286 
epicenter issued by the Coast and Geodetic 
Survey lists the magnitude of the earthquake 
of August 21 as 6.75 as determined at Pasa- 
dena and 7.0 as determined at Berkeley, in 
California. 
Field studies of the effects of the earth- 
quake indicate an intensity of 7 on the modi- 
fied Mercalli scale in the area near the 
epicenter, decreasing to 6 at Waiohinu and 
Naalehu, 5 in the vicinity of Kilauea Caldera 
and in Hilo, and 4 at Honokaa and in the 
Kohala district at the north end of the island. 
At Honolulu, 180 miles (288 km.) from the 
epicenter, the intensity was 2. Populated 
areas of the island of Hawaii are largely 
restricted to the periphery of the island. The 
interior portions of the island are almost 
wholly unpopulated, making it impossible to 
draw accurate isoseismal lines. Approximate 
isoseismals are shown in Figure 1. 
Given a single impulse, the minimum 
horizontal acceleration that can cause the 
sliding of a short stone column on a stone 
base is 71 per cent of the value of gravity, 
decreasing to 57 per cent at an angle of 
emergence of 35° to the horizontal (Imamura, 
1937: 105). Because the sliding of headstones 
and, especially, base plates was common in 
cemeteries during the August 21 earthquake, 
it might be concluded that the acceleration 
during the earthquake was at least six tenths 
that of gravity. However, Imamura (1937: 
106) also has shown that small, short-period 
vibrations in the epicentral areas of strong 
earthquakes, although they do not themselves 
cause the displacement of objects, may so 
lower the normal values of the coefficients of 
friction that sliding can be caused by longer 
period vibrations with accelerations much less 
than six tenths that of gravity. The presence, 
of such vibrations in the Kona area is sug- 
gested by local vagaries of displacement and 
by other behavior. The acceleration which 
caused the lateral displacement of objects 
during the Kona earthquake is not known 
but probably was much less than six tenths 
that of gravity. 
PACIFIC SCIENCE, Vol. VI, October, 1952 
CONCLUSION 
The earthquake of August 21, 1951, like 
most of its aftershocks, probably was caused 
by movement on the Kealakekua fault. This 
is one of a number of similar faults along 
which the lower slopes of Mauna Loa and 
Kilauea Volcanoes have moved relatively 
downward and outward toward the deep 
ocean. In this sense the earthquake was tec- 
tonic in origin. 
In one sense, of course, all earthquakes in 
Hawaii are volcanic in origin. However, the 
August 21 earthquake cannot be directly re- 
lated to any specific volcanic episode. It is 
possible that it is related in some way to the 
great extravasation of lava during the 1950 
eruption of Mauna Loa, but there is no 
evidence to demonstrate such a relationship. 
On September 16 a series of smaller earth- 
quakes originated on the Kaoiki fault system, 
a series of fractures corresponding to the 
Kealakekua fault, on the southeast slope of 
Mauna Loa. From mid-May until early July 
abnormally rapid eastward tilting at Kilauea 
Caldera indicated a tumescence of Mauna 
Loa Volcano. There is a possibility that both 
the August 21 earthquake and its aftershocks 
and the September 16 earthquakes were 
caused by a slight upward movement of the 
central portion of Mauna Loa in relation to 
the lower slopes. The August 21 earthquake 
has no known connection with any coming 
volcanic activity, though such a relationship 
may yet appear. 
The southern part of the island of Hawaii 
is subject to frequent earthquakes, but few 
are as intense as that of August 21, 1951. The 
great earthquake of April 2, 1868, judging 
from the descriptions of damage, was much 
more severe. Wood (1914) assigned to it an 
intensity of 10. Its epicenter was farther south, 
near Waiohinu in Kau, where extensive sur- 
face faulting took place. The earthquakes of 
March 28 and April 3, 1868, also were prob- 
ably at least as severe as that of August, 1951. 
The earthquake of October 6, 1929, centered 
beneath Hualalai Volcano, had a magnitude 
