Part 6 (1/2)

_f._ Campagnano.

_g._ Vezza.

_h._ Imperatore.

_i._ C. St. Angelo.

_j._ Lo Toppo.

_k._ Marecocco.

_l._ Zale.

_m._ Rotaro.

_n._ Montagnone.

_p._ Bagno.

_q._ Tabor.

_r._ P. Castiglione.

_s._ Cremate.

_t._ Arso.

_u._ Porto d'Ischia.

[22] It is possible that Monte Campagnano may form an exception to this statement.

[23] Shocks were felt in the island in 1559 and 1659, but one at least was of external origin.

[24] Prof. Mercalli, from the five estimates of the angle of emergence which he considered most reliable, found the mean depth to be about 3,280 feet.

[25] Professor de Rossi estimated the mean duration as not much exceeding ten seconds. Dr. Johnston-Lavis, on the other hand, considers the general estimate of fifteen seconds as far too low. In one case, at Casamicciola, he ranks it as high as thirty-one seconds.

[26] Quoted from the useful translation of Covelli's memoir given by Dr. Johnston-Lavis.

[27] Baldacci supposes that the thermal springs and fumaroles of Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie along a tangential fracture starting from Forio and pa.s.sing by Casamicciola to near Punta di Castiglione. Mercalli, however, argues forcibly against this inference.

[28] Professor Mercalli adds, as a fourth point of contact between Ischian earthquakes and volcanic phenomena, the changes in the fumaroles and hot springs which preceded or accompanied or followed the earthquakes of 1828, 1881, and 1883.

[29] ”Fontana,” he says, ”occupies the centre of the great crater of Epomeo..., and therefore lies immediately over the ancient chimney, which in all probability is filled by an old plug of consolidated trachyte, which must descend to the igneous reservoir. Any ma.s.s of igneous matter, that might determine the further rupture of a collateral fissure, would result in the conduction of any changes of pressure or vibrations, along the column of highly elastic trachyte; whilst the same earth-waves would be annulled or absorbed by the inelastic tufas surrounding it, so that the blow would be struck perpendicularly to the surface, and in a small area with well defined limits. The undulatory sensations, after the princ.i.p.al local shock, were those that arrived from the great centre of impulse beneath Casamenella.”

[30] The above paragraph is a summary of the reasoning stated with admirable clearness by Dr. Johnston-Lavis. It should be mentioned that the late Professor Palmieri, relying on the extremely limited disturbed area, dissented from this view; but his difficulty is met by supposing the focus to be small as well as shallow, a supposition that is supported by the shortness of the meizoseismal band, as well as by the elongation of the isoseismal lines in the direction perpendicular to this band.

CHAPTER IV.

THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884.

In most countries the princ.i.p.al seismic districts are of limited extent. Thus, in central j.a.pan, the east coast is frequently visited by earthquakes, while the west coast is relatively undisturbed. Of the earthquakes felt in the kingdom of Greece during the years 1893-98, 63 per cent. were observed in Zante, and were for the most part confined to that island. In the interior of the Iberian peninsula--in Leon and in New and Old Castile--destructive earthquakes are practically unknown; while the littoral regions of central and southern Portugal, Andalusia, and Catalonia are noted for their disastrous shocks.

During the eighteenth century seismic activity was chiefly concentrated in Portugal, and culminated in the great Lisbon earthquake of 1755. In the following century the seat of disturbance was transferred from the west to the south of the peninsula; Portugal remained throughout in comparative repose, while Almeria experienced destructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and 1864, leading up to the Andalusian earthquakes of 1884-85, described in the present chapter.