Seismic Velocity and Attenuation Tomography of the Tonga Arc and Lau Back-arc Basin

Abstract:

We apply various techniques to analyze seismic data from the 2009 – 2010 Ridge2000 Lau Spreading Center project to investigate the distribution of partial melt beneath the Tonga arc and Lau back-arc basin. The shear wave velocity structure is jointly inverted from the phase velocities of teleseismic and ambient-noise Rayleigh waves, as the former is inverted using the two-plane-wave method with finite-frequency kernels, and the latter is obtained from cross-correlation in frequency domain. Additionally, we determine the 3D attenuation structure from t* measurements of P and S waves from local earthquakes. In order to avoid the trade-off between t* and corner frequency, we analyze the spectral ratio of S coda to independently constrain the f_c for each event. The Q_P and Q_S structures are inverted separately, and Q_P/Q_S is jointly inverted from Q_P and t*(S). Tomographic results show strong signals of low velocity and high attenuation within the upper 100-km of the mantle beneath the back-arc basin, suggesting perhaps the lowest shear velocity (V_SV = 3.5 km/s) and highest seismic attenuation (Q_P < 35 and Q_S < 25) known in the mantle. These anomalies require not only the abnormally high temperature but also the existence of partial melt. The inferred partial melt aligns with the spreading centers at shallow depths, but shift westwards away from the slab, implying a passive decompression melting process governed by the mantle wedge flow pattern. The Tonga volcanic arc does not display as strong of velocity or attenuation anomalies as the spreading centers, suggesting less magmatism associate with the arc compared to the back-arc.