EARTHQUAKES 
251 
(Rep. Brit. Assoc., 1917, pp. 13-14; 1922, Phil. Trans. A., 
vol. 222, pp. 45-6). Prof. H. H. Turner, from the time of 
arrival of an earthquake shock at the opposite side of the 
sarth, assigns the centrum of most earthquakes to a depth of 
about 145 miles, and some to 310 miles; e.g. the Formosa 
Earthquake of 14th April, 1906, started at a depth of 280 
miles (Rep. Brit. Assoc., 1922, p. 255), and that in China on 
16th December, 1920, at more than 80 miles (ibid., p. 256). 
The Assam Earthquake of 12th June, 1897, which had a 
meizoseismic area of 150,000 square miles, was at first re- 
ferred to a depth of 5 miles, but is now assigned by R. D. 
Oldham to the depth of between 100 and 200 miles (G.S. 
India, Mem. xlvi, pt. 2, 1026, p. 62). 
Causes—Certain unstable belts in the earth’s crust are 
especially liable to earthquakes, which are of three kinds— 
tectonic, volcanic, and those due to variations in the load 
on the surface. Tectonic earthquakes are due to unequal 
movements of the material within or below the crust along 
great faults and thrust-planes, around subsidences, and along 
folds which are often traversed by cross-faults.” Volcanic 
earthquakes are due to the uprush of steam during eruptions 
which keep the adjacent ground in constant tremor, while 
single explosions may shake the whole world. Volcanic 
action often results in local earthquakes by the collapse of 
cavities left by the ejection of material or the shrinkage 
of the cooling rocks. Such earthquakes may be of intense 
violence, but of short range ; those in Ischia from 1881-3, due 
to subsidence in an extinct volcano, though they culminated in 
the destruction of the chief town of the island, were barely 
perceptible in Naples 18 miles away, and were not recorded 
in Vesuvius Observatory at the distance of 25 miles. 
Earthquakes have been attributed to landslips, such as 
the Pamir Earthquake of 1911; but Oldham (Q.%.G.S., 
xxix, 1923, pp- 243-4) has shown that the landslip at Usoi 
was not at the epicentre and was a result of the earthquake 
and not its cause. The slide of material down oceanic 
slopes often breaks telegraph cables; but the movement 
may not be recorded by seismographs and so does not cause 
appreciable vibration in the crust, although the repeated 
blows of the sutf on the coast of India disturbs the seismo- 
graph at Calcutta 500 miles away.