A two‐dimensional seismic investigation of crustal structure under the Hawaiian Islands near Oahu and Kauai

Abstract

Two expanding spread profiles (ESPs) were analyzed in detail to investigate the deep structure of the volcanoes and the underlying oceanic crust near Oahu. One ESP was located on the southeastern submarine flank of Kauai (ESP 10), and the other was located directly over the ridge axis between Oahu and Molokai (ESP 5). Two‐dimensional models of the entire crust were obtained by iterative comparison of both travel times and relative amplitudes of arrivals on the ray‐theoretical synthetic seismograms with those in the data. A 2.4‐km vertical offset in the preexisting crust, inferred to be a large fault zone, was found near the midpoint of ESP 10. This fault may form the southern wall of a graben or half graben in which Kauai sits. The layering and seismic velocities of the preexisting oceanic crust near Kauai have remained unmodified by the load of the volcanic pile. ESP 5 is located over a shallow 4‐km‐thick basin filled with volcaniclastics and possibly volcanic flows bordered by the islands of Oahu and Molokai, Penguin Bank, and two buried rift zones. Under this basin the main volcanic edifice is 4 to 5 km thick. The top of the preexisting oceanic crust is 8 to 9 km below sea level and is 3 to 4 km lower than the crust away from the islands. The underlying oceanic crust is 6 to 7 km thick excluding the Moho transition zone, which is of normal thickness and at a depth from 15 to 16 km. The existence of a large subcrustal plutonic body is refuted.