118 CHARLES DAVISON 
c) The sound and tremor, with which tectonic earthquakes 
usually begin, are either absent from volcanic earthquakes, or else 
of duration so short as to be almost imperceptible. 
d) The duration of the shock in volcanic earthquakes is natant 
short, being very often less than 5 seconds. 
e) The length of the focus is inconsiderable when compared 
with that of tectonic earthquakes. Even in the strongest of 
volcanic earthquakes, it rarely exceeds 4 or 5 miles, whereas the 
average length of the focus in British earthquakes (omitting the 
weakest) is about 12 or 13 miles. This conclusion is also supported 
by the brief duration of the shock. 
f) Some, but not all, volcanic earthquakes are preceded and 
followed by a rather large number of accessory shocks. In this 
they resemble tectonic earthquakes. But the after-shocks of 
volcanic earthquakes are distinguished by the short period of their 
action. For instance, the series of after-shocks in the Albano 
earthquake of 1906 lasted for 3 days, in the Ischian earthquake 
of 1883, for 7 days, in the Fleri earthquake of 1894, for about 3 
weeks, in the Linera earthquake of 1914, for nearly 4 weeks, and 
in the Fondo Macchia earthquake of 1865, for 5 weeks. 
(g) The after-shocks of volcanic earthquakes are practically 
confined to the epicentral area. They point to little, if any, 
tendency toward any extension of the original focus. 
h) In a volcanic system such as that of Etna or the Alban 
Hills earthquakes occur in many different zones, and seismic 
activity is subject to frequent and sudden migrations from one 
zone to another. Nevertheless, in any given zone there is often 
a certain fixity in the epicenters of successive earthquakes, as 
in those of Fondo Macchia (Etna) and Casamicciola (Ischia). 
The most characteristic of these features is the smallness of 
the disturbed area considering the intensity of the shock at the 
epicenter. In Great Britain we have no shocks to be compared in 
strength with those of Fondo Macchia and Ischia. The strongest 
of British earthquakes are just capable of causing slight damage 
to buildings (corresponding to the degree 7 of the Mercalli scale), 
and the average area disturbed by them is 65,900 square miles. 
Others of intensity 6, only slightly stronger than the Albano 
