THE NEW ZEALAND 
JOURNAL OF SCIENCE 
y x 
•:o 
a* 
\s 
AND 
TECHNOLOGY. 
a . n. 
*•> 
'"VbL. IV. 
Wellington, November, 1921. 
No. 5. 
A STUDY OF PACIFIC EARTHQUAKES.* 
By Professor G. Angenheister, Observatory, Apia, Samoa. 
(Communicated by the Hon. Minister of External Affairs.) 
It is well known that about two out of every three of the world-shaking 
earthquakes originate near the steeply sloping margins of the Pacific. The 
present paper deals with those whose epicentres are near that part of the 
ocean known as the Tonga Deep. Fortunately, although these earthquakes 
are numerous, there are few inhabitants in that part of the Pacific, and 
consequently the earthquakes have little destructive effect, more especially 
as most of them originate under the sea. The Samoa Observatory, because 
of its geographical situation, is particularly well suited for a study of these 
earthquakes. 
Before dealing with the instrumental records, however, it will be of interest 
to give such meagre information as is available of personal observations of 
these earthquakes. One very noticeable feature of personal observations of 
the earthquakes is that there often appear to be two shocks, so that observers 
declare that there are two separate quakes. The first of these shocks, due 
to the preliminary tremors, or longitudinal vibrations, generally lasts from 
five to ten seconds. It is succeeded by several seconds of relative quietness, 
and then again there is a sharp shock due to the main waves from the 
earthquake. According to information collected the area through which 
the shocks are felt is generally very large. That of the 26th June, 1.913, 
was felt through 8° from its centre, and that of the 26th June, 1917, which 
was felt in Samoa as the strongest for over a generation, was observed at 
6° from its epicentre. This latter earthquake was of strength VIII according 
to the 12-division scale of Mercalli, and according to instrumental observa¬ 
tions gave rise at Apia to a maximum acceleration of over 300 mm./sec. 2 . 
The area through which an earthquake is felt appears to be far more 
extended in the north-south direction than in the east-west direction, 
and the total area in the case of the latter earthquake was over a million 
square kilometres. The quake movement of the main phase of an earth¬ 
quake appears to be felt as many times stronger than that of the first pre¬ 
liminary tremor. In the case of the 1917 earthquake mentioned above the 
movement was felt for a minute and a half. It was not so strongly felt 
* I have to acknowledge my indebtedness to Professor E. Marsden for assistance 
in the preparation of this paper. 
14—Science. 
