformation front. This increases the likelihood of a large tsunami offshore SW Japan when a large earthquake happens. In addition, a pore pressure prediction approach accounting for consolidation state of sediments is developed. Compared with the normally consolidated sediment, overconsolidated sediments in the Nankai Trough generate 40%~50% less excess pore pressure. The shear strength along the Nankai décollement is below 2.2 MPa, obviously larger than the shear stress (<0.4 MPa). This implies that the accretionary prism is not at Coulomb failure. In the North Sumatra subduction zone, moderate excess pore pressure is revealed in the proto-décollement horizon. Pore pressure modeling shows that the excess pore pressure increases continuously and reaches a high level corresponding to a pore pressure up to 96% of lithostatic stress, as the sediment approaches the trench. The high excess pore pressure explains the 2004 Sumatra earthquake. In the Northern Hikurangi subduction zone, consolidation experiments show that the trench-wedge facies are underconsolidated. Pore pressure 77% of lithostatic stress is revealed at the top of the Pāpaku fault, whereas the pore pressure at the bottom increases to 93% of lithostatic stress. Slow slip events (SSEs) are simulated under this pore pressure condition. The modeling results are comparable to the natural SSEs.