Regional seismic observations of the Ontong Java plateau and East Mariana Basin

P-waves recorded on Ponape Island at the northern end of the Ontong Java plateau have been investigated. Different modes of propagation in the distance range 12° to 17° Δ between paths from Melanesian earthquakes across the Ontong Java plateau and paths from Mariana earthquakes across the eastern Mariana basin suggest that the mantle underlying the regions is not homogeneous. Travel-times of P-waves beneath the Ontong Java plateau are slower than for paths beneath the east Mariana basin. The frequency content of the first-arriving P-waves is lower for paths across the Ontong Java plateau than for the east Mariana basin. The disparate crustal thicknesses of the Ontong Java plateau and east Mariana basin may influence the relative amplitude of oceanic Pn, with smaller amplitudes corresponding to a thicker crust. Pn-P differential times for the Ontong Java plateau show a general decrease with the depth of earthquake focus but no comparable trend is seen in the Mariana data, possibly because of the scatter. The observation of high-frequency oceanic Pn propagation across the Ontong Java Plateau is suggestive that the plateau is not of continental origin.