VOLCANIC EARTHQUAKES 121 
must be appealed to as offering the best explanation of the various 
phenomena of volcanic earthquakes. 
For a long time the rending of the mountain mass has been re- 
garded as a probable cause of volcanic earthquakes. G. P. Scrope 
wrote: 
It may very plausibly be suspected that each of the shocks by which the 
environs of a volcano are so repeatedly agitated, during, and previous to an 
eruption, is occasioned by the rending of some part of the solid frame-work 
of the mountain or its supporting strata, by the action of the force—resulting 
from the pressure in all directions of the liquid which is in communication 
with that elevated within the volcanic chimney. The prolongation or widening 
of a fissure previously formed would have the same jarring, or vibratory 
effect, as the creation of a new one.t 
Nearly half a century later? Scrope repeated this paragraph 
with hardly any change except in the initial words, and these 
were altered to ‘‘there can be little doubt that,” etc. That the 
fracturing of the mountain mass would be attended with consider- 
able noise and some perceptible vibration cannot be disputed. 
Whether it is competent to produce earthquakes so strong as 
those which visit the southeastern flank of Etna or to give rise 
to the earthquake phenomena described above is less clear and 
remains, I think, unproved.3 
Professor Omori ascribes volcanic earthquakes without explo- 
sions to the underground expansive force which produces cracks 
at the depth of a few kilometers; and those with explosions partly 
to the upward extension of the cracks, and partly, as regards the 
slow movements, to the outward pushing of the mountain mass 
consequent to the explosion. If this be the case, it would seem 
that the earthquakes due to explosions would usually be of slight 
t Considerations on Volcanos (1825), p- 155- 
2 Volcanos (1872), p. 163. 
3 The explanation has received support from well-known seismologists, among 
others from G. Mercalli (Vulcani e Fenomeni Vulcanici in Italia [1883], pp. 354-55), 
J. D. Dana (Characteristics of Volcanoes [1890], p. 22), F. Omori (Bull. Imp. Earthquake 
Inv. Com., Vol. VI [1912], pp. 132-33), and A. Riccéd (Boll. Soc. Sis. Ital., Vol. XVI 
[r912], p. 32), but not from H. J. Johnston-Lavis (Monograph of the Earthquake. of 
Ischia [1885], p. 92). 
4 Bull. Imp. Earthquake Inv. Com., Vol. VI (1912), pp. 132-33- 
