32 NATURAL SCIENCE. Jan., 



by Messrs. Dutton and Hayden for the Charleston earthquake of 1886. 

 The evidence on which his calculations are based is not described, 

 but the result obtained for the depth is 34 kilometres or about 21 miles. 

 Another estimate has been made by Mr. Lacoine (7). This is founded 

 on the times of occurrence of the earthquake at different places, and 

 the result arrived at is the same. The coincidence is striking, but it 

 should not, I think, be inferred that the result is trustworthy, for both 

 methods of calculation are liable to the same source of error, namely, 

 the varying refractive powers of the different rocks traversed by the 

 earthquake waves in their passage from the focus to the surface. The 

 method of Dutton and Hayden is, moreover, open to a serious objection. 

 If it were correct, it would follow that, for all earthquakes originating 

 at the same depth, the distance from the epicentre of the line on the 

 surface (the "index-circle") at which the intensity declines most 

 rapidly would be the same. But it is conceivable that many earth- 

 quakes with the given depth of focus may not be perceptible so far as 

 the index-circle, perhaps not even at the surface at all. Since it fails, 

 then, in these cases, the method can hardly be expected to give correct 

 results in others. Indeed, the problem of finding the depth of a seismic 

 focus is one that at present lies beyond our range. The only guide 

 our knowledge gives us is to look with suspicion on any estimate so 

 great as twenty miles. 



After-shocks. — The principal earthquake was succeeded, as usual, 

 by many after-shocks, though these were less numerous than is often 

 the case. 1 One reason for this may have been that a large part of the 

 epicentral tract was submarine, and thus the district where they are 

 most frequent lay beyond the reach of observation. At Constantinople 

 four slight shocks were felt on July 10, the day of the earthquake, 

 three on the next day, and ten more before the end of the month. 

 After this they became still less frequent, though several months 

 elapsed before the district returned to its usual condition (1, 2). 



Origin of the Earthquake. — Mr. Eginitis points out that the axis 

 of the first isoseismal coincides with the line of depression which begins 

 at Ada-Bazar, and is marked by the Lake of Sabandja and the Gulf 

 of Ismid. He suggests that the earthquake was tectonic — in other 

 words, connected with the moulding of the earth's surface features. 

 This he infers from the absence of volcanoes in the district affected, 

 the great calculated depth of the focus, the elongated form of the 

 isoseismal lines, the intensity of the shock, and the immense area over 

 which it was observed. With this conclusion, probably, all geologists 

 will agree, though not unnaturally they may desire more detailed 

 information as to the way in which the moulding was effected. The 

 evidence is too scanty to provide this with any certainty, but what 

 there is seems to me to support the view that the earthquake was 

 caused by a great fault-slip, the effect of which has been to deepen 

 the Gulf of Ismid. 



1 See Natural Science, vol. vi., pp. 391-397, June, 1895. 



