1909.] THEIR CAUSES AND EFFECTS. 245 



vary indefinitely. The Charleston, San Francisco, Kingston and 

 many other quakes lasted only from thirty to forty seconds. Milne 

 states that the average duration of 250 earthquakes of moderate 

 intensity recorded by instruments in Tokyo between 1885 and 1891 

 was 118 seconds. The first shocks are almost always succeeded by 

 after shocks which may continue for weeks, months or even years. 



It has not been possible yet to determine the periodicity of 

 shocks or to predict with any degree of accuracy the time of the 

 occurrence of an earthquake. Some earthquake regions are subject 

 to frequent shocks, while others experience them only at long 

 intervals. The frequency of earthquakes, considering those of all 

 amplitudes, is not generally realized. The globe, indeed, may be 

 said hardly ever to be free from seismic disturbances of some kind 

 somewhere, since the average of all recorded shocks, according to 

 de Montessus de Ballore, is more than fifteen per day, and there are 

 between fifty and sixty heavy shocks per year. The bare enumer- 

 ation by this author of those occurring in 1903 alone fills a book of 

 six hundred tabulated pages, and he has compiled the data and 

 plotted the position of 159,781 earthquakes that have been recorded 

 up to the end of 1903. 



At the same time that important quakes are the result of tecton.c 

 movements in the earth's crust, they may themselves be the causes 

 of more or less important changes in the surface of the earth. 

 Sharp waves passing through mountain regions have been known to 

 produce land sHdes, shatter rocks, displace larger or smaller seg- 

 ments of clififs, open fissures in the soil or cause subsidence in: 

 alluvial regions. Springs, brooks, rivers and lakes have been 

 formed, altered or obliterated as a result of earthquake action. 

 Great earthquakes have usually produced important sea waves caus- 

 ing much destruction along the coast and, sometimes, permanent 

 changes due to erosion and transportation of material. 



Several scales for the purpose of indicating the severity of an 

 earthquake shock have been proposed. The one most commonly 

 employed is known as the Rossi-Forel scale, which distinguishes 

 ten degrees of intensity according to the effects produced upon 

 human observers and structures. Another widely used scale is that 

 which has been devised by Professor G. Mercalli. This likewise 



