The phenomena of earthquakes differ greatly in accordance with the number, duration, and intensity of the shocks, and with the distance of the place of observation from that of the origin of the disturbance. One of the greatest of modern earthquakes is that of northern India of 1897, which is well summed up in the official report.

"On the afternoon of June 12,1897, there burst upon the western portion of Assam an earthquake which, for violence and extent, has not been surpassed by any of which we have historic record. Lasting about .two and one-half minutes, it had not ceased at Shillong before an area of 150,000 square miles had been laid in Tuins, all means of communication interrupted, the hills rent and cast down in landslips, and the plains fissured and riddled with vents, from which sand and water poured out in most astounding quantities; and ten minutes had not elapsed from the time when Shillong was laid in ruins before about one and three-quarter million square miles had felt a shock, which was everywhere recognized as one quite out of the common." (R. D. Oldham).

Fig. 6. - Earthquake fissure in limestone, Arizona. (U. S. G. S).

A great earthquake usually begins suddenly and without warning. A rumbling sound, quickly becoming a loud roar, accompanies or slightly precedes the movement of the ground, which is at first a trembling, then a shaking, and finally a rapid swaying, wriggling motion, describing a figure 8, which is extremely destructive and overthrows the buildings affected, and even in the open country it is impossible to keep one's feet. The surface of the ground has been repeatedly observed to rise in low, very swiftly moving waves, somewhat like those on the surface of water, upon the crests of which the soil opens in cracks, closing again in the wave-troughs. When the earth-waves traverse a forested region, the trees sway violently from side to side, like a field of ripe grain in the breeze. In the details of movement earthquakes differ greatly from one another; sudden and extremely violent vertical shocks may come from below, or the surface may writhe and twist in every direction, instead of rolling in waves; there may be only a single shock, or many successive ones.

Fig. 8. - Fence broken and shifted horizontally 15 feet, San Francisco earthquake of 1906. but the time of formation of these enormous dislocations is not definitely known.

Violent earthquakes, which affect extensive areas, are almost always followed by a succession of after-shocks, which may continue for weeks, months, or even years. These may be very violent, though never equalling the primary shock in this respect, but gradually die away, until the region once more comes to rest.

In the sea the elastic waves producing shock soon die away in the water. Observations made on the several ships affected by the same quake frequently show a lineal arrangement of the disturbances. A special manifestation of earthquakes in the bed of the sea is the great sea-wave (sometimes erroneously called the tidal wave), which is a gravity wave produced by disturbances of the sea-floor or by a submarine volcanic eruption. The great sea-wave, though not strikingly displayed in the open sea, piles up on the coast into enormous breakers, which often are more terribly destructive than the earth-waves themselves.