Abstract
A small but explosive eruption occurred from Asama Volcano from 01:51 a.m. to around 08:00 a.m. on February 2, 2009 and the diluted tephra covered an elongated area extended SE direction passing Chichibu, Hamura, Fuchu, Inagi, Kawasaki and Yokohama Cities and reached Boso Peninsula. In Tokyo Metropolis and Saitama and Kanagawa Prefectures, we carried out a questionnaire survey about the distribution and amount of ash-fall deposit by this eruption. The result shows that most of the respondents found ash deposit in the area enclosed by the isopach of 1 g/m2 and the ratio of the respondents who recognize ash deposit decrease remarkably out of the area. In the questionnaire, the respondents reported the semi-quantitative volume of ash deposit, judged from comparing their observations with a photograph showing an ash fall of 6 g/m2. The result is almost consistent with the quantitative field survey by geologists, suggesting the possibility of semi-quantitative evaluation of ash fall with questionnaire.
