[CHAPTER XXXII]
SEAQUAKES
As earthquakes are shakings of the earth's crust in places where it is uncovered by the waters of the ocean, so seaquakes are the shakings of those portions that lie on the bed of the ocean.
Mallet points out that the earthquake wave may start either in the interior of the continent, or on the bed of the ocean; that the latter place is the more common, since on the land vents—rude safety-valves, as it were,—are provided by the craters of the volcanoes; that, when earthquakes start on the ocean bed, the impulses are conveyed in different forms of waves, i. e., those through the solid earth, those through the water, and those through the air, with varying sounds like bellowings and explosions, or like the rolling of wagons over rough roads.
To learn when quakes occur on the sea is a much harder task, since on the land we can use seismoscopes, seismographs, or seismometers to indicate, record, or measure the shakings of the crust, while on the sea, where the water is always in more or less motion and the surface so far from the ocean's bed this is impossible, or, rather shall it be said, has hitherto been found so; for that the mind of man may surmount this obstacle is not impossible to conceive.
To detect the wave produced by the quaking of the bed of the ocean is exceedingly difficult, since those in very deep water are flat or possess but a small height. Indeed, in the deepest parts of the ocean this height is probably to be measured only by inches instead of feet. When, however, the waves advance towards the shore they increase in height, and when they reach the shallows near the coast, they begin to curl over and break, thus creating the enormous waves mentioned so often as attending great earthquakes in the ocean.
During the great earthquake of Simoda in Japan, 1854, the waters of the bay were first greatly agitated, and then retreated, leaving the bottom bare in places where the water was formerly thirty feet deep. A wave, thirty feet high, then rushed in from the bay and, climbing the land, swept away everything in its path, covering the town with water to the tops of the houses. This monster wave then receded, but rushed back five times.
In 1751, an earthquake wave suddenly entered Callao, the port of Lima, Peru, sinking twenty-three vessels and driving a frigate inland, where it was left high and dry. This wave extended across the Pacific to the Hawaiian Islands, a distance of 6,000 miles.
On the 13th of August, 1866, an earthquake wave, that started a short distance from shore, produced a number of earthquake waves sixty feet high that reached the coast of Peru half an hour after the principal earthquake shock. These waves reached Coquimbo, 800 miles distant, in about three hours, and Honolulu, on the Sandwich Islands, 5,520 miles distant, in twelve hours, and the coast of Japan, more than 10,000 miles distant, on the next day. Le Conte remarks that these waves would have encircled the earth, had it not been for the barrier interposed by the Andes.