[1] The cross-correlation of multicomponent ambient seismic noise can reveal both the velocity and polarization of surface waves propagating between pairs of stations. We explore this property to develop a novel method for determining the horizontal orientation of ocean bottom seismometers (OBS) by analyzing the polarization of Rayleigh waves retrieved from ambient noise crosscorrelation. We demonstrate that the sensor orientations can be estimated through maximizing the correlation between the radial-vertical component and the phase-shifted verticalvertical component of the empirical Green's tensor. We apply this new method to the ELSC (Eastern Lau Spreading Center) OBS experiment data set and illustrate its robustness by comparing the obtained orientations with results from a conventional method utilizing teleseismic P and Rayleigh wave polarizations. When applied to a large OBS array, the ambient noise method provides a larger number of orientation estimates and better azimuthal coverage than typically is possible with traditional methods.

Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south, which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity. We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity or increase grain size, or both, thereby facilitating melt transport.

Determining the melt distribution in oceanic crust at mid-ocean ridges is critical to understanding how magma is transported and emplaced in the crust. Seafloor compliance-deformation under ocean wave forcing-is primarily sensitive to regions of low shear velocity in the crust, making it a useful tool to probe melt distribution. Analysis of compliance data collected at East Pacific Rise between 9°and 10°N through 3-D numerical modeling reveals strong along-axis variations in the lower crustal shear velocities, as well as temporal variation of crustal shear velocity near 9°48′N between measurements spanning 8 years. Compliance measured across the rise axis at 9°48′N and 9°33′N suggest a deep crustal low-velocity zone beneath the ridge axis, with a low Vs/Vp ratio consistent with melt in low aspect ratio cracks or sills. Changes in compliance measured at 9°48′N between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval, perhaps following the 2005-2006 seafloor eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast spreading ridge.