ON THE EARTHyUAKB AND VOLCANIC PHENOMENA OF JAPAN. 223 



Gyroscopes as adjancts to the solation of these problems have not 

 hitherto proved themselves successful. 



On the assumption that the earth-waves in alluvium were harmonic 

 in character and symmetrical in form, in the Report for 1892 it was shown 

 that they might be 20 feet in length ; and, knowing their length and 

 period, the velocity of propagation was determined. Even should these 

 waves have lengths several times this amount, some knowledge of their 

 form might be obtained by simultaneously measuring the difference 

 in movement between, say, the heads of a line of stakes at right angles to 

 the direction of the advancing waves and different points of a wire or rod 

 parallel to such a line, but only held in position at its two extremities. 



I am led to mention these latter experiments as indications of the 

 important problems which seismologists have yet before them. 



List of Earthquakes recorded in Japan in February 1893. 



Tlie list of earthquakes appended to this section of the Report is given 

 as an example of a catalogue which might be compiled from the material 

 which since 1885 has been accumulating at the Central Meteorological 

 Observatory in Tokio. 



The approximate centre of a disturbance is indicated by its latitude 

 and longitude, while the energy of the disturbance may approximately be 

 deduced from the figures which show in geographical miles the diameter 

 of the area shaken. 



Hitherto investigations respecting seismic activity, the periodicity of 

 earthquakes, &c., have been based upon catalogues where only the number 

 of shocks have been recorded, and where the disturbances of one seismic 

 area have been inextricably mixed with those from another. 



With a catalogue like the one suggested it would be possible to investi- 

 gate the rate at which seismic activity is decreasing or increasing either 

 in a given area or in Japan as a whole, giving values to the shocks pro- 

 portional to the area they had shaken. It would assist us in determining 

 whether there is any relationship between the frequency of earthquakes 

 in neighbouring areas. Inasmuch as many earthquakes seem to be the 

 result of sudden fractures or yieldings taking place during the process of 

 rock-crumpling, it does not seem unlikely that the relief of strain along 

 one axis should be altogether without effect upon neighbouring axes 

 where folding may also be in operation. One interesting investigation of 

 the records of a district which has very kindly been made by Mr. F. 

 Omori has been to plot the shocks which succeeded the great disturb- 

 ance of 1891 as a curve, the co-ordinates of which are equal intervals of 

 time and the number of shocks occurring during these intervals. 



It will be remembered that the immediate cause of the disturbance 

 was the formation of a large fault which can be traced some forty or fifty 

 miles, together with several minor faults. During the seven months 

 which followed the great shock no less than 3,000 shocks were recorded. 

 How many have been recorded up to date has not been calciilated, but 

 from the appended List for the month of February, that is sixteen months 

 after the first shock, sixty-two disturbances were noted. 



The curve representing this decrease in activity closely approximates 

 to a rectangular hyperbola, which now, with an average of two shocks 

 per day, is becoming asymptotic. 



With the law of decrease deduced from these records Mr. Omori 



