EARTHQUAKES 



287 



The Charleston (South Carolina) earthquake affected an area 1000 

 -niles in diameter. 



Instruments for Determining and Measuring Earthquakes. — 

 Earthquake instruments or seismographs have been established in 

 many parts of the world, and from them the location and intensity 

 of earthquakes are known. For example, seismographic records will 

 be made in Germany, the United States, and elsewhere of an earth- 

 quake in Java or the West Indies. Seismographs vary widely in 

 construction, but since they all endeavor to show the direction of the 

 vibrations, the essential feature consists of three pendulums arranged 

 so as to vibrate in mutually perpendicular directions, the record being 

 made on a sheet of paper which moves at a uniform rate (Fig. 28 1). 1 



Effects of Earthquakes 



Faults and Fissures. — We have seen that earthquakes are usually 

 the result of faulting. Sometimes the fault rift extends to the surface 

 as an open fissure, but more often the fissure is closed. When deep 



alluvial soil is shaken, many 



cracks are often formed, as __— — r — - ^1 ^ 



a result of the compacting 

 of the loose material and of 

 its slumping. Such fissures 

 are especially likely to form 

 in stream valleys parallel to 

 their course (Fig. 285), since 

 the alluvium is unsupported 

 on the stream side and 

 moves in that direction, if 

 at all. As a result of such 

 slumping cracks are formed 

 and valleys are narrowed. 

 Fissures formed during the 

 Mississippi Valley earth- 

 quake of 1811-1812 are still 

 visible. One such fissure 

 diverted the course of the 

 Mississippi River so that an 

 oxbow (p. 121) was cut off. 



Fig. 285. — Diagrams showing the effect of 

 earthquake shocks upon loose material. The 

 bridge girder has remained in place, but the 

 piers have moved inward at the bottom. 



1 For a more complete description of seismographs see : Hobbs, Earthquakes, pp. 257-275. 

 CLELAND GEOL. — 1 9 



