DISTRIBUTION OF EARTHQUAKES IN TIME. 245 



quakes in Japan, remarks that' if periods of seismic 

 activity do not occur every 490 years, there is a repetition 

 of the cycle after 980 years, but there is much variability. 

 A period of 68 years is very marked. On the average, 

 large earthquakes have occurred every 5*9 years. Fuchs 

 gives some interesting examples of the repetition of 

 earthquakes at definite intervals, of which the following 

 are examples. Sometimes earthquakes appear to have 

 repeated themselves after 100 years. One remarkable 

 example of this is that of Lima, on June 17, 1578, 

 which was repeated on the same day in the year 1678. 

 In Copiapo it is believed that earthquakes occur every 

 twenty-three years, and examples of such repetitions are 

 found in the years 1773, 1796, and 1819. In Canada, 

 near to Quebec, earthquakes lasting forty days are said 

 to occur every twenty-five years. The plateau of Ardebil 

 is said to be regularly shaken by earthquakes every two 

 years. 



A. Caldcleugh, writing on the earthquake of Chili, in 

 1835,^ remarks that the Spaniards first had the idea that 

 a great earthquake occurs every century. Afterwards they 

 thought the period was every fifty years. As a matter of 

 fact, however, there were large earthquakes in 1812, at 

 Caraccas ; in 1818, at Copiapo; in 1822, at Santiago; in 

 1827, at Bogota ; in 1828, at Lima ; in 1829, at Santiago; 

 and in 1832, at Huasco. 



The average period of seismic disturbances in any 

 country probably depends upon the subterranean volcanic 

 activity of that country. "When the activity is great the 

 large earthquakes may occur at short intervals ; but when 

 the activity is small, as in England, shocks of moderate 

 intensity may not be felt more than once or twice per 

 century. A general idea of the relative frequency of the 

 » Phil. Tram. vol. i. 1836. 



