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 approximately 2.0 km subb ottom which was not required by the travel time data. This transition zone likely represents the oceanic layer 2-layer 3 boundary. The analy sis also identified zones of relatively high attenuation (Q(E) approxi mately 20-100) at subbottom depths of 0.3-2.0 km beneath the ridge axi s 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 associate d with axial hydrothermal circulation within the extrusives and sheete d dike complex.