284 Y. OINOUYE 



Furthermore, there is a remarkable change of height in Usu 

 proper, as is shown by the map of the Imperial Geological Survey, 

 July, 191 1. In the topographical map published by Hokkaidocho, 

 the height of O-usu is recorded as 595 m. and that of Ko-usu as 

 580 m., while the Imperial Geological Survey reports 736 m. and 

 609 m. respectively. This difference is too great to be regarded as 

 an error in surveying and must mean that some igneous intrusion 

 produced the irregular change of elevation. The writer presumes 

 that the present height of Usu would be found to differ materially 

 from that recorded by the Imperial Geological Survey. 



One year after the eruption Dr. Omori 1 observed that there were 

 local elevations and depressions of the ground in the vicinity of 

 the mountain and over an area of 150 sq. km. The Military 

 Survey Department undertook the determination of height at the 

 request of Dr. Omori and found that Mount Usu was raised, while 

 the western foot was depressed. In the following year the same 

 surveyor recorded contrary results; that is, the previously ele- 

 vated portion had been depressed, while the depressed part was 

 uplifted. 



VI. SUMMARY 



i. Earthquakes and roaring before an eruption. — As a rule the 

 eruption of Usu is preceded by the foreshocks. This, in the opin- 

 ion of the writer, suggests that the lava reservoir was located nearly 

 at the same depth in the case of the recent eruptions. The magma 

 in the reservoir, becoming highly heated, could not retain the 

 involved gases, and so the maximum strain under the crust was 

 produced by the continuous heating process. The explosion took 

 place when the interior and exterior pressures were not counter- 

 balanced. Thus the pressure of the highly heated magma over- 

 balanced both the atmospheric and the crustal pressures. The 

 ground beneath the surface burst, owing to the intense strain, and 

 produced the loud sound. The speed of the earthquake waves 

 is greater than that of the sound traveling in the ground and the 

 air. The minute tremor which normally precedes the sound was 

 not noticed because there was no seismograph at hand. Hence, 



1 Bulletin of the Imperial Earthquake Investigation Committee, V, No. 3 (1913). 



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