Mav. 1<)I- 



KN()\\Li-:i)r,i:. 



173 



It is important to notice that, from the flat ground 

 forming the southc-rn margin of the To\a lake and 

 the scene of the new mountain, these craterlets were 

 entirely absent. 



The first two craterlets, as alread\- noticed, were 

 formed at the west end of the zone of disturbance. 

 The third explosion took place at the east end, 

 then followed three more at the west end, and then 

 another at the east end. 

 After this, they occurred 

 along the /one C through 

 West Maru-yama, and 

 in the centre of tlu- 

 principal band. A A. 

 There was thus an alter- 

 nation of activity from 

 end to end of the area, 

 terminating bv confine- 

 ment to the central 

 region. 



.Ml of the craterlets 

 were formed in a thick 

 laver of soft earth, and 

 none of the explosions 

 was of great importance. 

 Most of the craterlets 

 quietlv" ejected ashes 



and. except when explosions occurred, were 

 unaccompanied by loud detonations or by earth- 

 tremors. There was no outflow of la\'a from any one 

 of them. Most were in action for a few days or 

 hours only, ami then became completely quiescent. 

 A few, hos\e\er, remained active for several days, 

 and consequently increased in size until they were 

 more than two hundred yards in diameter. When 

 the eruption was at its height, the scene is said to 

 have been magnificent, as smoke-columns could be 

 seen issuing at once 

 from six or seven 

 different craters. 

 Figure 203 shows 

 eruptions taking 

 place simultaneous- 

 ly from three crater- 

 lets, that on the left, 

 known as the 

 Kumantsubo cra- 

 terlet, being one of 

 the most active 

 craters during the 

 whole course of the 

 eruption. 



Some interesting 

 observations on the earthquakes originating from 

 the volcano, especially at the times of the eruptions, 

 were made by Professor Omori. He erected a 

 portable horizontal tromometer and a vertical 

 motion recorder, from July 30th to August 6th, 



Figure 204 



new Mountains seen on the 

 column of smoke. 



davs of August, the number of shocks recorded at 



Xishi-Monbets was about two hundred and ninety, of 



which ten were sensible to observers in the same 



town. The number of volcanic earthquakes recorded 



being about twenty-nine times the number of 



perceptible shocks, it follows that, if the tromometer 



had been at work on July 24th, when earthquakes 



were most frequent, the total number recorded 



would have been about 



thrt'c thousand eight 



hundred, or one ever}' 



twcnty-twi) and a-half 



scconils. In other words, 



the ground must have 



been in a state of almost 



continual trembling. 



.\ special feature of 

 the records obtained 

 near the Kumantsubo 

 craterlet is the occur- 

 rence of w ell - defined 

 quick unfelt vibrations 

 in addition to the 

 proper volcanic earth- 

 (|uakes. These small 

 \ ibrations are termed by 

 Professor Omori micro- 

 frcinors. Thc\- were entirel\- absent at Nishi-Monbets, 

 and rajjidlv die out with increasing distance from the 

 origin. .\s a rule, moderate explosions in the 

 Kumantsubo craterlet were not accompanied by 

 marked micro-tremors, but violent explosions from 

 this and other craterlets in the vicinity were 

 accompanied, and often preceded for a few minutes, 

 by well-pronounced micro-tremors. They also 

 occurred when the smoke ejections from the 

 different craterlets were very insignificant or even 



had completely 

 ceased. It would 

 seem that, if ex- 

 plosions were pre- 

 \ented by some 

 temporary obstruc- 

 tion in a craterlet, 

 the pent-up steam 

 and gases produced 

 in consequence a 

 series of minor 

 earthquakes result- 

 ing in the micro- 

 tremors. 



On August 6th, 

 Professor Omori 

 discovered accidental!} that the eastern part of the 

 south coast of the Toya lake (from D to E, 

 Figure 202) had risen about a yard, causing the lake- 

 margin to retreat about seven }-ards. At the foot of 

 East Maru-vama, the movement continued, but at a 



at Nishi-Monbets, five miles from the crater, and decreasing rate, until, at the end of August, the total 



from August 6th to 10th, in a school at the foot of rise of the coast was about four feet, and the 



East Maru-yama and about three-fifths of a mile recession of the coast line about twenty-three yards, 



from the Kumantsubo craterlet. On the first two The elevation was not. however, confined to the 



