ON SEISMOLOGICAL INVESTIGATION. 157 



that at least one of the latter must have been accompanied by enormous 

 dislocations in order to have produced the former. 



These earthquakes, as recorded on land, were comparatively small, 

 which, from what we know of the dissipation of earthquake energy as it 

 radiates from its origins, indicates that the earth vibrations must have 

 travelled at least 100 miles. 



The least interval of time that we can give between the arrival of the 

 vibratory wave and the sea-waves is that observed at Miyako, which is 

 21 minutes. 



If we assume a mean depth for the ocean off the north-east coast of 

 Nippon, along an easterly line, to the origin of the disturbance at 2,000 

 fathoms, then the distance from the land to the origin may be expressed 



V 12000x^x21 x60, 



or about 130 geogi-aphical miles. 



Again, if we assume ■y., to be the velocity of the sea-wave, which may 

 be taken at 500 feet per second, this being a somewhat low observed 

 velocity for earthquake sea- waves approaching this coast; d, the velocity 

 of the vibratory waves, which over a short range has often been observed 

 at 7,000 or 8,000 feet per second ; and T the observed interval of time 

 between the arrival of the two waves, then the distance of their origin 

 from the coast is 



or in this case about 113 geographical miles. 



If we make i;2=600 feet per second, the distance of the origin becomes 

 about 140 geographical miles. 



Because we have taken the least interval that can be assigned to the 

 difference in the times of the arrival of the land and sea- waves, it may 

 be concluded that the origin of the Japan disturbance of June 15 was 

 along a submarine line at a distance of 1 20 to 1 40 geographical miles off 

 the coast of Noi-th-east Nippon. 



Such a locus is at a depth of 4,000 fathoms, and, so far as we know 

 the sub-oceanic contours, exactly at the bottom of the Nippon slope, 

 forming the western boundary of the Tuscarora Deep, a well-known 

 origin for many large earthquakes (see map, fig. 14). 



Although much evidence may be adduced to show that early in June 

 1896 the ocean currents were deranged in direction and intensity, the 

 cause of the submarine dislocation was probably seismic. 



Velocity of Propagation of Earth-waves. — Assuming the origin to lie 

 120 geographical miles east of Miyako, to which place it travelled at a 

 rate of 8,000 feet per second, which fairly well accords with the velocity 

 it travelled from the Miyako isoseist to Tokio, and velocities of propaga- 

 tion of similar earthquakes over short ranges, the time, within a few 

 seconds, at which the earthquake occurred was, in G.M.T., June 14 

 22h. 31m. Os. 



G.M.T. — Times at ivhich Preliminary Tremors commenced in Europe. 



