424 



REPORT — 1888. 



The mean number of earthquakes which occurred per hour is approxi- 

 mately 40. Calling the time from 6 p.m. to 6 A.M. night, and the remain- 

 ing hours oat of the twenty-four, day, we find that 86 more earthquakes 

 were recorded during the night than during the day. The probable 

 explanation of this observation is, that during the night persons are more 

 lavourably situated for the observation of small disturbances than they 

 are during the day. During the day, when a person is engaged in active 

 work, especially if out of doors, it has often been observed that earth- 

 quakes are passed by unnoticed, while at night, if not sleeping too 

 soundly, the slightest tremor may be recorded. 



Under the heading ' Seismic Survey of Tokio ' reference is again made 

 to this subject. 



(d) Area shaken by Earthquakes in 1885 and 1886. — Inasmuch as 

 there are many reasons for believing that the depth at which an earth- 

 quake originates is usually incomparable with the radius of the area 

 shaken, the area shaken may be taken as a very fair estimate of the 

 intensity of any given disturbance. The following table gives the sum 

 of the areas shaken per month and the mean area per shock. The unit 

 is one square ri, or 5-9553 square miles. 



The total area, 92,050 square ri, shaken in 1886 is 3"8 times the area 

 of the empire, which, exclusive of small islands and Loo Choo, is 24,352 

 square ri. There were only QQ earthquakes which exceeded the mean 

 value of 189 square ri. 



From the following table we see that the greater intensity of 1885 was 

 due to the fact that in that year there were more shocks of large exten- 

 sion than in 188G, when the majority of shocks (349) were local in their 

 character. It also gives the number of earthquakes of different intensities, 

 reckoned as areas shaken, felt during different months. 



