Usu-san, Japan. 249 



The main features in the topograpliy of the mountain are 

 summarized in the map, Plate XII. It will be noticed that two 

 prominent summits, 0-usu and Ko-usu, rise within the crater hollow. 

 These two are apparently of prehistoric origin, but are ascribed by 

 Omori to "the pushing up of molten lava mass which solidified during 

 the process". A neighbouring volcano, Tarumai-san, built up 

 a similar lava-dome, 134 m. high, during an eruption which lasted 

 three months of the year 1909. Kompira-yama and the East and 

 West Maru-yamas, on tlie northern flanks of Usu-san, are also 

 regarded as lava-domes, but evidence in regard to their geological 

 relationships is very meagre, since the whole of the lower portion of 

 the volcano is swathed in soil of great depth. The crater of Usu-san 

 took no part in the 1910 eruption, but Kompira-yama and West 

 Maru-yama were both the sites of active explosions. 



There are records of previous eruptions at Usu-san for the years 

 1663, 1769, 1822, and 1853. In the case of both the 1663 and 1822. 

 eruptions, of which detailed accounts have been preserved, we learn 

 that earthquakes preceded any smoke-explosions by an interval of 

 about three days. The occurrence of warning shocks of this character 

 was a feature once more of the 1910 eruption, and seems to be 

 characteristic of volcanic action in general, at any rate in Japan. In 

 other respects Omori has no analogy to point to between the early 

 eruptions and that of 1910. For convenience of description the 

 phenomena of this last eruption may be grouped under three main 

 headings: (1) earthquakes, (2) explosions, and (3) elevation. 



(1) Earthquakes were first felt on July 21, and obtained their 

 maximum frequency during the fifteen hours which elapsed between 

 6 p.m. on the 24th and 9 a.m. on the 25th. They then fell off 

 rapidly, and in fact were only about half as numerous by the time of 

 the first explosion, 10 p.m. on the 25th. They continued on the 

 decrease the following day also, but thereafter remained irregularly 

 constant during the ensuing months of active eruption. 



The earthquakes which preceded the first explosion were not only 

 more frequent than those which followed it, but also more severe. 

 The strongest sliock occurred at 3.49 p.m. on the 24th, and the next 

 strongest at 4.39 p.m. on the 25th. A small amount of damage 

 resulted from these disturbances in Abuta, a town 6 km. distant from 

 Usu-san ; but, in accordance with the general rule for volcanic 

 earthquakes, neither shock was of really destructive violence, con- 

 sidered, that is, in relation to wooden buildings of Japanese type. 

 The main earthquake penetrated as a sensible tremor to a distance of 

 60 km. towards the north-east and 140 km. towards the south-west. 



(2) As already stated, the eruption proper was initiated by an 

 explosion at 10 p.m. on July 25. The outbui'st was a single 

 small affair, and took place on the north-west side of Kompira-yama. 

 Ashes and rock fragments were discharged, and a rude crack fashioned 

 in the soft earth of the hillside (numbered 1 , Plate XII). Other 

 eruptions from new foci followed at short intervals, some of them 

 enjoying a paroxysmal career of several days duration. By August 10 

 the number of craterlets had increased to at least twenty-eight. The 

 opening up of fresh orifices continued, though with diminished vigour. 



