There are three kinds of waves produced by earthquakes; namely, the earthquake waves proper through the earth; the sound waves in the air, and great forced waves in the sea.
The sound waves of course reach the air from the point of origin below the earth's surface through the solid materials of the crust, and take on the curious varieties already described in connection with the sounds accompanying earthquakes.
We have already briefly described the manner in which the earthquake waves travel through the materials of the earth's crust. There remain to be discussed the great waves that are rolled up in the ocean during an earthquake shock. These waves are, perhaps, among the most destructive phenomena of great earthquakes. The following are only some of the more remarkable of such waves, and have been taken from Mallet's collection of earthquake data.
During some of the great earthquakes on the coasts of Chile and Peru, huge waves from the ocean did great damage when they reached the land. In the earthquake of 1590, ocean waves rushed for several leagues inland over the coast of Chile, carrying with them ships that were left high and dry as the wave receded. In the earthquake of 1687, Callao was inundated by a great wave from the Pacific Ocean, and ships were carried a full league into the country. During the earthquake of 1746, Callao was again swept away by a huge ocean wave. At later times earthquake waves have caused great damage to several other parts of the coast of South America.
Ocean waves of this character are formed by successive upward and downward movements at the bottom of the ocean, following each other at very brief intervals. Le Conte points out that the sudden upheaval of the bed of the ocean forms a huge mound in the surface of the water which results in a large wave that spreads rapidly in all directions. Waves produced in this manner sometimes reach a height of fifty to sixty feet. They are not readily observed in the deep ocean, but as soon as they reach the shallow waters near the shore they rush forward, forming waves from fifty to sixty feet in height and, rushing over the land, sweep everything before them.
During the great Lisbon earthquake of 1755 a huge wave started at a point fifty miles off the coast of Portugal. Half an hour after the earthquake was over several waves, the largest of which was sixty feet in height, rushed over a part of the city and greatly increased the ruin already wrought by the earthquake. According to Le Conte the great waves so formed moved in all directions across the Atlantic Ocean. They were thirty feet high when they reached Cadiz, eighteen feet in height at Madeira, and five feet on the coast of Ireland. They even crossed the Atlantic, being observed on the coasts of the West Indies.
A great ocean wave accompanied the Japanese earthquake in 1854. As in the case of the Lisbon earthquake this wave started in the bed of the ocean off the coast of Japan and only reached the island half an hour afterwards. It was thirty feet in height, and completely swept away the town of Simoda.
Owing to water's greater freedom of motion earthquake waves travel greater distances through the water than they do on land.
Of course, great earthquake shocks as a rule cause a very large loss of life. The following figures from Mallet give some idea of the extent of this loss, which is generally a matter of a few moments.
In the Lisbon earthquake, where the worst shock lasted a few seconds, 60,000 people were killed. During other earthquakes the losses have been as follows: 10,000 at Morocco; 40,000 in Calabria; 50,000 in Syria, and probably 120,000 in earthquakes that occurred in Syria in a. d. 19 and in a. d. 526.