VOLCANIC EARTHQUAKES 117 
(zone 10), followed a week later (on November 13) by one with 
its epicenter near Velletri (zone 9); on July 19, 1899, the strongest 
shock of the series (intensity 8), causing some damage at Frascati 
(zone 4) and rather less at Marino, the mean duration of the 
shock 3% seconds, registered at Lubiano (298 miles to the north) 
and Catania (318 miles to the south); on October 12, 1902, a 
shock of intensity 6 at Genzano (zone 10); on February 21, 1906, 
one of intensity 5-6 at Albano (zone 6); and, lastly, on October 
6, 1909, a shock at Frascati (zone 4). Thus, of these ten earth- 
quakes, four originated on the north side, and six on the south side, 
of the volcano; while six of the ten seismic zones were in action. 
II. CHARACTERISTICS OF VOLCANIC EARTHQUAKES 
a) While volcanic earthquakes often occur in close connection 
with eruptions—either shortly before, during, or shortly after 
them—they seldom coincide with an outburst, except in the case 
of the weak tremors with which the Japanese records have made 
us familiar. Nor do the almost contemporaneous earthquakes 
and eruptions always occur in the same part of the volcano. The 
Fondo Macchia earthquakes of 1865 and 1o11 were felt in a district 
to the east of the central crater of Etna, while they followed closely 
on eruptions from the northeastern flank. Some earthquakes, 
again, occur in active volcanic regions without the accompaniment 
of any change in volcanic activity; others in regions in which the 
volcanoes have been extinct for many centuries. 
b) In volcanic earthquakes the intensity of the shock is 
often great near the center of an extremely small disturbed area. 
The relation between the intensity and the area is by no means 
constant. On the one hand, we may have an earthquake like 
that of Nicolosi in 1901, destroying houses in a minute epicentral 
area and yet imperceptible at a distance of more than 4 miles, or 
one like the Ischian earthquake of 1883, leveling every building 
within an area of 3 square miles, and yet felt within an area of 
not more than 1,250 square miles. On the other hand, we may 
have an earthquake like the Albano earthquake of 1906, not 
strong enough to damage houses at the epicenter and yet felt 
over an area of 535 square miles. 
