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 undei stood, 



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 the surface 

 of the earth were thrown violently upward ; bodies of men were hurled 



