VOLUME XxXIx NUMBER 2 
THE 
WOURNAL OF GEOLOGY 
FEBRUARY-MARCH 1921 
VOLCANIC EARTHQUAKES 
CHARLES DAVISON 
Birmingham, England 
Volcanic earthquakes, according to the late Professor Mercalli, 
are those which have their centers of maximum intensity under 
or close to the cones of active or semi-extinct volcanoes.* Pro- 
fessor Omori somewhat enlarges this definition. “A volcanic 
earthquake,” he says, ““may be defined as a seismic disturbance, 
which is due to the direct action of the volcanic force, or one whose 
origin lies under, or in the immediate vicinity of, a volcano, whether 
active, dormant or extinct.”? Of the two definitions, the latter 
is the wider in its scope, for it includes the earthquakes which visit 
extinct volcanoes, such as the Alban Hills near Rome—earthquakes 
which differ in no important particular from those of an active 
volcano like Etna or of a dormant volcano such as M. Epomeo 
in the island of Ischia. 
Mercalli follows up his definition of volcanic earthquakes by 
describing their important properties. The earthquakes, he says, 
(rt) are felt many times in an area which is very restricted, although 
the shocks are violent; (2) they precede slightly, but sometimes 
accompany or follow, the eruptions of the neighboring volcano; and 
(3) they are repeatedly felt in the same area with similar characters 
*G. Mercalli, Vulcani e Fenomeni Vulcanici in Italia (1883), p. 355. 
2F. Omori, Bull. Imp. Earthquake Inv. Com., Vol. VI (1912), p. 8. 
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