part 3] pamIb EAbthquakE or febbuary 1911. 241 



miles, and those of the area included by the VII1° isoseist being 

 about 40, the ratio between the two is about 6 to 1. The corre- 

 sponding ratio in the case of other destructive earthquakes works 

 out at various values, between the extremes of 12 to 1 and 3 to 1, 

 the usual value being about 5 or 6 to 1. In this respect, therefore, 

 the earthquake shows no abnormality, and there is nothing to 

 suggest a radical difference in origin from other earthquakes. 



Another feature which marks this earthquake as of the usual 

 type and origin, is the occurrence of aftershocks. From the 

 central region we have no certain records, but, both at the Pamir 

 Post and Iskashim, subsequent shocks are definitely reported. 

 Great earthquakes vary extremely in the number of aftershocks 

 and the duration of the period covered by them ; in some cases 

 the aftershocks are few and soon over, in others they are numerous 

 and prolonged, and there is no definite relation between the magni- 

 tude of the earthquake and the number of aftershocks. In a 

 general way, however, the greater earthquakes are followed by 

 more numerous aftershocks than the smaller, and the Pamir one 

 of February 1911, though it must be classed with the great world- 

 shaking earthquakes, was, in reality, a small one of its class. To 

 this must be added that the nature of the country and its in- 

 habitants precludes the possibility of anything like a complete 

 record being obtained. The published reports are sufficient to 

 show that there were at least some aftershocks, and that in this 

 respect the earthquake was of normal type. 



So far nothing has been said of the landslips, these having been 

 reserved for separate consideration. Of the largest of them we 

 have fairly full particulars, in the description and survey made by 

 Col. Spilko's expedition. This slip fell from the mountains north 

 of the Murghab valley, just above the village of Usoi (Usaid of 

 the Indian Survey maps), which was overwhelmed and all the 

 inhabitants destroyed, except two, who were away on the night of 

 the earthquake. The debris of the slip formed a heap in the 

 valley, measuring about 19,000 feet in length along the bed of 

 the valley, about 12,500 feet in width across the valley, and very 

 little short of 2500 feet in maximum depth, the total bulk 

 being about 100,000,000,000 cubic feet, and the weight about 

 7.500,000,000 tons. These figures are necessarily approximate, as 

 it is not possible to determine, from the map, the exact limit of 

 the slip, nor is the original contour of the ground known. The 

 barrier, formed hj the slip, gave rise to a lake which, at the time 

 of Col. Spilko's survey, had attained a length of 26 versts (about 

 17 miles) and a maximum depth of 131 sashin (917 feet) ; at 

 the time of his visit the water-level was still rising, but this was 

 probably seasonal, for two years later Sir Aurel Stein, traversing 

 the same route, estimated the length at 15 miles. As the 

 upper end of the. lake was very narrow, merely a flooded river- 

 channel, when surveyed by Col. Spilko, widely differing estimates 

 of length might easily have been made by different observers; 



