396 NATURAL SCIENCE. June, 



11.49 p.m., and disturbed an area of more than 38,000 square miles. 

 Up to the end of 1893, 922 after-shocks were recorded at Kumamoto. 

 One of these, five days after the principal shock, was violent, seventy- 

 six were strong, and 845 weak or feeble shocks, or else merely sounds. 

 The curve representing the decline in frequency is somewhat similar 

 in form to that of the Mino-Owari earthquake, and shows that, after a 

 total lapse of about seven or eight years, the annual number of shocks 

 will once more have attained the value which it had before the earth- 

 quake of 1889, namely, three or four a year. Thus, the disturbed 

 condition of the earth's crust in this district is now approaching its 

 end, and the complete number of after-shocks will probably be about 

 950 or 1,000. 



The Kagoshima earthquake, which took place on September 7, 

 1893, at 2.46 a.m., disturbed an area of about 30,000 square miles. 

 The record of after-shocks at Chiran, in the epicentral district, began 

 at about 9 p.m. In the interval, there were probably about a hundred 

 shocks, so that by the end of January, 1894, the total number must 

 have been nearly 480. The annual number will probably return to 

 its original value after a lapse of three or four years, by which time 

 some six hundred shocks may have been felt. 



The number of earthquakes discussed is too small to draw any 

 general conclusions ; but so far it would appear that the more intense 

 the original earthquake the greater is the number of its after-shocks. 



Distribution of After-Shock Frequency in Space. — When many earth- 

 quakes occur in the same district, it does not follow that they all 

 proceed from one centre of disturbance. Probably, a continual dis- 

 placement of the centre, resulting from a relief and transference of 

 stress at each successive shock, is the more general rule. To make 

 our history of an earthquake-series approach completeness, we ought, 

 therefore, to know the disturbed area and epicentre of every shock, 

 and this would enable us to trace the laws according to which the 

 disturbed portion of the earth's crust gradually settles down into 

 equilibrium again. But, as already remarked, this is in many cases 

 a task of peculiar difficulty, and, with the Mino-Owari earthquake, 

 one that it was impossible to perform. The large number of after- 

 shocks in this case, however, renders another method applicable, for 

 the suggestion of which we are, I believe, indebted to Mr. Omori. 

 This is the distribution of after-shock frequency with regard to 

 space. 



Besides the seismographic records from Gifu and Nagoya, earth- 

 quake reports were sent in from district offices and other stations in 

 the three provinces of Mino, Owari, and Mikawa. These give, with 

 a general approach to accuracy, the monthly numbers of after-shocks 

 from November, 1891, to February, 1894. Plotting such numbers on 

 the seismic map, curves may be drawn through places where equal 

 numbers of after-shocks occurred during stated intervals. Mr. Omori 

 gives three such maps — one for 1892, another for 1893, and the third 



