IV. 



The After-Shocks of Earthquakes. 



NO severe earthquake ever occurs without numerous attendants of 

 a slighter character, a few of them preceding, but by far the 

 greater number following it, and continuing to be felt for many 

 months afterwards. Their unusual number, their immediate succes- 

 sion, and their gradual decline in frequency with the lapse of time, as 

 well as their geographical distribution, point unmistakably to the 

 existence of an intimate connexion between the earthquake and its 

 after-shocks. 



From whatever point of view we look at earthquakes, but 

 especially if we regard them as the results of fault-slipping, the study 

 of after-shocks is of great importance. At the same time, it is a 

 study frequently encompassed with serious difficulties. In the 

 presence of a terrible disaster, slight shocks are only too likely to be 

 neglected. Their number is often so great that even a cool-headed 

 observer may give up in despair the attempt to chronicle them. And, 

 again, supposing his record to be nearly complete, the interval between 

 successive shocks may be so small that their identification with those 

 noted by other observers may be nearly, if not quite, impossible. 

 Thus, the distribution of after-shocks, with regard to space, is a 

 subject on which we must at present be content to remain in partial 

 ignorance, except in those cases where the shocks are few, or where 

 seismographs are abundant, and escape injury during the chief 

 disturbance. 



The distribution of after-shocks, with regard to time, is a 

 subject which lies more within the range of investigation. It has 

 recently been discussed with great ability by Mr. F. Omori, of the 

 Seismological Institute at Tokio (3, 4). Within the last six years 

 there have been three earthquakes of unusual severity in Japan : those 

 of Kumamoto. on July 28, 1889, Mino-Owari, on October 28, 1891, 

 and Kagoshima, on September 7, 1893 5 an d> fortunately, in each case 

 there has been an observatory provided with a seismograph in the 

 immediate neighbourhood of the epicentre. 1 Except within a few 

 hours of the earthquakes, the records of these instruments may be 



1 The "epicentre" is the area on the earth's surface vertically above the 

 seismic focus or centre of disturbance. 



