THE SEVEBE JAPAN EARTHQUAKE OF THE 15th OF JANUARY, 1887. 



315 



the seismic waves propagated nearly 200 miles to the west and north- 

 east along the Pacific seaboard. On the northwest they approached 

 but do not quite reach the shore of the Japan Sea. They shook, in 

 all, about 32,000 square miles of land area. The limit of the dis- 

 turbance is indicated on Plate XXII. 



In Tokyo the disturbance began at 6^ ol"" 59' p.m., with slight J^^^^l^^f^^'^i 

 tremors. After thirty seconds from the commencement, the greatest 

 horizontal motion (21mm.) was recorded. The time taken to 

 complete one to-and-fro motion of the ground was 2.5 seconds. 

 The maximum vertical motion was only 1.8 mm., being, as usual, 

 very small compared.; with the horizontal movement. The principal 

 motion continued for more than two minutes, during which time 

 no less than sixty distinct shocles occurred. The maximum velocity 

 and maximum acceleration, which measure the overthrowing and 

 shattering power of earthquakes, have been calculated from the above 

 numbers, and found to be respectively 26 mm. and 66 mm. per 

 second. These numbers, considering the range of motion, are small; 

 or, in other words, the oscillations of the ground were comparatively 

 gentle and slow, wliich serves to explain the fact that but little 

 harm was done to property in the Capital. In Yokohama Hipp's 

 Seismograph registered a horizontal motion, of 35 mm. 



The origin of the shock was in a narrow band of country I'un- ^'^^^ause of 

 ning from west to east in the province of Sagami, parallel to the 

 coast, at a distance from it of about seven miles. The red shading 

 on Plate XXII indicates this band. It emanates from the western 

 or mountainous parts of the province, passes through the southern 

 foot of Oyama (4125 feet above the sea-level), and reaches the Bay 

 of Yokohama in a total distance of about thirty miles. I believe 

 the most probable cause of the shock to have been faulting or 

 dislocation of the earth's crust alono^ the band above named. This 



