2018 年 39 巻 1 号 p. 1-14
This paper reviews previous studies of volcanic sector collapse, with particular attention to the relationship of the collapse direction to the regional stress field in Japan. The large number of examples of Quaternary age in Japan has allowed analysis of the factors that control the direction of collapse, but researchers have not reached a consensus about those controls. Early researchers concluded that most collapses have been perpendicular to the alignment of parasitic cones and dikes on the volcanic edifice. Subsequent research identified no clearly preferred direction of collapse. More recent research has proposed the following relationship between sector collapse and the regional tectonic stress field. Under regional compressional stress, collapse is subparallel to the axis of maximum horizontal principal stress in response to a local extensional stress field developed along mountain ridges. Under regional extensional stress, collapse is subparallel to the axis of maximum horizontal principal stress. Furthermore, if in an extensional regime the difference between intermediate and minimum principal stresses is small, the relationship of the two can be temporarily reversed in response to stress released due to volcanic activity. Another model, based on analogue experiments, has indicated that slope failures occur at an angle to the axis of regional maximum horizontal principal stress. A strike-slip model for generation of crater breaches can explain this phenomenon, although the model is not applicable to Japanese volcanoes yet. Much of the previous research has suffered from the inclusion of too few volcanoes to provide statistically sound results. Further consideration of the strike-slip model and research using larger datasets published during the past decade may overcome these shortcomings.
