INTERESTING PHENOMENA ON THE ERUPTION OF USU 267 



that the preliminary warnings were the symptoms of an eruption, 

 and, watching the crater every moment during the day and night 

 and the following day, he observed in detail the phenomena. The 

 number of earthquakes, as well as their vigor and intensity, in- 

 creased. No one could indulge in sound sleep in the neighborhood 

 of the volcano. Cannonading and trembling developed till the 

 introductory explosion took place. Among the several hundred 



Fig. 11. — A great column of smoke at the top of Nishimaruyama beyond the 

 steaming mud flow. Taken August 2, 19 10. 



earthquakes two violent ones are worthy of mention, one at 

 4:30 P.M. on July 24, and the other at 5:00 P.M. on July 25. Monu- 

 ments fell, houses were badly damaged, the earth was ruptured, 

 and many mud cones were formed around the volcano (Figs. 2-6). 

 The earthquake wave reached an average radial distance of 65 km. 

 outward from Usu, except in the southwesterly direction, where 

 it reached 140 km. The earth's shaking abruptly decreased after 

 the first explosion, suggesting that strains which had been accumu- 

 lating were then relieved. 



