Kona Earthquake — MACDONALD AND Wentworth 
285 
Fig. 13. Seismogram of aftershock recorded at 18:32, August 21, on the Bosch-Omori seismograph at Kilauea. 
Letters indicate the points of arrival of primary (P), secondary (S), and long (L) waves. The amplitude of 80 mm. 
on the seismogram corresponds to approximately 0.7 mm. of ground motion at Kilauea, 47 miles from the epicenter. 
the result of lagging behind as the ground 
moved eastward under them. To some extent 
also, the general east-west azimuth of fall 
undoubtedly reflects the direction of the epi- 
center. However, the prevailing east-west 
slope appears to have been still more impor- 
tant in determining the direction of fall of 
objects. Its effects on various types of damage 
have already been indicated. 
It has already been pointed out that the 
prevalent direction of rotation of columns in 
cemeteries indicates a location of the epicen- 
ter within the shaded offshore area in Figure 
12. This area contains the seaward extension 
of the Kealakekua fault. 
As a result of the consideration of all lines 
of evidence, the probable epicenter of the 
earthquake is placed approximately 3 miles 
west of Napoopoo, at latitude 19°29' N, lon- 
gitude 155°58' W. 
intensity of the earthquake 
There are in common use two different 
methods of determining and expressing the 
strength of an earthquake. The older method 
is based on the observed effects of the earth- 
quake on structures and various other objects. 
Based on these effects, a numerical value is 
assigned which is termed the intensity of the 
earthquake at any one point. Obviously, since 
the effects are less at greater distances from 
the origin of the quake, the intensity de- 
creases away from the epicenter. Various 
scales of intensity have been proposed. That 
used in the present study is the modified 
Mercalli intensity scale of 1931 (Wood and 
Neumann, 1931), in which values range from 
1, at which the earthquake is not felt except 
by a very few persons under especially favor- 
able conditions, to 12, at which damage is 
total. The second method assigns a value 
called magnitude to the earthquake, based on 
the effect on standard seismographs at known 
distances from the origin of the quake (Rich- 
ter, 1935). The magnitude is a measure of the 
amount of energy in the earthquake at its 
point of origin and, consequently, should be 
essentially the same at all measuring stations. 
The notice of preliminary determination of 
