FORESHOCKS AND AFTERSHOCKS. 357 



they are numerous and of an order that would entitle many of them 

 by themselves to be ranked as violent earthquakes. They may conti- 

 nue for days, weeks and months after the parent shock ; whilst, in the 

 case of earthquakes of the magnitude of the Kangra earthquake, they 

 persist for years — often with recurring periods of maxima and minima. 

 Very frequently they affect various parts of the epicentral area at diff- 

 erent times, as if one portion after another of the originally disturbed 

 crust felt the need of further relief. Frequently also a more than 

 usually violent aftershock, originating at one locus of the first-affected 

 area, is succeeded by a set of aftershocks that are evidently secondary in 

 origin, and belong more properly to that aftershock than to the* first 

 grand seismic disturbance. 



Most of the above points are clearly illustrated in the list of after- 

 shocks that followed the earthquake of 4th April 

 imperfections of! S 1905. In that list, in spite of its size, the prevail- 

 ing character is its imperfections. It is imperfect 

 for many obvious reasons and along many lines, but principally for 

 the reason that, with the exception of the larger aftershocks that were 

 noticed in the newspapers, no record was commonly kept or submitted 

 by observers when once the earthquake-forms had been filled in and 

 finally despatched to the Geological Survey Office. The dates when 

 this was done varied considerably according to circumstances, and as a 

 consequence the records break off suddenly and irregularly here and 

 there over the enormous area reported on, without there having been 

 any corresponding real cessation of the shocks. 



There were, however, a few other sources from which the list was 



Other sources from com P^ e <i- Such were the seismoscopes and seismo- 



which the list has graphs in ordinary use in the meteorological obser- 



een comp ed. vatories, 1 the newspapers, and letters from many 



private persons who interested themselves in the matter. A large 



number of aftershocks were also collected by Lady Holland at **Kenil- 



1 Early manuscript copies of the monthly list of earthquake shocks at Simla were sup- 

 plied me by the courtesy of Dr. Gilbert T. Walker, M.A., F.R.S., Meteorological Reporter 

 fcu Government of India and Director-General of Indian Observatories. 



