Attenuation in the Upper Mantle Beneath the Juan de Fuca Plate Using Rayleigh Wave Tomography

We have measured shear attenuation beneath the Juan de Fuca plate using Rayleigh waves from teleseismic earthquakes propagating across the Cascadia Initiative ocean-bottom seismometer (OBS) arrays. We employ the two-plane-wave technique to account for multi-path interference arising from velocity heterogeneities outside the array, the Born approximation to account for focusing and defocusing within the study area, and station corrections to account for site response and errors in instrument corrections. We solve simultaneously for phase velocity variations within the plate and average Rayleigh wave attenuation coefficients across the entire plate. Rayleigh wave attenuation coefficients extend from periods of 20 s to 143 s. The age of the seafloor ranges from 0 to about 10 Ma. The Juan de Fuca area is slightly more attenuating than seafloor of similar age near the East Pacific Rise in the GLIMPSE and MELT experiments, and the broader period range gives better depth resolution in the asthenosphere than in those studies. The minimum shear quality factor Q is found centered at about 80 km, just below the expected dry solidus and coinciding roughly with the minimum in shear velocity. Q averaged over the well-resolved depth range of 70 to 110 km is 45-50. We compare these observations to predictions of thermal models and various attenuation models. Lateral variations in velocity reveal asymmetry across the ridge axis associated with seamount asymmetry and a minimum in velocity in the vicinity of Axial Seamount.