The Koolau Volcanic Plug — Adams and Furumoto 
301 
tude and character can be seen. This occurrence 
of strong bubble pulse was especially annoying 
in this study in which secondary arrivals were 
expected to play a prominent part in the results. 
From the travel-time plot, it is seen that the 
first arrivals align well out to 7.27 km. Beyond 
this point this line can be extended on the basis 
of secondary arrivals at distances of 8.8 km, 12.7 
km, 18.1 km, etc. 
The portion of the plot showing a downward 
shift in the arrival of first motion, initially at- 
tributed entirely to diffraction effects, is com- 
posed of notably strong arrivals and the bubble 
pulses duplicate the first motion in this sector. 
The segment of first motion from 11.7 km out- 
ward is definitely early and is parallel to the 4.74 
km/sec segment obtained at distances less than 
7.27 km. Considering the complexity of the 
geology in a volcanic terrain, this extreme 
parallelism was unexpected. (Note, however, 
that the arrivals at 15.2 km are about 0.085 sec 
late for both the initial signal and bubble pulse, 
and that the arrival at the 19.5 km station is 
0.123 sec late. Note also that the first arrival 
at 19.5 km, denoted with a question mark, was 
definitely a repick and was similar in character 
to noise in the leading part of the seismo- 
gram.) 
The computer program for obtaining the 
best fitting line by least squares has been run 
on the first arrivals, the bubble pulses, and the 
early first arrivals. The values obtained are given 
in Table 3. 
Assuming that the top of the plug is approx- 
imately parallel to the surface of the ground, 
the dependency of the first occurrence of refrac- 
TABLE 2 
Arrival Times from the Refraction Seismograms 
DISTANCE 
(km) 
RECORD 
NO. 
1ST 
ARRIVAL 
(sec) 
2nd 
ARRIVAL 
(sec) 
3rd 
ARRIVAL 
(sec) 
4th 
ARRIVAL 
(sec) 
3.79 
41606 
0.840 
1.260 
1.845 
4.55 
41605 
0.960 
1.398 
1.870 
5.35 
41604 
1.155 
1.290 
1.610 
1.940 
5.97 
41603 
1.295 
1.505 
1.770 
2.090 
7.27 
41607 
1.560 
1.755 
2.060 
2.770 
7.99 
41601 
1.640 
1.800 
2.140 
2.380 
8.78 
42602 
1.790 
1.910 
2.240 
2.500 
9.62 
42603 
1.848 
1.948 
2.288 
10.43 
42604 
1.988 
2.408 
2.758 
2.848 
11.70 
42605 
2.115 
2.270 
2.470 
2.970 
12.70 
41901 
2.310 
2.730 
3.010 
3.720 
13.50 
42607 
2.495 
2.778 
2.875 
3.375 
14.80 
41903 
2.790 
2.890 
3.140 
3.270 
15.20 
41904 
2.950 
3.140 
3.350 
3.540 
18.10 
42902 
3.520 
3.642 
4.050 
4.350 
18.90 
42904 
3.680 
4.090 
4.180 
4.580 
19.50 
42903 
3.920 
4.290 
4.570 
4.940 
