応力比からみた陥没カルデラ構造と応力場

概要

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An attempt to estimate stress ratio Φ= (σ_2-σ_3)/(σ_1-σ_3) for determining the slip direction of caldera bounding fault have been made to clarify influence of regional stress on the caldera formation. The variation of stress ratio provides two important conclusions; particularly in the piston-cylinder subsidence (coherent floor subsidence), the slip-direction of caldera bounding fault (ring fault) is considered as uniformly subvertical one, indicating the lowest value of c. In the trap-door subsidence of coherent floor, however, the oblique-slip of caldera bounding fault is present, and it indicates larger value of the c, rather than the ordinary ring faulting. An example at dissected trap-door caldera reveals a coincident result with the conclusion, by use of a reduced stress tensor method. This analogue also provides an apparent stress ratio (Φ_H=1-(σ_3/σ_2)), which is performed to compare characteristics of the subsidence structure at respective calderas. In the piston-cylinder subsidence, Φ_H is almost equal to the Φ. In the case of piecemeal subsidence, however, the Φ_H is larger than the Φ, indicating that there is in need of larger stress σ_1 than the piston-cylinder subsidence. This relationship between Φ and Φ_H Probably suggest that the piecemeal caldera needs to a deeper roof of the underlying magma chamber, rather than the piston-cylinder caldera.
特定非営利活動法人日本火山学会の論文
2000-08-28