Seismic Refraction Studies of the Crustal Structure 
of the Hawaiian Archipelago 1 
Augustine S. Furumoto and George P. Woollard 
In recent years several seismic refraction 
studies have been made of the crustal structure 
in and near the Hawaiian Islands. Earlier in- 
vestigators were Raitt (1956) and Shor (I960). 
A crustal analysis of earthquake travel time was 
made by Eaton (1962). Shor and Pollard (1964) 
made refraction studies to the north of the island 
of Maui in connection with Mohole site selec- 
tion investigations. Results of recent refraction 
studies over the Koolau volcano on the island of 
Oahu are reported in other papers in this issue 
(Furumoto et ah, p. 306; Adams and Furu- 
moto, p. 296). Some unpublished results in- 
clude those from the seismic refraction survey 
made in 1963 by Western Geophysical Com- 
pany, over the area as outlined by Shor and 
Pollard, that showed a subnormal depth to the 
mantle, and from the refraction surveys con- 
ducted by the U.S. Geological Survey across 
the island of Hawaii during the summers of 
1963 and 1964. Of these, the authors had access 
to the results obtained by Western Geophysical 
Company and, through verbal communication 
with D. Hill, to those of the Geological Survey. 
Because seismic results can best be considered 
in terms of their location, they will be divided 
on the basis of their geomorphic associations. 
Three crustal and geomorphic provinces are 
represented: the Hawaiian Arch, the Hawaiian 
Deep, and the Hawaiian Ridge. The locations 
of the measurements relative to these three fea- 
tures are shown in Figure 1, along with depth 
to bottom and depth to mantle. The recording 
stations of the refraction surveys are designated 
by numbers or letters in Figure 1. Two num- 
bers are listed next to the station designation; 
the upper number gives the depth to the ocean 
bottom, and the lower the depth to the Moho 
discontinuity. 
1 Hawaii Institute of Geophysics Contribution No. 
91. 
THE HAWAIIAN ARCH 
The Hawaiian Arch is a broad topographic 
feature lying to the north of the Hawaiian 
Ridge and separated from it by the north Ha- 
waiian Deep or Trench. 
Shor and Pollard (1964) found that over 
the Hawaiian Arch the average depth to the 
Moho is 10.4 km. At one location (Station 29 
of Fig. 1) the depth to the Moho was as shal- 
low as 9 km. These workers claim an accuracy 
of =i= 0.5 km for their depth estimates. The 
repeat studies in the same area by Western 
Geophysical Company and a reanalysis of 
Shor and Pollard’s data by the National Sci- 
ence Foundation confirm these results in gen- 
eral. The new data and the reanalysis show that 
the average depth to the mantle in this area is 
10 km. The structure of the crust in the area 
is defined by four distinct layers having the fol- 
lowing velocity values: Layer a, 2.15 km/sec; 
layer b, 4.20 km/sec; layer c, 5.56-6.41 km/sec; 
layer d, 6.82-7.01 km/sec. 
The underlying mantle has a velocity of 7.97- 
8.68 km/sec, which appears to vary according 
to azimuth. East-west oriented measurements 
have a higher velocity than those oriented 
north— south. This suggestion of anisotropy is 
definitely established in one section where cross- 
oriented measurements show a velocity of 8.1 
km/sec in a north-south direction, and of 8.7 
km/sec in an east-west direction. As all the 
measurements were reversed to obtain true 
velocity values, anisotropy appears to be well 
established. On the basis of the combined stud- 
ies of the Western Geophysical Company and 
Shor and Pollard, a site for a potential drill 
hole to the mantle was designated at approxi- 
mately 22°22'N and 155°28'W. The site, as 
seen in Figure 1, lies on the southern flank of 
the Hawaiian Arch at a location where the 
depth of water is about 2380 fathoms (4350 
315 
