230 THE ITALIAN EARTHQUAKE OF AUGUST 1895. [vol. lxxix, 



well marked, of another line joining the extremities of these two, 

 and running from near Como towards Pisa. If the origin of the 

 earthquake really was of the nature of two, possibly three, fractures 

 or series of fractures, it is not conceivable that they could have 

 been due to any cause directly connected with the tectonics of the 

 surface-rocks : for, with the exception of the east-to-west northern 

 line, they run across the structural features as seen on the surface. 

 But it is conceivable that, if any general change of bulk had taken 

 place in the material underlying that portion of the crust over 

 which the earthquake was felt, it might be the determining factor 

 in producing fractures in the crust, which would be unconnected 

 with the surface tectonics, and in this way the isolated ai'ea in 

 Elba, where an earthquake was noticed in various parts of the 

 island, could be brought into relation with the main area in a 

 manner not otherwise easy to explain. This change of bulk might 

 be of equal area with the earthquake, in which case it need not lie 

 at a great depth from the surface, but so large an area of almost 

 uniform change is not easy to understand ; it becomes more 

 intelligible if we consider the original change of bulk to be of 

 more restricted dimensions, and the effect, immediately below the 

 solid crust, to be the transmitted effect of such change. This 

 transmission would not be merely vertically upwards, but would 

 spread outwards at a certain angle which might be as much as 45°, 

 and in this case the ultimate origin would lie at a depth of the 

 order of 120 miles if the dimensions were inconsiderable ; but, if 

 considerable, the depth would be proportionately diminished, and 

 thus for an origin having one-quarter of the dimensions of the 

 earthquake it would become about 90 miles, and 60 miles for 

 one-half the dimensions. On the other hand, the angle of 

 spreading of the effect would probably not be as much as 45°, and, 

 if only 30° (corresponding to an apical angle of 60°), the depths 

 would be nearly one and three-quarters as great as those mentioned. 



From these facts and considerations it will be seen that no very 

 positive conclusions can be drawn, nor any precise estimate made, 

 of the depth of origin ; this much, however, is clear, that the 

 earthquake was more akin to those which give good distant 

 records, than to the ordinary type of local earthquake ; and that 

 the depth of the ultimate origin of the shocks was great, pro- 

 bably of the order of about 100 miles or so, below the outer 

 surface of the earth. This conclusion is of interest in its accordance 

 with the deductions, recently announced by Prof. H. H. Turner, 

 that the origins of many of the disturbances, which give rise to 

 good long-distance records, lie at a depth of about 200 kilometres 

 (125 miles). 



