264 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



1910, during which the ground was elevated and 45 new craterlets were 

 developed ; (7) the eruption in 1943-45, with the birth of a new moun- 

 tain, Syowa Sinzan. 



The newest activity of TJsu began on the evening of December 28, 

 1943, with a series of severe earthquakes at the northwestern foot of 

 the mountain, and essentially ended in September 1945 upon the 

 completion of the growth of the newly formed mountain. The devel- 

 opment of the recent eruption falls into three stages of activity; (1) 

 The pre-eruption stage, from December 28, 1943, when the first earth- 

 quake was felt, to June 22, 1944; (2) stage of paroxysmal eruptions, 

 with the formation of small eruptive craters, from June 23, 1944, when 

 the first explosion took place, to October 31, 1944; and (3) the birth 

 and development of the lava dome from November 1, 1944, to Sep- 

 tember 1945. 



PRE-ERUPTION STAGE 



The recent catastrophic activity started with the first perceptible 

 earthquake on December 28, 1943. This was followed by numerous 

 succeeding earthquakes, which for 3 days increased in frequency and 

 intensity. Although these earthquakes were perceptible throughout 

 the whole area around Usu Volcano, the shocks during December and 

 the early part of January were felt most strongly at Toya Hot Springs 

 at the northwestern foot of the mountain. As a result of these shocks, 

 some damage was caused to water pipes and plastered walls of dwell- 

 ing houses at Toya Hot Springs. Other earthquakes, felt only at 

 Yanagihara and Kami-Osaru located near the eastern foot of Usu, 

 increased in frequency and intensity and continued without diminu- 

 tion until the middle of April. During the period from December 28 

 to January 5 the earthquakes were felt more frequently at the north- 

 western base of Usu (Toya Hot Springs) , but later the seismic activity 

 was more marked at the northeastern village of Sobetu. Accordingly, 

 it is evident that the hypocenters of the shocks shifted from the 

 northwestern or central part of Usu to its eastern foot. By comparing 

 the perceptible areas it was apparent that the earthquakes originating 

 in the first area were deeper than those at the later place. And rum- 

 blings accompanying the earthquakes were most strongly heard at 

 the northwestern skirt of the mountain during the initial seismic 

 activity but were more frequently heard in Yanagihara and Kami- 

 Osaru after the hypocenters had shifted toward the eastern foot of 

 Usu. 



Soon after most of the earthquakes were localized at the eastern 

 side of Usu, a most remarkable phenomenon occurred. The ground 

 about the epicenter of the shocks began to rise rapidly. The roads, 

 railways, irrigation canals, and the Osaru River and its tributaries 



