VOLCANIC EARTHQUAKES II5 
after-shocks were felt in Casamicciola. This, however, was not 
the only center in action, for several were recorded at the town of 
Ischia only.* 
4. ALBAN HILLS 
About 15 miles to the southeast of Rome lies a group of extinct 
volcanoes containing several circular lakes which evidently occupy 
old craters of the system. The principal hill, Monte Cavo, of 
which the other and smaller hills may be regarded as lateral cones, 
rises to a height of nearly 3,000 feet. ‘There is no certain evidence 
of activity during historic times. 
The earthquakes of this district are less frequent and destructive 
than those of the much larger Etnean region, but in other respects 
they resemble them closely, and especially in the constant migration 
of activity from one part of the volcano to another. 
One of the most recent in the district is that of February 21, 
1906. It was preceded by six very slight fore-shocks, one on 
February 20, at Ariccia, the others on February 21, at Albano. 
The principal shock occurred at 9:49 P.M. on February 21, and, 
from that time until February 23, there were 6 very slight after- 
shocks. The isoseismal lines of the principal earthquake, which 
was of intensity 5—6 (Mercalli scale), are represented approximately 
in Figure 7.. They are rather close together, indicating a rapid 
decline in the intensity of the shock from the epicenter, though 
a decline less rapid than in other earthquakes of the same district 
or of the Etnean region. The mean duration of the shock was 
nearly 4 seconds.’ 
Dr. M. Baratta defines the following seismic zones among the 
Alban Hills (Fig. 7): (2) Colonna; (2) Montecompatri; (3) 
Monte Porzio; (4) Frascati; (5) Rocca di Papa-Monte Cavo; 
(6) Albano; (7) Ariccia; (8) Nemi; and (9) Velletris To these 
districts should perhaps be added: (10) Genzano-Civitalavinia. 
=G. Mercalli, L’isola d’Ischia ed il terremoto del 28 luglio 1883 (Milan, 1884); 
H. J. Johnston-Lavis, Monograph of the Earthquakes of Ischia (1885). A summary 
of these and other memoirs on the Ischian earthquakes is given in my Study of Recent 
Earthquakes, pp. 45-73. 
2G. Agamennone, Boll. Soc. Sis. Ital., Vol. XXI (1918), pp. 47-101. 
3 Terremoti d'Italia, pp. 778-80. 
