EARTHQUAKES 



285 



Fig. 283. — Pier driven into 

 the ground by an earthquake 

 shock. 



trated in some earthquakes. In one case (Calabria, Italy), the stone- 

 work of a well was thrown out of the ground, and in its new position 

 resembled a small tower. In an Icelandic earthquake in 1896 persons 

 lying on the ground near a cliff were thrown over the edge. More 

 commonly stones are thrown into the air and overturned (Assam, 

 India). Sometimes heavy objects such as gravestones are embedded 

 more deeply in the ground (Fig. 283). The reason for this can be 

 shown by another simple experiment. If 

 a ball of soft clay upon which a pebble 

 rests is subjected to a sudden upward 

 movement, the pebble will be embedded, 

 to some extent, in the clay. 



The amplitude of the vibration of a 

 rock particle should be distinguished from 

 the earth waves which are produced in 

 loose alluvium. For example, during the 



earthquake which shook the Mississippi Valley in 181 1, and which 

 was probably the most violent that has taken place in North 

 America since its settlement by Europeans (although not the most 

 destructive because of the sparseness of the population and the 

 character of the buildings), the ground is described as having been 

 thrown into great waves, so that the branches of the trees inter- 

 locked as the waves passed under them. In this case, the ampli- 

 tude of the vibrations of the rock upon which the thick alluvial soil 

 rested probably did not exceed a few centimeters. 



Vorticose and Twisting Movements. — After earthquakes, pictures 

 are often found with their faces toward the wall, furniture has been 



turned partly or completely 

 around, statues have been 

 twisted on their pedestals 

 and chimneys have been 

 partly turned about. No 

 one cause can be assigned to 

 such movements. In many 

 cases the turning was due to 

 a simple motion backward and forward ; in others the rotation prob- 

 ably resulted from " a combination of shocks from separate faults. ,, 

 (Hobbs.) The latter is given as the cause of the turning of a bronze 

 angel in Belluno, Italy, through an angle of 20 , and the rotation of 

 the statue of Queen Victoria in Kingston, Jamaica. 



Fig. 284. — Diagram showing a railroad track 

 bent during the Charleston earthquake. 



