Seismic attenuation structure beneath the Juan de Fuca Ridge from tomographic inversion of amplitudes

Tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the Juan de Fuca Ridge to further constrain a velocity structure previously obtained by travel time‐velocity tomography and to obtain a model for attenuation variations within the shallow crust. Amplitude analysis has identified an additional boundary, defined by a decrease in vertical velocity gradient, at ∼2.0 km subbottom which was not required by the travel time data. This transition zone likely represents the oceanic layer 2‐layer 3 boundary. The analysis also identified zones of relatively high attenuation (QE ∼ 20–100) at subbottom depths of 0.3–2.0 km beneath the ridge axis which correlate well with low‐velocity zones (velocities reduced by <0.4 km/s) in the velocity model. The subaxial zone is interpreted as a region of increased fracture porosity and/or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex.