Upper mantle shear velocity structure beneath Eastern Lau back-arc Spreading Center from ambient noise tomography of ocean bottom seismometer data

We analyze ambient seismic noise data collected from 50 broadband ocean bottom seismographs (OBS) during a 13-months deployment over the Eastern Lau Spreading Center (ELSC). The ELSC is a 400-km-long back-arc spreading center lying close to the Tonga subduction trench in the southwestern Pacific. Fluids expelled from the subducting slab cause melting of the upper mantle above the slab, strongly influencing the structure of the southern ELSC. Seafloor morphology, crustal thickness and lava composition data show systematic variations along the ridge as the ridge migrates away from the volcanic arc front to the north, indicating a decreasing influence of the subducting slab. Phase velocities for Rayleigh between 3-20s and for Love waves between 3-12s period were obtained from inter-receiver coherences of ambient seismic noise. The phase velocity measurements have been inverted for 3D phase velocity maps and shear velocity models. Preliminary inversion results suggest two separate low velocity zones (LVZs) beneath the back-arc basin. The shallow LVZ lies beneath the Tofua Volcanic Arc closer to the trench. The deeper LVZ underneath the back-arc spreading center widens and shifts laterally with respect to the ridge as the ridge migrates away from the volcanic arc. The two LVZs overlap to the south of 21°S but become distinct to the north, consistent with a decreasing influence of the slab on the spreading ridge as the ridge propagates away from the trench. Other components of the ELSC experiment include body wave tomography and measurements of surface wave phase velocities, body wave attenuation and shear wave splitting. Our results will be combined with these other results to improve our understanding of this rifting-spreading transition process.