Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust

Abstract

Recent seismic reflection and ocean bottom seismometer (OBS) studies reveal broad regions of low seismic velocity along the Nankai subduction plate boundary megathrust offshore SW Japan. These low velocity zones (LVZ's) extend ∼55 km landward from the trench, corresponding to depths of >∼10 km below sea floor. Here, we estimate the in‐situ pore pressure and stress state within these LVZ's by combining P‐wave velocities obtained from the geophysical surveys with new well‐constrained empirical relations between P‐wave velocity, porosity, and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment. We document excess pore pressures of 17–87 MPa that extend ∼55 km into the subduction zone, indicating that trapped pore fluids support ∼45–91% of the overburden stress along the base of the upper plate and surrounding major fault zones. The resulting effective stresses in the LVZ are limited to ∼1/3 of the values expected for non‐overpressured conditions. These low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary. The region of lowest effective stress coincides with precisely located very low frequency earthquakes, providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure.

Frictional properties of the shallow Nankai Trough accretionary sediments dependent on the content of clay minerals

“…In addition to its intrinsic weakness due to the abundance of clay minerals, the low porosity and permeability of the claystone sample likely resulted in an increase in pore pressure during its compression, which may have further weakened this sample and promoted its slow failure (Takahashi et al 2013). It has recently been suggested that the slow slip events or very low frequency earthquakes observed within the Nankai Trough accretionary prism or along the shallow décollement (Ito and Obara 2006;Sugioka et al 2012) occur in regions of elevated pore fluid pressures (Park et al 2010;Kitajima and Saffer 2012). Because claystone is likely under a high pore water pressure during its compression, and also because it fails slowly, faulting in claystone layers is a possible source for such slow slip events or very low frequency earthquakes.…”

Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

“…Studies using seismic reflection, ocean bottom seismometers, and seismic tomography suggest that the source regions of low-frequency seismic events correspond to zones of elevated fluid pressure, suggesting a very low stress drop during these events (Ito and Obara 2006b;Shelly et al 2006;Matsubara et al 2009;Kitajima and Saffer 2012). Low-frequency tremors and earthquakes at depths of 30 to 45 km and temperatures of 400°C to 550°C under increased fluid pressures are generally assumed to result from dehydration of the subducting oceanic plate (Obara 2002;Shelly et al 2006;Matsubara et al 2009).…”

Shear strength of sediments approaching subduction in the Nankai Trough, Japan as constraints on forearc mechanics

“…Based on our observation that residual shear strength generally increases with depth, significant excess pore pressures would be required in this unit to make it weak enough to be mechanically favorable for hosting a major thrust fault [Le Pichon et al, 1993]. Recent estimates of in situ pore pressure at the base of the outer wedge determined by relationships between P wave velocity, porosity, and effective mean stress document excess pore pressures of 45-91%, or ¼ $0.7-0.95 [Kitajima and Saffer, 2012]. We employ these values of , , and in a Coulomb wedge calculation for the outer wedge area.…”