this archipelago varies extremely in depth, having many shallows and reefs, with deep passages and basins giving a highly diversified profile and that relief which De Montessus associates with seismicity.

1

We find a similar relation between relief and seismicity in the long seismic belt which begins in Venezuela and extends into the Windward and Leeward Islands of the West Indies and through San Domingo and Jamaica. Caraccas, the "City of Earthquakes," is situated among the mountains of one of the great spurs of the Andean system, and the descent is rapid to the coast at La Guayra and thence to the depths of the Caribbean. Throughout the Windward and Leeward Islands the frequency is high, though the intensity is seldom great. The earthquake of 1867 at St. Thomas, with its great sea-wave, however, will be recalled in this connection, and also the fact that just north of this island is the deepest abyss of the Atlantic Ocean within a distance of about forty miles. San Domingo, Jamaica, the eastern part of Cuba, and Porto Rico are also frequently shaken, and their seismicity may be put into relation with the fact that the sea-bottom in the vicinity of those islands is one of the most rugged and highly diversified in its profiles of any part of the earth.

1 An interesting account of the earthquakes of Caraccas is given by Mr. H. D. Warner in the Atlantic Monthly of March, 1883, in an article entitled “A City of Earthquakes."

CHAPTER XVI

SEAQUAKES

The Sea has its Quakes as Well as the Land-Difficulty of Securing WellObserved Data-Dr. Rudolph's Investigations of Seaquakes-Description of the Quake on the Water-Varying Intensity of Seaquakes -Sounds Heard in Seaquakes-Explanation of Tremors in the Water, which can Transmit only Normal Waves-Energy of Sound-Waves in Water-Other Disturbances than Normal Vibrations are Rare-A Few Instances of Extraordinary Agitation of the Water Not Attributable to Normal Vibration-Submarine Seismic Regions in the Atlantic-St. Paul's Rocks-The Equatorial District-Submarine Quakes near the Azores-In the West India Deep-Sea-Waves of Seismic Origin-Those of the Peruvian-Chilian Coasts-The Great Arica Quake and Sea-Wave of August 13, 1868-Its Record on the Tide Gauges of Japan, Australia, and California-Its Speed of Propagation-Causes of Such WavesSudden Uprise and Downfall of Considerable Areas of the Ocean-Bed— Downfalls More Frequent-The Simoda Wave of December 29, 1854 -Bay of Bengal, December 31, 1881-Dr. Rudolph's Views of the Causes-Krakatoa Wave

HE sea has its quakes as well as the land. Our oppor

THE

tunities for recognising and studying the two categories are, however, widely contrasted. By means of delicately poised instruments we can detect and measure the minutest amplitudes and the slowest and gentlest periods of the landquake, while on the water the use of such instruments is impossible. The seaquake can be detected only when the water is put into a state of vibration sufficiently energetic to cause the ship and its loose objects to tremble and thus

affect the senses. The vast waves rolled in upon the shores of continents are much too flat and too slow in period to be perceptible to the senses of the traveller on shipboard. But sometimes the vibration of the water imparted to the ship is sharp enough to arouse the attention of passengers and Sometimes it is forcible enough to cause alarm or terror. In very rare cases it causes utter dismay and the feeling that destruction is at hand.

crew.

Dr. Emil Rudolph has devoted much time and labour to the research of reports from ships' logs and other sources bearing upon the subject of quakes observed at sea.' From the very large number of reports he has brought together we may select a few which will illustrate the kind of sensations experienced by the officers and crews of vessels during a seaquake.

Captain Gales, of the ship Florence Nightingale, reports

that

"on January 25, 1859, while in N. Lat. 0° 48' and W. Long. 29° 16', St. Paul's Rocks being about ten miles N. W. by N. of us, we felt a strong shock of an earthquake. It began with a rumbling sound like distant thunder and lasted about forty seconds. I was quite well acquainted with earthquakes, as I had experienced a good many on the west coast of America, but never had I felt so Glasses and dishes rattled so vigorously that I was surprised to find them uninjured. A good many objects fell down and it was as if the ship were grinding upon a reef. At once arose from all sides the cry, The ship has struck!' and the watch came tumbling up in hot haste. I was much alarmed and looked over the side of the ship in order to see the reef, but quickly formed my opinion and quieted the commotion by the

severe a one.

1 These researches are embodied in two long articles in Beiträge zur Geophysik, vols. i. and ii., and form very interesting and instructive reading.

explanation that it was only an earthquake."- Petermann's Geogr. Mittheil., xv., 1869, p. 97.

Another report from a locality not far from the above is the following: “On January 28, 1883, in N. Lat. 1° 38', W. Long. 27° 40', in clear weather and a light sea, suddenly we heard, about 7.47 P.M., a strange submarine noise not unlike distant thunder or still more like the distant firing of heavy guns. At the same time there was a vibration of the ship as though the anchor had been let go, or as if one were standing on the after-deck of a screw steamer. The entire phenomenon lasted about a minute. A peculiar sensation came upon everybody as if electrified. The crew thought there must be a large stick of timber rubbing alongside. The lookout thought that the ship had struck bottom."

The foregoing are representative of the large majority of the reports of seaquakes. The ship quivers, vibrates; loose objects chatter and tremble. There is a strange noise in the sea like distant thunder or distant artillery. The first impression is as if the ship were grinding upon the bottom, and there is an instinctive rush of the crew to the deck and the bulwarks to see if the ship is not aground or on a reef. But the situation is soon recognised. The ship is seen to move steadily onward with unchecked speed, she rises and falls to the swell of the sea without shock, the water is dark and fathomless. The tremor soon passes and the nature of the phenomenon is at length apparent.

Although the trembling of the ship and the strange roar from the sea are the most common and exclusive indications of the seaquake, there occur more forcible indications in a few instances. As might be expected there are degrees of

intensity in seaquakes just as there are in landquakes, though the means and agencies by which they are made sensible are much more limited. Among many hundreds of reports from ships at sea which Dr. Rudolph has collected. are a few which indicate intensities of a high order. Thus one master of a vessel reports: "We felt a shock so strong that the entire crew was brought to its feet at once; the wheel flew from the hand of the steersman and I myself was flung down upon the deck." He quotes Virlet d'Aoust, a French geologist, who in a paper on earthquakes states that in an earthquake experienced on the coast of Asia Minor: "Our ship was over the epicentre and was so severely shaken that at first the Admiral feared the complete destruction of the corvette." Heavy objects including cannon and their carriages were thrown up from the deck. The corvette itself seemed to be hurled upwards. The statement that heavy objects have been lifted from the deck and the vessel itself lifted as if projected upwards is by no means unique, for Dr. Rudolph has collected a considerable number of them. The exact amount of credence we ought to concede them or the precise interpretation we ought to give them is another matter. We seem to be justified in believing that in rare cases the power of the shocks may be great enough to render standing on the deck as difficult as it sometimes is on land. It may even be great enough to cause the fear that the vessel is being shaken to pieces.

The tremors imparted to the vessel from the water and the strange sounds from ocean depths are readily explained. The only form of elastic wave which a fluid medium can transmit is the normal wave. This mode of vibration the