246 EARTHQUAKES. 



large earthquakes in various parts of the world may be 

 easily obtained by an inspection of the table on page 240. 



Between the years 1850 and 1857 Kluge found that 

 in the world there had been 4,620 earthquakes, which is, 

 upon the average, nearly two per day. This estimate of 

 the frequency of earthquakes of sufficient intensity to 

 be recorded without the aid of instruments is, however, 

 much below the truth. In Japan alone there probably 

 occurs, as a daily average, a number at least equal to that 

 which has been just given for the whole world. Boussin- 

 gault considered that, in the Andes, earthquakes were 

 occurring every instant of time.^ 



To state definitely how many earthquakes are felt in 

 the world on the average every day is, from the data 

 which we have at our command, an impossibility. Per- 

 haps there may be ten, perhaps there may be 100. The 

 question is one which remains to be decided by statistics 

 which have yet to be compiled. 



After a large earthquake, smaller shocks usually occur 

 at short intervals. At first the succession of disturbances 

 are separated from each other by perhaps only a few 

 minutes or hours. Later on, the intensity of these shocks 

 usually decreases, and the intervals between them become 

 greater and greater, until, finally, after perhaps a few 

 months, the seismic activity of the area assumes a qui- 

 escent state. 



The great earthquake which overtook Concepcion on 

 February 20, 1835, was followed by a succession of 

 shocks like those just referred to, there being registered, 

 between the date of the destructive shock and March 4, 

 300 smaller disturbances. 



During the twenty-four hours succeeding the des- 

 truction of Lima (October 28, 1746), 200 shocks were 



^ Am Jour, of Sci. vol. xxxvii. p. 1. 



