Upper mantle Q structure beneath old seafloor in the western Pacific

The Pacific Lithosphere Anisotropy and Thickness Experiment ocean bottom seismometer array south of the Shatsky Rise allows imaging the attenuation structure of the upper mantle in the oldest parts of the Pacific Ocean. The array consisted of eight seismometers with a lateral extent of 200 km by 600 km. Intermediate-and deep-focus earthquake sources in the Mariana and Izu-Bonin subduction zones provide paths that probe different depths beneath the seafloor. Acceleration spectra from the vertical component of P waves are inverted for the attenuation operator and corner frequency. P-n propagation indicates very high Q(p) values in the lithosphere, of order 1000, while path-average Q(p) values in the upper mantle are on the order of 400. The variety of source epicentral distances and depths enables us to deduce the vertical Q(p)(-1) structure of the upper mantle. Using the amplitude spectra directly in a weighted least squares problem that accounts for the effects of triplications in the traveltime curves, we invert for Q(p)(-1) as a function of depth. We demonstrate that Q(p) is frequency dependent and adopt a power law dependence with coefficient alpha = 0.27. We resolve three distinct layers using a reference frequency of 1 Hz with Q(p) on the order of 145 at depths 100-250 km, Q(p) on the order of 350 at depths 250-410 km, and very high Q(p) with attenuation indistinguishable from zero for depths below 410 km. The P waves have a component of scattering, and thus, Q(p)(-1) represents the maximum intrinsic attenuation beneath normal undisturbed oceanic lithosphere.