310 
1200 2400 3600 4000 6000 7200 8400 
FIG. 5. Section structure under Kawainui Swamp 
from seismic refraction surveys. 
terms of sloping beds, rather than in terms of 
high-velocity layers. The presence of infinite 
apparent velocity over the spread at point C-l 
gives support to this interpretation. 
An interpretation of the travel-time plots 
and data gave the shallow structure shown in 
Figure 5. The water layer in the canal (average 
depth, 4-6 ft) was undetected and hence is dis- 
regarded in this discussion because all explo- 
sions were set off at the canal bottom. The first 
layer detected had a seismic velocity of 1.7 
km/sec and extended to a depth of 300-400 ft. 
The second layer had a velocity of 2.8 km/sec. 
the 2.8 km/sec layer dips 38° under site C-l 
to give an apparent infinite velocity across the 
geophone spread. The third layer, the deepest 
layer detected, was assigned a velocity of 4.63 
km/sec. This value was obtained from the re- 
sults of gashouse series. The 4.63 km/sec layer 
dips 8° in the midsection between C-l and C-2. 
For probing the deeper structures in the cal- 
dera the GASHOUSE series (Fig. 2) was carried 
out. Details of the field work, data, and inter- 
pretation of this series are given in another 
paper (Adams and Furumoto, p. 296 in this 
issue). Analysis showed that a layer of velocity 
4.64 km/sec was intruded by a plug that had 
a velocity of 7 km/sec as a lower bound. The 
depth to the top of the plug from the ground 
surface was estimated to be 1.6 km. 
Northwest Rift Zone 
To investigate the northwest rift zone, series 
able and K-K were carried out. For the K-K 
PACIFIC SCIENCE, Vol XIX, July 1965 
series recording units were placed at China- 
man’s Hat Island and at Kailua High School 
(Fig. 1). The site at Kailua High School is 
identical with site H-2 described in the discus- 
sion on the caldera area. For the ABLE series 
recording was done at Kahuku and again at 
Kailua High School (Fig. 1). 
The travel-time plots for K-K and ABLE series 
are given in Figure 6. Notice in the figure that 
the 5.58 km/sec line is common to the able 
and K-K series. For the other lines, however, 
such a happy coincidence does not occur. It is 
best to consider the two series separately for 
analysis. 
For the K-K series the 5.58 km/sec layer 
matches the 6.74 km/sec layer in the reversed 
profile. On the Chinaman’s Hat recording a 5.2 
km/sec layer is evident and this corresponds to 
the 4.9 km/ sec layer in the Kailua High School 
recording of the series in the caldera area. How- 
ever, the pairing of 5.2 and 4.9 should not be 
done to determine true velocity, because the 
value of 4.9 is the result of complicated dipping 
and faulting in the caldera zone. Instead, as was 
done for the caldera interpretation, the value of 
4.64 km/sec, obtained from the GASHOUSE 
series, was assigned as the true velocity for 
this layer. 
For the layer just below the water the value ' 
of 3.0 km/sec was taken from the Chinaman’s 
Hat recording. This layer becomes 2.8 km/sec 
on land, as was seen in the caldera area data. 
Calculations made from the travel-time data 
yielded the 
following: 
Layer 
Velocity 
Km to Top of Layer 
(km /sec) 
Ciiinaman’s 
Hat 
Kailua 
a 
3.0 
0 
0 
b 
4.64 
0.7 
0 
c 
6.1 
1.7 
0.8 
The ABLE series form a reversed profile and 
a split profile on the northern end. For the 
Kahuku recording the layer that should corre- 
spond to the 5.58 km/sec layer of the Kailua 
recording is discernible only as second arrivals. 
The arrivals match well with a set of second 
and third arrivals with an apparent velocity of 
6.8 km/sec on the north side of the Kahuku 
station. 
