P-wave attenuation structure of the Lau back-arc basin and implications for mantle wedge processes

Abstract

We image 3-D seismic P-wave attenuation structure of the Tonga subduction zone and the Lau back-arc basin using local earthquake waveforms recorded by land and ocean bottom seismographs of the 2009–2010 Lau Spreading Center Imaging project. The amplitude spectra of P waves from local earthquakes are inverted for the path-average attenuation operator (t⁎), with a variety of frequency-dependence exponent values (α). Analysis shows that the data are best fit by the assumption of a weak frequency dependence (α≈0.3), consistent with laboratory results modeled as the effects of grain boundary sliding. The t⁎ measurements are inverted for a 3-D tomographic QP−1 model. The results show high P-wave attenuation within the upper 100 km of the back-arc mantle, with a sharp boundary at the Tofua arc between high attenuation sub-arc mantle and the low attenuation fore-arc. Perhaps the highest P-wave attenuation (QP−1≈0.036 or QP≈28) known in the mantle occurs at 20–60 km depth beneath the back-arc spreading centers. High attenuation anomalies form an inclined zone dipping from the spreading centers to the west away from the slab, indicating abnormally high temperature and the existence of hydrous partial melt. These observations suggest that hot materials supplied from the Australian mantle upwell along with the mantle wedge flow pattern, triggering extensive decompression melting near the back-arc spreading centers. The along-strike variation of P-wave attenuation is different from that of SV-velocity, possibly implying the different effects of partial melt on seismic attenuation and velocity.