Multiple-Scs probing of the Ontong-Java Plateau

We use multiple-ScS phases recorded at Pohnpei from a 1996 deep earthquake under the Solomon Islands to investigate the shear velocity and attenuation characteristics of the upper mantle under the Ontong-Java Plateau in the Western Equatorial Pacific. Cross-correlation results indicate a vertical one-way travel-time under the plateau 3 s slow with respect to PREM, in agreement with previously obtained tomographic results, which revealed the existence of an anomalously slow root under the plateau. Additionally, the study of stacked spectral ratios suggests an unusually high Q_ScS = 366, with a lower limit of Q_ScS ≥ 253. This value is considerably higher than elsewhere in the Pacific, suggesting the absence of an asthenospheric layer under the edifice. The combination of slow shear-wave speed and lower than average shear attenuation rules out a temperature anomaly as the source of the Ontong-Java Plateau (OJP) root, which must then be related to a chemical or mineralogical heterogeneity.