Crustal structure of the Island of Hawaii from seismic-refraction measurements

Abstract

In August of 1964 the U. S. Geological Survey established seismic-refraction profiles along the northeast, southwest, and west coasts of the roughly triangular-shaped Island of Hawaii. Shots were fired at 10-km intervals along each coast from the U. S. Coast Guard Cutter CAPE SMALL and were recorded on shore by five refraction units spaced at approximately 25 km intervals along each coast. Most of these shots were also recorded on the 13 seismograph stations maintained on Hawaii by the U. S. Geological Survey's Hawaiian Volcano Observatory. These data were supplemented by recordings on the 13-station seismograph network and two mobile systems of three 500-ton chemical explosions deronated by the U. S. Navy on Kahoolawe as part of the SAILOR HAT program and by a re-evaluation of arrivals recorded on the seismograph network from seismic-refraction profiles shot off the northeast coast of Hawaii by Scripps Institution of Oceanography in 1962. Interpretation of the resulting seismograms suggests that the crust under Hawaii can be divided into two principal layers: (1) a basal layer 4 to 8 km thick with P-wave velocities of 7.0 to 7.2 km/sec, and (2) an upper layer 4 to 8 km thick in which P-wave velocities increase with depth from 1.8 to 3.3 km/sec at the surface to 5.1 to 6.0 km/sec at depth. The basal layer is probably the original oceanic crust under Hawaii plus the intrusive system associated with central vents and rift zones, and the upper layer is the accumulated pile of lava flows that form the bulk of the island. The crust along the northeast and southwest flanks of Kilauea is 11 to 12 km thick with P-wave velocities increasing in the upper layer from 1.8 km/sec at the surface to 5.1 km/sec at depth. The basal layer is 4 km thick and has a P-wave velocity of 7.1 km/sec. A 7.0-km/sec layer at depths of 3 to 5 km under the northeast flanks of Mauna Kea and Kohala Mountain masks first-arrival evidence for deeper structure, but secondary arrivals interpreted as reflections from the M discontinuity suggest that the underlying crust may be anywhere between 12 and 20 km thick. This shallow 7.0-km/sec layer is probably associated with the nearby rift zones of Kohala Mountain and Mauna Kea. The crust increases in thickness along the west coast of Hawaii from about 14 km under the flanks of Kohala Mountain and Hualalai to about 18 km under the flank of Mauna Loa. P-wave velocities along this coast increase with depth from 2.5 km/sec at the surface to 6.0 km/sec at about 10 km, and the lower 4 to 6 km of the crust has a P-wave velocity of about 7.2 km/sec. The upper mantle P-wave velocity under most of the island is 8.2 km/sec but may decrease to 8.1 km/sec under the southeast flank of Kilauea. Material with mantle-like P-wave velocity appears to bulge up under the summit of Kilauea to a depth as shallow as 10 or 11 km. Early P-wave arrivals associated with the summits and major rift zones of the volcanoes indicate that material with velocities as high as 7.0 km/sec approaches within 2 or 3 km of the surface under these structures and merges at depth with the 7.1- to 7.2-km/sec layer forming the base of the crust.