Earthquakes and other earth movements

by John Milne

Fig. 28.—Record of Tide Gauge at Port Point, San Francisco. Showing Earthquake Waves of May 1877.

The sea wave of the Iquique earthquake of May 9, 1877, like many of its predecessors, was felt across the basin of the whole Pacific, from New Zealand in the south, to Japan and Kamschatka in the north. And but for the intervention of the Eurasian and American continents would have made itself appreciable over the surface of the whole of our globe. At places on the South American coast, it has been stated that the height of the waves varied from twenty to eighty feet. At the Samoa Islands the heights varied from six to twelve feet. In New Zealand the sea rose and fell from three to twenty feet. In Australia the heights to which the water oscillated were similar to those observed in New Zealand. In Japan it rose and fell from five to ten feet. In this latter country, the phenomena of sea waves which follow a destructive earthquake on the South American coast are so well known that old residents have written to the local papers announcing the probability of such occurrences having taken place some twenty-five hours previously in South America. In this way news of great calamities has been anticipated, details of which only arrived some weeks subsequently. Just as the destructive earthquakes of South America have announced themselves in Japan, in a like manner the destructive earthquakes of Japan have announced themselves upon the tide gauges of California. Similarly, but not so frequently, disturbances shake the other oceans of the world.

For example, the great earthquake of Lisbon propagated waves to the coasts of America, taking on their journey nine and a half hours.

Sea waves without earthquakes.—Sometimes we get great sea waves like abnormal tides occurring without any account of contemporaneous earthquakes. Although earthquakes have not been recorded, these ill understood phenomena are usually attributed to such movements.

Several examples of these are given by Mallet. Thus, at 10 a.m. on March 2, 1856, the sea rose and fell for a considerable distance at many places on the coast of Yorkshire. At Whitby, the tide was described ebbing and flowing six times per hour, and this to such a distance that a vessel entering the harbour was alternately afloat and aground.

In 1761, on July 17, a similar phenomena was observed at the same place.

A like occurrence took place at Kilmore, in the county of Wexford, on September 16, 1864, when the water ebbed and flowed seven times in the course of two hours and a half. These tides, which appear to have taken about five minutes to rise and five minutes to fall, were seen by an observer approaching from the west as six distinct ridges of water. The general character of the phenomena appears to have been very similar to that which was produced at the same place by the Lisbon earthquake of 1755; and the opinion of those who saw and wrote about their occurrence was that it was due to an earthquake disturbance. Such phenomena are not uncommon on the Wexford coast, where they are popularly known as ‘death waves,’ probably in consequence of the lives which have been lost by these sudden inundations.

They have also been observed in other parts of Ireland, the north-east coast of England, and in many parts of the globe. They will be again referred to under the head of earth pulsations.

Cause of sea waves.—Mallet, who in his report to the British Association in 1858, writes upon this last-mentioned occurrence at considerable length, whilst admitting that many may have originated from earthquakes, he thinks it scarcely probable that an earthquake blow, sufficiently powerful to have produced waves like those observed at Kilmore, should not have been felt generally throughout the south of Ireland. He, therefore, suggests that sometimes waves like the above might be produced by an underwater slippage of the material forming the face of a submarine bank, the slope of which by degradation and deposition, produced by currents, had reached an angle beyond the limits of repose of the material of which it was formed. Mallet does not insist upon the existence of these submarine landslips, but only suggests their existence as a means of explaining certain abnormal sea waves which do not appear to have been accompanied by earthquakes.