560 Dr. C. Davison on Sea-Waves connected 



exception of a report presented to the Japanese Government, 

 of which no adequate summary has, I believe, appeared in any 

 European language. In the present paper, I shall therefore 

 confine myself to considering the propagation of the sea- 

 waves in an easterly direction, one of my objects being to 

 compare the mean depth of the ocean as obtained by sound- 

 ings with that obtained from the formula usually employed 



where H is the uniform depth of the ocean in which waves 

 would travel with the same mean velocity V as that of the 

 seismic sea-waves. I have shown in a previous paper* that 

 the value so found must be considerably less than the true 

 mean depth ; the calculated value in a particular case con- 

 sidered being 1900 fathoms instead of 2420 fathoms, i.e. 

 about | of the real value. 



In tig. 1 are shown the parts of Central and Northern 

 Japan that were chiefly affected by the earthquake, the town 

 of Kamaishi being practically destroyed by the sea-waves. 

 The continuous curves represent isoseismal lines. Within 

 tbe curve marked 2, the shock was " weak," the motion 

 being well-pronounced , but not sufficiently severe to cause 

 general alarm ; between the two curves 1 and 2 the shock 

 was " slight, "" and only just strong enough to be felt. The 

 dotted curves are sub-oceanic contour-lines, the figures 

 annexed to each giving the depth in fathoms. According to 

 Mr. T. Iki, the epicentre was situated in lat. 39° N. and 

 long. 144° 30' E., or about 240 km. E.S.E. of Miyako. It 

 is indicated on the map by a cross, and lies at a depth of 

 4000 fathoms near the foot of the western slope of the Tusca- 

 roora Deep. 



Prof. Omori informs me that the surface- velocity of the 

 earthquake-waves within the disturbed area was about 2*7 km. 

 per second, and, as the time of occurrence at JVliyako was 

 7h. 32m. 30s. p.m. (mean time of 135° E.), this would give 

 7h. 31m. (or lOh. 31m. a.m., G.M.T.) as the approximate 

 time at the epicentre. This result is probably correct to 

 within a minute. 



Though I made inquiries at all the harbours known to me 

 where self-recording tide-gauges are established, records of 

 the sea-waves appear to have been obtained at only two 

 stations, namely, Honolulu and Sausalito (San Francisco Bay). 

 Copies of these records are given on a reduced scale in fig. 2 

 and Plate V. For these and for other information given 



* Phil. Mag. vol. xliii. (1897), pp. 33-36. 



