Crustal and upper mantle structures in the West Philippine Basin, along 17-18°N, have been determined using explosions as sources and ocean bottom seismometers to measure refracted compressional waves. Seismic refraction profiles out to nearly 500 km were completed. Shallow structure was measured using small shots, 1-240 lbs., and the deeper structure was probed with large explosions, 0.9-1.8 tons. A velocity-depth inversion using short range data shows the upper crust to have strong velocity gradients which gradually decrease with depth. The lower crust is characterized by a nearly constant velocity gradient of 0.24 sec⁻¹. Standard delay-time functions and a modified function accounting for lateral velocity gradients were also used in travel time inversion. Results from the two methods are comparable and yield ~1.5 km transitional zone thicknesses in the basin. Although they vary slightly in magnitude between methods, West Philippine Basin oceanic layer thicknesses are abnormally thin, by about 2 km, when compared to average crust. Total crustal thicknesses are shown to be thinner in the eastern part of the basin, approaching only 3 km. Crustal thinning toward the east is consistent with the Palau-Kyushu Ridge being a remnant transform fault connecting Philippine and Kula-Pacific ridges in the past. Predicted water depths in the basin are about 300 meters shallower than observed depths when compensated to average mantle depths found for the Western North Pacific. The depth anomaly cannot be fully reconciled by thin crust, and requires a deeper-seated anomaly to be present in the West Philippine Basin. Temperature and pressure modeling using experimental measurements from proposed mantle constituents indicate high seismic gradients in the upper mantle and may suggest that a multi-component or graded mantle exists beneath the marginal sea. / Graduation date: 1983