EARTHQUAKE, the term applied to any tremor or shaking of the solid crust of the earth. The frequent occurrence of earthquakes, their destruction to life and property, their influence upon the solid surface of the earth, and the mysteriousness of their cause, force them upon our attention. It is estimated that 12 or 13 earthquakes, destruc tive more or less of life and property, occur every year, and it is well known that the surface of the globe is never free from sensible evidence of the continued operation of E. agency that in some quarter or another tremors or slight shakings are always taking place. When these are of a serious nature, whole cities have been destroyed; fertile districts, with all their fruits and grain, have been laid waste; and enormous masses of human beings have lost their lives. No less than 60,000 perished in the great Lis bon E.; while in that of Calabria, in the end of last century, 40,000 were destroyed. It is estimated that as many as 13,000,000 of the human race have thus perished' The great changes which earthquakes produce on the earth's surface deserve the careful attention of the geologist. They disclose to him an agency which seems to have been at work during every period of the earth's history, and which has altered the earth's surface to an extent that can scarcely be imagined. The observed results of earth quakes which more immediately demand his attention are such as these: the new lakes and river-courses which they form, at the same time obliterating the old ones; the new valleys which they hollow out; the fissures of various sizes they form; and the immense landslips they frequently produce. But the mysterious nature of the pro ducing cause of earthquakes is also a strong incentive to their study. It is unfor tunately true, that the most popular scientific inquiries are those in which' the tion has large play: dry inductions from observed phenomena are not suited to the genius of popular modern science. Consequently, earthquakes, where every attempt at explaining their origin is theoretical. from the impossibility of obtaining direct obser vation, affording as they do a wide field for the play of the poetic faculty, find plenty of students.
No portion of the earth's surface is exempt from the influence of earthquakes. Egypt has been less visited than perhaps any other country, but even here we have the record of one which took place in 1740 A.D. ; and Holland, with its loose alluvial deposits, has also felt their power. Nor is the bed of the ocean exempt; records of many sub aqueous earthquakes exist, taken by vessels at sea, sometimes passing over the point of greatest disturbance at the moment of the shock. In like manner earthquakes have been active at every period of the earth's existence, breaking up its solid crust, elevat ing or depressing its surface, and as much as any other single agent to bring it into its present condition. They have been probably at some periods more active than at others, just as we find that some districts are now more liable than others to their visitation. So well defined, indeed, are the localities where earthquakes occur, that it
is easy to exhibit their limits on a map. They are most frequent around the present lines or centers of volcanic action (see VOLCANO); and their frequency and violence seem to bear some relation to the activity and intensity of the associated volcanoes. Observers of volcanic phenomena have noticed that every great eruption, in whatever part of the world observed, and whether from a volcanic vent on land or beneath the ocean, is accompanied by E. shocks of greater or less violence and duration; while, on the other hand, those observing earthquakes speak of them as accompanied by volcanic eruptions, and of their often being stopped on the opening of volcanic vents. It is, however, an important fact that, although regions of active volcanic action are those of most frequent E. movements, yet the most violent earthquakes do not appear to have occurred in these regions, but, on the contrary, in districts lying some degrees away from the nearest volcanic action, as, for instance, in the famous E. of Lisbon. Districts in which there are extinct volcanoes are not more liable to such visitations than non-volcanic regions. • The phenomena connected with earthquakes have been variously described. Many writers refer to appearances in the heavens, or changes in the atmosphere, which to them seem to have some connection with the catastrophes they narrate. They tell of irregularities in the season preceding or following the shock, of sudden gusts of wind interrupted by sudden calms, of violent rains at unusual seasons, or in countries where such phenomena are almost 'unknown, of a reddening of the sun's disk, of a haziness in the air often continued for months, and similar phenomena. But these are so irregu lar in their appearance, and have been so seldom observed associated with more than a single E., that; in the absence of any decided reason to the contrary, there seem good grounds for believing they have no real connection with the earthquake. It is differ ent with underground noises, which frequently precede, accompany, or succeed the occurrence of earthquakes, or some of the shocks of them. They are undoubtedly inti mately donnected with the shock, yet earthquakes occur, even of the greatest violence, which are unaccompanied by any sound whatever. Different descriptions have been given of these subterranean noises. In some earthquakes, they are likened to chains pulled about, increasing to thunder; in others, the, sound is like the rumbling of car riages, growing gradually louder, until it equals the loudest artillery; or like heavy wagons running away upon a road; or distant thunder; or like the hissing produced by the quenching of masses of red-hot iron in water; or like the rush of wind under ground. As there have been earthquakes without subterranean noises, so there are fre quently, in South America and elsewhere, underground sounds which are not followed by earthquakes.