Consolidation behavior, permeability, and compressional wavespeeds of sediment entering the eastern Aleutian subduction zone: Implications for rock properties along the plate interface

Abstract:

Subduction zones host the world’s largest and most damaging earthquakes. The oceanic sediment subducted and entrained along the plate interface plays a key role in controlling the physical properties and in situ conditions along the megathrust and within surrounding wall rock in these systems. The evolution of porosity and pore structure with progressive loading is particularly important for understanding both permeability and elastic properties, and their distribution down-dip. These properties, in turn, are primary controls on drainage, pore pressure, effective stress, fault strength, and rupture propagation. Deformation begins with uniaxial consolidation outboard of the trench where pore collapse and dewatering are driven by sedimentation and vertical loading. Sediments that are accreted and incorporated into the upper plate at the trench undergo progressive lateral loading, whereas material underthrust at the trench undergoes dominantly vertical loading, but with a component of shear associated with transfer of stress across the plate interface.

We report on the consolidation behavior of sediment entering the Eastern Aleutian subduction zone, sampled during Integrated Ocean Drilling Program (IODP) Expedition 341 at Site U1417, located ~60 km seaward of the trench. We tested samples from 340 mbsf to 700 mbsf that consist of muds with interbedded silt and diamict. We report on the evolution of compression behavior, permeability (k), compressional wavespeed (Vp), and acoustic transmission amplitude as functions of stress state. We measure P-wave velocities ranging from 1.8 km/s at 500 kPa mean effective stress to 3.0 km/s at 90 MPa; porosities decrease from 60% to 28%, and permeabilities decrease from 10^-17 to 10^-20 m² over this stress range. We use the relationships defined by our laboratory experimental data to estimate Vp, permeability, porosity, and resistivity extrapolated down-dip along the megathrust to ~10 km depth, for a series of scenarios representing a range of drainage conditions. Estimated values of Vp are broadly consistent with those defined by from seismic reflection data ranging from 2.2 to 3.2 km/s at 10 km depth, and suggest partly drained conditions/are consistent with modest fluid overpressure.