6 4 o SCIENCE PROGRESS 



any rate, it is in this direction that hope lies. If we could but 

 ascertain certain phenomena, one or more of which invariably 

 precede a great earthquake, we should be on the high road to 

 success. The phenomena may belong to different categories. 

 They need not be the same in every case. Nor need they be 

 manifest without instrumental or other aid. In all probability, 

 the phenomena that herald an earthquake will only be revealed 

 by careful and detailed study ; otherwise they would have been 

 discovered long ages ago. Thus, we may at once rule out the 

 favourite portents of past times, such as the appearance of a 

 comet or of strange lights in the sky, the arrival of unusual birds, 

 a dull, heavy condition of the atmosphere formerly known as 

 " earthquake weather," or the depressed feelings of neurotic 

 persons. 



If we reflect on the mechanism of a great earthquake, on the 

 enormous masses that are displaced in some cases, on the 

 stupendous forces that are involved, and on their gradual 

 increase until they are sufficient, and more than sufficient, to 

 sweep away the resistance opposed to them, it would seem that 

 there must be some indication of their growth, some sign of 

 incipient motion, that might be revealed by painstaking investi- 

 gation. 



Most earthquakes probably originate at a depth of several, 

 though not many, miles below the surface. The sliding move- 

 ment along a fault or fracture, to which they are due, usually 

 dies out before the surface is reached, so that no visible effect of 

 the motion is manifest, and it is only by the study of the evidence 

 available that we can determine the position of the fault respon- 

 sible for the earthquake. But, in a few great earthquakes, the 

 displacement underground is so considerable that it is continued 

 right up to the surface, and there it remains and displays to us 

 something of the magnitude of the region in which the earth- 

 quake originates, something also of the nature and extent of the 

 initial movement, until, by the gentle but continual action of the 

 weather, the fault-scarps are worn down and all traces of 

 the disturbance are obliterated. 



In the earthquake which visited San Francisco and many 

 another coast-town of California in 1906, the surface displace- 

 ments exceeded in one respect those of every other earthquake 

 with which we are acquainted. The destruction of San Francisco 

 was due, not so much to the strength of the shock, as to the 



