Seismic Formation Fluid Pressure Observations Reveal High Anisotropy of Oceanic Crust

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Summary:
Fractures and faults in oceanic crust host hydrothermal circulation, affecting heat and volatile advection and subducting slab hydration. Constraints on these crustal fabrics’ distribution are therefore important, and such information has been provided by seismic velocity measurements. Many studies suggested that seismic P wave speeds in the direction of plate spreading—thus across local fault trends—are typically less than 20% lower than those along the fault trends. In a new study, Sun, Davis, and Heesemann reported more direct determinations of the anisotropy of oceanic crust at the 3.6-Ma Juan de Fuca plate, by using formation fluid pressure variations recorded in sealed subseafloor boreholes caused by surface waves from distant large earthquakes. Their observation—sensitive to the upper hundreds of meters of the igneous crust—reveals a large contrast in formation compressibility across versus along the structural trend by roughly a factor of five, equivalent to a much higher degree of seismic anisotropy (larger than 50%) than determined from standard seismic measurements.

Authors:
Tianhaoze Sun et al.

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