THE DEATH-RATE OF EARTHQUAKES 247 



two elements it is of greater value to know the area that will be 

 mainly affected than the time when a shock will take place. 

 The time alone would be of little service, for sixty " world- 

 shaking " earthquakes occur on an average every year, so that 

 as a rule few weeks will pass by without the visit of an earth- 

 quake somewhere or other upon the globe. 



What is required for the solution of this problem is more 

 definite knowledge than we at present possess of the operations 

 which precede the occurrence of a great earthquake. On this 

 subject, some light has been thrown by recent disasters. A 

 displacement of the earth's crust along a fracture more than two 

 hundred miles in length, like that which caused the Californian 

 earthquake of 1906, cannot be the work of an instant of time. 

 For many years, the strain must have been increasing until it 

 reached the point when rupture and sliding could no longer be 

 averted. By the erection of pillars along a line at right angles 

 to such a fracture and by careful observation subsequently of 

 their relative positions, the first deformations may be detected 

 and measured. Or, again, before a great movement can take 

 place, small obstacles to motion must be cleared away along the 

 surface of the fracture and every such removal must give rise 

 to a tremor more or less pronounced. The outlining of the 

 course of a fault by the centres of numerous slight shocks, as 

 happened before the Japanese earthquake of 1891, should reveal 

 the preparation that is being made for a great movement — a 

 movement which may, as in that case, take place within the 

 next two years. 



For the present, it would seem advisable to direct attention to 

 those conditions which are partially within our control, so as to 

 lessen, if we cannot avert, the destructiveness of an earthquake 

 shock. In a few cases, there can be little doubt that the 

 Government should interfere and prohibit the rebuilding of a 

 town that has been frequently ruined. In permitting the re- 

 erection of Casamicciola after the Ischian earthquake of 1883, 

 the Italian Government incurred a grave responsibility, not- 

 withstanding all the precautions taken. Here, there is no 

 reason to suspect any migration of the seismic focus. Time 

 after time, the same small district has been the seat of renewed 

 shocks of increasing violence. The central volcano of Epomeo 

 may have been extinct during the historical period but outbursts 

 have occurred along radial fissures. The violent shocks which 



