Attenuation in the Upper Mantle Beneath “Normal” Old Seafloor in the Western Pacific
Abstract:The Pacific Lithosphere Anisotropy and Thickness Experiment (PLATE) ocean bottom seismometer array south of the Shatsky Rise allows imaging the attenuation structure of the upper mantle beneath the oldest (~ 150 Ma) parts of the Pacific seafloor that have not been affected by subsequent volcanism. The array consisted of eight seismometers plus three stations with only differential pressure gauges covering a lateral extent of 200 km by 600 km in seafloor with an average depth of about 6000 m. Intermediate- and deep-focus earthquakes in the Mariana and Izu-Bonin subduction zones provide paths for P waves that probe different depths beneath the seafloor and globally distributed shallow earthquakes serve as Rayleigh wave sources. Q estimates from P waves are based on frequencies from 1 to 15 Hz, primarily 2 to 10 Hz, while Rayleigh wave attenuation is measured in the 0.007 to 0.045 Hz range. Qp is statistically required to be frequency dependent, with a best-fitting power law coefficient α = 0.40. A different form of frequency dependence is required to reconcile Rayleigh wave and P wave attenuation.
Not surprisingly, both P wave and Rayleigh wave data sets require a very low attenuation, high Q lithosphere. Two other aspects of the model are more surprising. First, Q in the asthenosphere in the depth range 100-250 km is quite low, with attenuation comparable to that in the highly attenuating wedges above subducting plates in back arc regions. This observation suggests the accumulation of a small melt fraction beneath the lithosphere. Second, attenuation in the transition zone deeper than 410 km is negligible, suggesting that the transition zone seaward of the subducting Pacific plate is dehydrated.