THE PREVISION OF EARTHQUAKES 643 



The second method, now to be described, rests on a more 

 definite foundation. It depends on observations actually made 

 on the slight shocks which preceded the great earthquake that 

 devastated the provinces of Mino and Owari in Japan on 

 October 28, 1891. As in the Californian earthquake of fifteen 

 years later, this earthquake was accompanied, or rather caused, 

 by unusual fault-displacements which to a great extent left 

 visible traces on the surface of the ground. The actual length 

 of the displacement was less than in the Californian earthquake. 

 The part of the surface-fault affected was traced for forty 

 miles, though it is probable that its total length was not 

 less than seventy miles ; but in this case the vertical displace- 

 ment was considerable. In one place it attained a height of 

 about twenty feet. The horizontal movement was less notable, 

 and was variable in amount from one up to about thirteen 

 feet. In addition to the displacement which resulted in this 

 fault-scarp, there must have been other movements along a 

 more deeply seated fault, which is roughly parallel to, though 

 possibly branching from, the more conspicuous fault. Both 

 faults, as well as other minor fissures which may have been in 

 action, will be referred to here as the fault-system. 



A very marked feature of this earthquake was the great 

 number of shocks that followed it. All of them were much 

 slighter than the original earthquake, but many, if they had 

 occurred alone, would have attracted attention as strong or 

 violent earthquakes. At first they occurred with great fre- 

 quency, more than a thousand being recorded at Gifu during 

 the first week. They were felt in all parts of the fault-system, 

 though more frequently in some than in others. But, after 

 the lapse of a few months, they occurred more rarely and 

 became almost limited to definite portions of the fault-system, 

 such as the central and terminal regions, and finally to the 

 central region alone. It is chiefly in these two respects— great 

 frequency and concentration of activity — that the after-shocks 

 of this earthquake were distinguished from those that pre- 

 ceded it. 



We are indebted mainly to the labours of the late Prof. Milne 

 for our knowledge of the earthquakes of this district, his great 

 catalogue of 8,331 Japanese earthquakes during the years 

 1885-92 providing all the materials necessary for our present 

 purpose. The area mainly affected by the earthquake of 1891 



