The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

"But Democritus says, that the earth being full of water, and receiving much also by means of rain, is moved by this. For when the water increases in bulk, because the cavities cannot contain it, in its struggles it causes an earthquake. And when the earth becomes partially dried up, the water being drawn from the full reservoirs into those which are empty, in passing from one to the other, by its movements it causes an earthquake also.

"Anaximenes, however, says that the earth, when parched up and again moistened, cracks, and by the masses thus broken off falling on it, is shaken; wherefore earthquakes occur in drouths and again in times of rain; in drouths, because, as we have said, it cracks, when highly dried, and then, when moistened over again, it cracks and falls to pieces. Were this the case, however, the earth ought to appear in many places subsiding. Why then is it that hitherto many places have been very subject to these convulsions which do not present any such remarkable differences from others? Yet such ought to be the case. And, moreover, those who think thus must assert that earthquakes constantly become less and less, and at last cease altogether. For the continual condensation of the earth would cause this. Wherefore, if this be not the fact, it is plain that this is not the correct explanation."

Besides the above, there are numerous references to earthquakes in the works of other writers. Thales, Seneca, and Pliny all speak of these phenomena and appear to describe correctly the movement of the earth in waves both in the solid land, as well as on the sea.

Coming down to less ancient writers, Mallet refers to a book by Fromondi, published in Antwerp, in 1527, that contains much valuable and interesting information. Among other things Fromondi declares that in the year 369, in the reign of Valentinian, there was a great earthquake that shook nearly the entire world and that another earthquake of almost equal severity occurred in 1116. He also states that in 1601 an earthquake continued for nearly forty days; that a great earthquake in Italy, in 1538, lasted fifteen days, and that another, in Spain, lasted for nearly three years.

This does not mean that these earthquakes actually continued to shake the earth violently for the times mentioned. These are only the times during which, at intervals of greater or less length, successive shocks were felt in these localities.

Another of the less ancient writers referred to by Mallet is Travagini, who published a book in Venice in 1683. This book contains a description of a terrible earthquake occurring in Italy on the 6th of April, 1667, which affected large portions of the country adjacent to Ragusa.

Without attempting at present to discuss the various theories of earthquakes, it will suffice to say that earthquakes can be divided, according to their origin, into two classes: volcanic earthquakes, or earthquakes that are caused by practically the same forces that cause volcanoes, and tectonic[3] earthquakes, or those produced by the slipping of a large mass of rock lying along the lines of old or new fractures.

Earthquakes of the first class are found especially in volcanic districts, while those of the second class are found in all parts of the world, whether in volcanic districts or elsewhere. According to Dana, earthquakes of the second class generally start in the neighborhood of mountains, where old lines of fractures are especially abundant.

As regards the direction of the shaking movements of the earth, earthquakes can be divided into three different classes: explosive earthquakes, or those in which the force acts vertically upwards; horizontal earthquakes, or those in which the force moves in a more or less horizontal direction, or parallel to the general surface of the earth, and rotary earthquakes, or those in which the earth rotates or moves in great eddies or whirls.

When the earthquake wave is started below the earth's surface, it spreads through the crust in all directions. The direction these waves will have on emerging, or coming out of the surface, will depend on the distance of this point from the place the waves started. When a place is situated directly over where the wave started, the waves will emerge so as to move vertically upwards, so that the earth at this point will be shaken by an explosive earthquake. As the point where the waves pass out is situated further and further from the place where the waves start, the waves will emerge more nearly horizontally, the greater the distance from the source.