. Elements of geology : a text-book for colleges and for the general reader. Geology. EARTHQUAKES. 117 thus far spoken of the earth-wave as a simple spherical wave of longi- tudinal vibration like a sound-wave, but the seismograph shows that there are transverse as well as longitudinal vibrations. Thus, the earth- movement is very complex and produced by the superposition of several waves of different kinds. But the wave which we have been discussing is the dominant one and the one whose origin is most easily undeistood,. Fig. 99—Model showing the Path of a Particle during an Earthquake (after
. Elements of geology : a text-book for colleges and for the general reader. Geology. EARTHQUAKES. 117 thus far spoken of the earth-wave as a simple spherical wave of longi- tudinal vibration like a sound-wave, but the seismograph shows that there are transverse as well as longitudinal vibrations. Thus, the earth- movement is very complex and produced by the superposition of several waves of different kinds. But the wave which we have been discussing is the dominant one and the one whose origin is most easily undeistood,. Fig. 99—Model showing the Path of a Particle during an Earthquake (after Sekiya). and is therefore called the normal wave. Fig. 99 is a wire model rep- resenting the actual motion of a point during an earthquake. It was made by combining the records of the three pendulums of Ewing's seismograph (Fig. 108). Explanation of Earthquake-Phenomena.—Earthquakes have been divided into three kinds, viz., the explosive, the horizontally progres- sive, and the vorticose. The first kind is described by Humboldt as a violent motion directly upward, by which the earth-crust is broken up, and bodies on the surface are thrown high in the air. The shock is extremely violent, but does not extend very far. In the second, the shock spreads on the surface like the waves on water to a great dis- tance. In the third there is a whirling motion of the earth entirely different from ordinary wave-motion. These three kinds are sometimes supposed to be essentially distinct, and possibly produced by different causes; but we will attempt to show that the difference is wholly due to the different conditions under which the waves emerge on the sur- face. The three kinds are, in fact, often united in the same earth- quake. The most remarkable example of explosive earthquake is that which destroyed Eiobamba in 1797. In this dreadful earthquake the shock came suddenly, like the explosion of a mine. Not only was the earth broken up and rent in various places, but objects lying on
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