P-and S-wave velocities of exhumed metasediments from the Aleutian subduction megathrust: Implications for the interpretation of low velocity zones and fault reflectivity

Abstract:

Subduction zones are host to the planet’s largest recorded earthquakes and tsunamis. The strength and slip behavior of the megathrust plate interface is tightly linked to the in situ conditions and physical state of sediments that are entrained along the plate interface. The mechanical properties of these sediments change as they are heated, consolidated, and metamorphosed with progressive subduction and burial beneath the forearc; understanding this evolution of rock properties is essential toward quantitative interpretation of geophysical signatures in the context of stress state, pore fluid pressure, and composition down-dip along the megathrust. In particular, observed zones of high-amplitude seismic reflectivity, low P- and S-wave velocities (Vp and Vs), and high Vp/Vs along subduction megathrusts have been widely interpreted to indicate near-lithostatic pore fluid pressure, yet the role of low-velocity and highly anisotropic metasediments along and beneath the plate interface remain weakly constrained.

To address this question, we conducted triaxial deformation experiments on a suite of argillaceous metasedimentary fault zone rocks exhumed from ~12-15 depth in the Aleutian subduction zone, in order to define Vp and Vs at mean effective stresses up to ~100 MPa. In our experiments we observed a rapid increase in Vp with increasing effective stress that we interpret to be microcrack closure. We also observed significant velocity anisotropy in samples tested perpendicular and parallel to foliation with respective Vp of 5.0 km/s and 5.9 km/s (17% anisotropy) at peak stress. Velocity predictions based on whole rock composition and simple assumptions about pore structure and volume tend to over predict observed velocities, but this discrepancy may be reconciled if pores are highly elongate. Synthetic waveform modeling shows that these wavespeeds are broadly consistent with observed reflectivity and low-frequency low velocity zones at 20-40 km depth (10-30 km from the trench) at this margin, raising the possibility that these signatures could be explained by the presence of optimally foliation of weak metasediments, without requiring highly elevated pore fluid content or pressure.