NATURE OF THE SHOCK.
In the meizoseismal band, preliminary tremor and rumbling sound were alike absent. So sudden, indeed, was the onset of the earthquake, that the survivors generally found themselves beneath the ruins of their houses before they were conscious of any shock. The destruction, practically instantaneous, was wrought by four or five vertical blows, so powerful that, according to some observers, Casamicciola seemed to jump into the air. Then followed undulations, not noticed by all, that appeared to come from every direction. The shock lasted altogether fifteen seconds or more,[25] and was accompanied by a rumbling noise, in the midst of which were detonations as of thunder or of great blows given upon an empty barrel.
In the immediate neighbourhood of the meizoseismal area, at Perrone, Pennella, and Lower Lacco, the subsultory movement was still the more prominent; but, farther away, as at Panza, Testacchio, Barano, Ischia, and Bagno, the subsultory motion was followed by distinctly horizontal undulations, while outside the island of Ischia only slow undulatory movements were perceptible.
LANDSLIPS.
The dotted areas in Fig. 16 indicate the sites of the only landslips of importance that were precipitated by the earthquake of 1883. Two of these occurred on the north slope of Epomeo, and the third on the west flank of Monte Rotaro. The materials of the Epomean landslips had evidently been separated for some time by shallow fissures from the adjoining rock, for the surfaces of the fissures were discoloured by fumarolic action. Immediately after the earthquake a cloud of dust was seen to rise from the spots; the masses, already detached laterally, were merely set in motion by the shock; and they continued to slide down during the following days either through the action of the after-shocks or of the heavy rains that followed.
All over the island, however, fissures and minor landslips occurred. At two places on the north coast the steep cliffs of incoherent tufa were so much damaged that, according to Dr. Johnston-Lavis, "large quantities of their materials were thrown into the sea. The water then sorted out the pieces of pumice, which in many cases were of very large size, and were seen floating about in the neighbourhood for some days," giving rise to the supposition that a submarine eruption had taken place to the north of the island.
AFTER-SHOCKS.
The after-shocks in 1883 were much more numerous than in 1881. Between 9.25 P.M. on July 28th and noon on August 3rd, twenty-one slight shocks were recorded at Casamicciola. At 2.15 P.M. on August 3rd, a violent shock occurred that caused further damage at Forio, and even at places so far from the epicentre as Fiaiano, Barano, and Fontana, and increased the displacements of the landslips on Epomeo. This shock was also registered at the observatory on Vesuvius.
After this the shocks became less frequent and slighter, twelve being felt at Casamicciola during the remainder of the year, and six in the first half of 1884. Several shocks and rumbling noises were also observed in other parts of the island. Among them may be mentioned noises heard at Fontana on August 12th and 15th, and a slight shock at the same place on August 17th; also on September 4th, at 10.30 and 10.40 A.M., slight shocks at Barano, Serrara, and Forio. On March 27th, 1884, at 2.7 P.M., another strong shock occurred; strongest at Serrara, where the shock was subsultory and accompanied by noise; and less strong, though still subsultory, at Ciglio, Panza, Forio, Fiaiano, and Casamicciola, and very slight at Ischia. The series seems to have ended during the following summer, with a slight shock at Casamicciola on July 21st, and a stronger one on July 23rd, felt from Casamicciola on the north to Serrara on the south.