Seismic Formation Fluid Pressure Observations Reveal High Anisotropy of Oceanic Crust.

Fractures and faults in igneous oceanic crust affect hydrothermal circulation and hydration of the oceanic plates. Seismic observations have provided information about lithospheric fabrics, but direct measurements of oceanic crust's elastic properties were not made. We report determinations of compressibility of the upper igneous crust (of 3.6 Ma) of the Juan de Fuca plate, using observed formation fluid pressure oscillations in sealed boreholes associated with passing surface waves from distant earthquakes. We determine an azimuthal variation of formation‐matrix compressibility by a factor of ∼5, with the crust being most compressible in the plate‐spreading direction across the structural fabric inherited from crustal creation. This is equivalent to a seismic compressional wave speed anisotropy of ∼50%–60%, much greater than that of standard seismic measurements (typically <20%). This likely reflects a previously unresolved degree of fracturing of the uppermost oceanic crust, consistent with existing observations that suggest a high degree of hydraulic permeability anisotropy. Plain Language Summary: Hydrothermal circulation in the oceanic crust plays fundamental roles in the exchange of water, heat, and chemical constituents between the oceanic hydrosphere and lithosphere, in the hydration of the crust and upper mantle, and in the support of the subseafloor microbial biosphere. Water is carried primarily in faults and fractures created at the time of seafloor spreading, and thus constraints on their distribution and orientation are important. Such information has been provided by active‐source seismic surveys, and results have shown that compressional wave speeds in the directions of spreading—thus across local faults and fractures—are typically <20% lower than those along the structural fabric. Here, we report more direct determinations of the anisotropy at the 3.6‐Ma Juan de Fuca plate, using formation fluid pressure variations in sealed subseafloor boreholes caused by seismic surface waves from distant earthquakes. These observations are sensitive to formation compressibility of the upper couple of hundreds of meters of the igneous crust. The observed azimuthal variation in fluid pressurization reveals a contrast in compressibility across versus along the structural trend of roughly a factor of five, and a much higher degree of seismic anisotropy (>50%) than that determined from standard seismic velocity measurements. Key Points: Offshore borehole monitoring shows formation fluid pressure variations caused by Rayleigh waves from distant large earthquakesPressure records reveal highly anisotropic oceanic crust, with the formation being most compressible in the plate‐spreading directionThe anisotropy surpasses values determined from standard seismic measurements and implies denser fracturing of uppermost oceanic crust [ABSTRACT FROM AUTHOR]