210 
The N.Z. Journal of Science and Technology. 
[Nov. 
on a sandy beach as on rocky ground in the higher parts of the island. All 
observers except one felt the motion as a horizontal vibration, and the 
exception, an educated white man, stated that the movement was an 
undulation like sea-waves. This latter observer was at Saluafata, on a 
small steep hill situated in the middle of a large crater the bottom of which 
is 250 metres above sea-level. 
There was an interesting psychological effect of the earthquake in 
question, in that the sensibility of observers to earthquakes was tem¬ 
porarily increased for several weeks after the event. Observers in a 
resting position in an upper story of a house were sensitive to quakes 
in which the acceleration according to seismograph records was only 
1 mm./sec. 2 — i.e., an earthquake of strength I in the above scale, and 
normally insensible except on a seismograph. 
The earthquake of the 31st March, 1907, was registered by persons in 
Tonga as of degree VI for a minute to a minute and a half ; the quake of 
the 30th April, 1919, was felt in Haapai for two minutes and a half, and 
in Tonga Tabu (South Tonga) for only fifty seconds. The epicentre, as 
will be seen from Table I, is much nearer to the former place. Changes 
in shape of the surface of the earth— e.g., holes of several metres depth 
and width—were reported from the surrounding islands, especially in 
ground filled in artificially. Landslides were experienced in Samoa. Only 
in one case was a large permanent change of level produced—namely, in 
Keppel Island, in the earthquake of the 26th June, 1917, where it is 
reported that the lagoon was raised and is now dry. The comparatively 
large area of shake of the more intense Tongan earthquakes suggests 
a considerable depth of epicentre. On the other hand, some surface 
changes take place, and suggest a more superficial epicentre, for 
tidal waves were associated with several of the earthquakes—namely, 
those of the 1st May, 1917, the 26th June, 1917, the 16th November, 
1917, and the 30th April, 1919. After the earthquake of the 26th June, 
1913, pumice was washed up by the sea in Tonga, and after the earthquake 
of the 26th June, 1917— i.e., exactly four years later—a large pumice-field 
was observed in the open sea near the Tongan Islands. 
II. Tilting of Ground in Earthquake Area. 
It is upheld by several authorities that during the passage of an earth¬ 
quake-wave there is sensible tilting—for example, in the main phase of a 
distant earthquake. To interpret the record of a seismograph it is necessary 
to distinguish between apparent and true tilting. The progress of a vertical 
displacement along a horizontal surface gives rise to undulations. If the 
motion is harmonic, we may write for the displacement 
(! + «.) 
z = z sin 2 tt y y + ^ 
where z is the maximum amplitude of displacement, x the distance from 
the origin, A the wave-length, and S the phase. The true tilting of the 
ground is given by 
dz 2 t - /2tt 
i * = dt = X 2 C0S l A + S i 
the maximum angle of tilt being i x = — z. 
A 
A horizontal displacement, however, will give rise to a horizontal 
acceleration, which, combined with the normal gravitational acceleration, 
