High-frequency Pnand Snphases recorded in the western Pacific

Abstract

One hundred sixty-four (164) high-frequency Pn phases and 126 high-frequency Sn phases recorded on a short-period seismograph temporarily operated at Ponape are used to complement and extend prior investigations of the Western Pacific mantle. Pn and Sn phases are well recorded out to distances of 20° for moderate-sized earthquakes (Mb ≃ 5) for travel paths entirely within the relatively homogeneous geophysical provinces of the Northwestern Pacific Basin or the Ontong Java Plateau. For comparable events, Pn and Sn phases are generally more prominent for travel paths in the Northwestern Pacific Basin than in the Ontong Java Plateau. Pn and Sn propagation is much less efficient but still possible for travel paths that cross island arc-trench systems or island chains. For distances beyond 20°, Pn and Sn are observed only for large earthquakes (Mb ≥ 6.0) or for moderate-sized earthquakes having substantial focal depths (>200 km). At all distances, Pn is generally more prominent on the seismograms than Sn. The maximum distance at which Pn and Sn phases are recorded is 37.84° (4208 km). Sn velocities for the Northwestern Pacific Basin (>130 m.y. old) and for the Ontong Java Plateau (60 to 130 m.y. old) are 4.77±0.03 km/s and 4.70±0.07 km/s, respectively. Corresponding velocities for Pn are 8.33±0.05 km/s and 8.52±0.14 km/s. A suggestion of more than one Pn, Sn wave guide is provided by Pn phases at distances in excess of 30°. The Pn velocity for these phases is 7.87±0.33 km/s.