THE RECENT ERUPTIONS OF THE ASAMA-YAMA 



(JAPAN). 



By CHARLES DAVISON, Sc.D., F.G.S. 



The eruption of the Usu-san in 1910, of which an 

 account is given in " KNOWLEDGE " for May, 1912, 

 was by no means of unusual violence, but was 

 interesting from the extraordinary rise of a new 

 mountain more than six hundred feet in height, 

 with which the volcanic operations were closely 

 connected. The Usu-san is situated in the northern 

 island of Hokkaido. Some four hundred and fifty 

 miles to the south-south-west of the Usu-san, near 

 the centre of the main island of Japan, is another 

 volcano, larger in size and much more violent and 

 devastating in its outbursts. The Asama-yama is 

 well known in Japan as the most active of all its 

 volcanoes, and as the scene of an eruption which 

 ranks as one of the greatest yet known to us. This 

 took place in 1783, and, as is usually the case, was 

 followed by a prolonged period of quiescence, which 

 at last seems to be drawing to a close. Numerous 

 explosions during the last few years, with a marked 

 increase of frequency in 1911, tremors and local 

 earthquakes in large numbers, and a gradual rise in 

 the floor and the crater, these are all premonitory 

 symptoms of a coming period of violent eruption, and 

 possibly of disaster, at some epoch which may be close 

 at hand, but can hardly be delayed for many years. 

 The Japanese Government is fortunately alive to 

 the danger that awaits them. By the timelv pre- 

 cautions of the police in the summer of 1910, the 

 eruption of the Usu-san was attended by no loss of 



From a photograph of a model. 



Figure 90. 



The Volcano, Asama-yama. 



life. Very different was the fate of the dwellers on 

 and around the Taal Volcano in Luzon in January, 

 1911. Here, the early signs of the coming eruption 

 were neglected, no attempts were made to remove 

 the inhabitants, with the result that eleven hundred 



Figure 91. 

 The eruption of Asama-yama, May 8th, 1911. 



lives were lost in a single night. A watch on the 

 Asama-yama has already been set. On its south- 

 western slope, a seismological observatory has been 

 erected, every throb of the volcano is being recorded, 

 and, under the guidance of Professor Omori, the 

 able director of the Seismological Institute in Tokyo, 

 the observers have learned to distinguish between 

 the tremors resulting from the volcanic explosions 

 and those which are unattended by visible efforts. 

 Professor Omori has recently made three ascents of 

 the mountain and has studied the detonations which 

 have accompanied the explosions and the areas 

 within which the ashes have been deposited. The 

 results of his enquiries are contained in a valuable 

 memoir,* of which a brief summary is given in the 

 following pages. 



The position of the Asama-yama is indicated by 

 the small triangle in Figures 92 and 9.3. It rises 

 from a plateau-region to a height of eight thousand 

 one hundred feet above sea-level. As the height of 

 its base is three thousand nine hundred feet on the 

 north, and three thousand six hundred feet on the 

 south side, the mountain proper is still more lofty 

 than the present cone of Vesuvius. The crater is 

 nearly four hundred feet in depth and about a 

 quarter of a mile in diameter. The form of the 

 mountain is shown in Figure 90, which is repro- 

 duced from a photograph of a model of the volcano, 

 and in Figure 91, which represents the eruption 

 that took place on May 8th, 1911. From Figure 

 90 it is evident that the present crater is surrounded 

 on the west side (that is, to the left in the figure) by 

 the remains of the wall of an old crater once about 

 a mile in diameter. 



The eruptions and earthquakes of the Asama-yama: Bulletin of the Imperial Earthquake Investigation Committee (Tokyo), 



Vol. VI, 1912, pages 1-147. 



94 



