REGIONAL SEISMIC OBSERVATIONS OF THE ONTONG JAVA PLATEAU AND EAST MARIANA BASIN RHETT BUTLER Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, U.S.A. (Accepted 7 September, 1985) Abstract. 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~ 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. 1. Introduction The Ontong Java Plateau is an oceanic platform located north of the Solomon Islands in the equatorial western Pacific. With an area of about 1.7 x 106 km 2 above the 2000 fathom isobath, the Ontong Java plateau is one of the largest oceanic plateaus on earth. The geology and tectonics of the plateau are discussed by Kroenke (1972) and Coleman and Kroenke (1981). The crustal thickness of the central part of the plateau was measured in a two-ship refraction experiment in 1970 and ranged from 35 to 42 km (Furumoto et al., 1976). Further evidence for a thick crust is seen in the surface wave dispersion study of Seneff (1978). Sub-Moho velocities of 8.0 to 8.6 km sec -~ were observed by Furumoto et al. (1976). Walker (1977) found oceanic Pn velocities around 8.5 km sec -~ for travel paths within the plateau. The thick crust of the plateau has prompted speculation that it is a continental fragment (Nur and Ben-Avraham, 1978, 1982). Hussong et al. (1979), however, noted that the crustal layers of the plateau are proportionately about 5 times thicker than oceanic basin crust and could be regarded as an expanded section of normal oceanic crust. Hussong et al. (1979) concluded that the Ontong Java plateau probably formed as an integral part of the lithosphere at an accreting plate margin. The Ontong Java plateau was compared with other oceanic plateaus and submerged continental fragments by Carlson et al. (1980). Hawaii Institute of Geophysics Contribution No. 1658. Marine Geophysical Researches 8 (1986) 27-38. 9 1986 by D. Reidel Publishing Company.