EARTHQUAKES AND SEAQUAKES 



87 



___Epicenlrum of 1883. 



Intense destructive Area 



1796 

 1828 

 1881. 

 1883 



hidden structure, as is the sensitized plate under the magical in- 

 fluence of the X rays. 



When, at the time of an earthquake, blocks are adjusted with 

 reference to their neighbors, the movements of oscillation are 

 greatest in those marginal portions 

 of direct contact. Corners of blocks 

 the intersecting points of the im- 

 portant faults should for the same 

 reason be shaken with a double 

 violence, and this assumption ap- 

 pears to be confirmed by observation. 

 Upon the island 

 of Ischia, off the 

 Bay of Naples, 

 the shocks from 

 recent earth- 

 quakes have 

 been strangely 

 concentrated 

 near the town of 

 Casamicciola, 



which was last destroyed in 1883. This un- 

 fortunate city lies at the crossing point of 

 important fractures whose course upon the 

 island is marked by numerous springs and 

 suffioni (Fig. 81). 



Seismotectonic lines. The lines of im- 

 portant earth fractures, as will be more clearly 

 shown in the sequel (p. 227), are often indi- 

 cated with some clearness by straight lines in 

 the plan of the surface relief (Fig. 82). Lines 

 FIG. 82. -A line of earth of this na t ure are easily made out upon the 



fracture indicated in ,, TTT , T ,. , ... 



the plan of the relief, ma P of the West Indies, and if we represent 

 which may at any time upon it by circles of different diameters the 



become the seat of combined intensities Of the recorded earth- 

 movement and result- , ., . ... ., A . 

 ant shock. quakes in the various cities, it appears that 



the heavily shaken localities are ranged upon 

 lines stamped out in the relief, with the most severely damaged 

 places at their intersections (Fig. 83). These lines of exceptional 



FIG. 81. Map of the island of 

 Ischia to show how the shocks 

 of recent earthquakes have been 

 concentrated at the crossing 

 point of two fractures (after 

 Mercalli and Johnston-La vis) . 



Scale. 



