Seismic Refraction Studies of the Crustal Structure of the Hawaiian Archipelago

1 Hawaii Institute of Geophysic s Contribution N o. 91. IN RECENT YEARS several seismic refraction studi es have been made of the crustal structure in and near the Hawaiian Islands. Earlier inTh e Hawaiian Arch is a broad ropographic vesrigato rs were Raitt (1956) and Shor (1960). feature lying to the north of the Hawaiian A crustal analysis of earthquake travel tim e was Ridge and separated from it by the north Hamade by Eaton (1962) . Shor and Pollard (1964) waiian Deep or Tr ench. made refraction studies to the north of the island Shor and Pollard ( 1964) found that over of Maui in connection with Mohole site selecthe Hawaiian Arch the average depth to the tion investigations. Results of recent refraction Moho is 10.4 km. At one location (Statio n 29 studie s over the Koolau volcano on the island of of Fig. 1) the depth to the Moho was as shalOahu are reported in other papers in this issue low as 9 km. These workers elaim an accuracy ( Fururnoto et al., p . 306 ; Adams and Furuof -+0.5 km for their depth estimates. Th e moro, p. 296) . Some unpublished results inrepeat s~udies in the same area by Western elude ~hose from the seismic refraction survey Geophysical Con;pany and a rea~alysis of . made 111 1963 by_W estern. Geoph ysical Corn>-. ~ho! ~n~. ~()I}~r4.s . dl! tfL by . th.e. N atlonaI Sci_ pany, over the area as outlined by Shor and ence Foundation confirm these results in genPollard, that showed a subnormal dep th to the eral. The new data and the reanalysis show that mantle, and from the refr action surveys conthe average depth ro the mantle ·in this area is ducred by the :"U.S. Geological Survey across ~O km. The structl~re of the crust in the area the island of Hawaii dur ing the summers of is d~fined by . four distinct layers having the fol1963 and 1964. Of these, the authors had access lowing velocity values: Layer a, 2.15 km /sec; to the results obtained by Western Geoph ysical layer b, 4.20 krrr/sec: layer c, 5.56-6.41 krrr / sec: Company and, through verbal communication layer d, 6.827. 01 kru / sec. with D. Hill, to those of the Geological Survey. The und erlying mantl e has a velocity of 7.97. Because seismic results can best be considered 8.68 ~m/sec, which appears to vary according III terms of their location, they will be divided ro azimuth, East-west oriented measurements on the basis of their geomorphic associations. have a high er velocity than those oriented Three crusta l and geomorph ic provinces are north-south. This suggestion of anisotropy is represented: the Hawaiian Arch, the Hawaiian definitely established in one section where crossDeep, and the Hawaiian Ridge. The locations oriented measurements show a velocity of 8.1 of the measurements relative to these three feakm /s ec in a north-south direction, and of 8.7 tures are shown in Figure 1, along wi th depth krn/ sec in an east-west dir ection. As all the to bottom and depth to mantle. Th e recording measurements were reversed to obta in true stations of the refraction surveys are designated velocity values, anisotropy appears to be well by numbers or letters in Figure 1. Two num established. On the basis of the combined studbers are listed next ro the station designation ; ies of the Western Geophys ical Company and the upp er number gives the depth to the ocean Shor and Pollard , a site for a potent ial drill bottom, and the lower the depth to the Moho hole ro the mantl e was designated at approxidiscontinuity. mately 22 °22 'N and 155°28'W . Th e site, as seen in Figure 1, lies on the southern flank of the Hawaiian Arch at a location where the dep th of water is about 2380 fathoms (43 50