High Anisotropy of Fractured Oceanic Crust Revealed by Seismic Formation Fluid Pressure Variations
Abstract
Fractures and faults in igneous oceanic crust host vigorous hydrothermal circulation and thus affect heat and volatile advection, hydration of subducting slabs, and the deep-sea biosphere. Despite the wide use of seismological observations for studying lithospheric fabrics, in-situ determinations of formation-scale elastic properties of oceanic crust have not been made. Here we report direct measurements of matrix compressibility, using formation-fluid pressure (Pf) oscillations measured in sealed boreholes caused by strain associated with Rayleigh waves from distant large earthquakes. At seismic frequencies, Pf variation acts as an ideal proxy for dynamic volumetric strain. By comparing the Pf response with co-located seismic ground velocity measurements, formation compressibility can be inferred on the basis of linear poroelasticity. We study data from three Ocean Drilling Program boreholes that are connected to the Ocean Networks Canada cabled observatory system. Two are drilled tens of meters into the upper igneous crust of the Juan de Fuca plate beneath low-permeability sediments, and the third hole is drilled 300 m into the outer Cascadia subduction accretionary prism. Data from four onshore Plate Boundary Observatory holes in the North American continental crust are also considered. Comparison among these sites show site-specific formation compressibility values that are lithology dependent. Values at the prism and the continental crust do not vary with arrival azimuth of seismic waves and agree with earlier estimates based on tidal loading (where available). In contrast, values at the two igneous crustal sites show azimuthal variation by a factor of ~5, with the formation being most compressible in the plate-spreading direction, across the structural fabric inherited from crust creation. This is equivalent to a seismic compressional wave speed anisotropy of ~50-60%, much greater than determined from standard seismic measurements (typically <20%). This likely reflects a previously unresolved degree of fracturing of the uppermost igneous oceanic crust, consistent with existing observations showing a high degree of hydraulic permeability anisotropy. Such a high degree of fracturing must play a key role in the hydration of the oceanic lithosphere.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMMR43A..04S