Can a narrow, melt-rich, low-velocity zone of mantle upwelling be hidden beneath the East Pacific Rise? Limits from waveform modeling and the MELT Experiment

One of the goals of the Mantle Electromagnetic and Tomography (MELT) Experi ment is to determine whether a narrow zone of enhanced melt concentration c onsistent; with focused upwelling exists beneath the East Pacific Rise. Usi ng SKKS, sScS, and S phases from two intermediate-depth earthquakes in the Banda Sea and the Tonga-Kermadec region, we demonstrate that there is no po sitive evidence for the existence of such a zone and that travel time delay s for shear waves traveling through it must be <0.5 s. To test whether diff raction and wave front healing could obscure evidence for its existence, we employ a pseudospectral method to simulate finite frequency teleseismic wa ves propagating through narrow, vertical low-velocity zones. A rich set of reflected, diffracted, and guided waves is generated when S waves encounter such a low-velocity channel, particularly at high frequencies. Limiting th e frequency content to the lower-frequency bands with good signal-to-noise ratios in the observed phases obscures these waveform complexities. The tra vel time anomaly is broadened and reduced in amplitude but remains detectab le unless the low-velocity zone is very narrow or has only modest velocity contrast. The lower limit of detectability corresponds to a 5-km-wide chann el of partial melt extending from 10 to 60 km below the seafloor at the rid ge axis with a shear velocity contrast of 0.5 km/s. Although these limits a re severe, 3 to 4% melt retention might cause a large enough viscosity redu ction and anomalous buoyancy to dynamically focus upwelling into a 5-km-wid e channel that falls within the limits. Strongly focused, dynamic upwelling beneath the ridge, however, is probably not compatible with the existence of a broad region of very low shear velocities in the surrounding mantle.