Hunting for Ocean Island Moho: The Snipe Bites Back.

Previous studies of oceanic crust in the Pacific using seismic refraction have modeled average properties of the plate, neglecting regions of abnormal topography (White et al. 1992). Subsequent refraction studies (eg. Mutter and Mutter, 1993) have shown that, though the average thickness of the Pacific crust follows White et al.'s 7.1 km, it ranges in thickness from 2-37 km in anomalous regions. We investigate crustal structure underneath Rarotonga (Cook Islands), Tahiti (Society Islands), and Christmas Island using multi-taper correlation (MTC) receiver function (RF) methods in conjunction with data from Global Seismic Network (GSN) stations (RAR, PPT, XMAS respectively). 415 teleseismic P coda comprise our data set, of which 166 were recorded at RAR, 182 at PPT, and 67 at XMAS. Robust receiver functions to 2.5 Hz are computed at each station, at a broad distribution of back-azimuths and epicentral distances. We find that the crust and upper mantle under these islands are significantly more complex than a simple oceanic lithosphere model predicts, and that this structure is very similar from one island to the next. We observe Ps conversions arriving at less than 0.5-s delay that we attribute to conversions from the base of the volcano. We also see three conversions arriving at roughly 1, 2, and 3 seconds delay. Several mechanisms could explain our results, including crustal underplating, partial serpentinization of the upper mantle, topographic scattering, and partial-melt phenomena. We also consider the possibility that this structure is broadly associated with the Pacific superswell, because the North Pacific stations POHA (Hawai'i Island) and WAKE (Wake Island) exhibit markedly different receiver functions. Mutter, C.Z. and J.C. Mutter, Variations in thickness of layer 3 dominate oceanic crustal structure, Earth Planet. Sci. Lett., v117, 295-317, 1993. White, R.S., D.P. McKenzie, and R.K. O'Nions, Oceanic crustal thickness from seismic measurements and rare earth element inversions, J. Geophys. Res., v97, 19683-19715, 1992.