河成段丘の縦断形から波状変形の変位量を推定する一方法

抄録

The present study presents a new method to estimate the amount of displacement in an active wavelike deformation on the basis of the data obtained from longitudinal profiles of a river terrace and the dip of the basement strata.
Examination is made on the following two assumptions.
Assumption l: The strata which are overlain by terrace deposits were horizontally layered before the neotectonic deformation. After the emergence of the river terrace, the wavelike deformation proceeded in the mode that developed the geological structure of the basement strata at a constant ratio (Fig. 3-B, C).
Assumption 2: The original shape of the river terrace showed a longitudinal profile concave upward (Fig. 3-A).
Assumption 1 means that one of the following equationss holds good. 1. For the displacement which preserves a constant proportion of the change in gradient,
α(x)=kβ(x). (1-1)
2. For the displacement which preserves a certain proportion of the amount of vertical displace-ment per unit length,
tan α(x)=k tan β(x), (1-2)
3. For the displacement which preserves a certain proportion of the amount of shortening per unit length,
1-cos α(x)=k(1-cos β(x)). (1-3)
In these equations, (1) α(x) is the change in the gradient at point x after the emergence of the river terrace, (2) j3 (x) is the dip of the layers overlain by the terrace deposits at point x, where k is the constant of proportion, and x indicates the horizontal distance measured from a point located on the lower side of the objective structure along the river course.
Assumption 2 is expressed as follows:
θ₀(x_p)>θ₀(x_q), x_p>x_q (2)
where θ₀ (x) is the original gradient of the river terrace.
The recent gradient of the river terrace, θ(x), is revealed by the following equation:
θ(x)=θ₀ (x)+ α(x). (3)
α(x) can be obtained from Equations (2) and (3) and one of Equations (1-1), (1-2), (1-3).
Because this method depends on the Assumptions 1 and 2, thorough examination of whether these assumptions hold good or not is required for the application of the method. The present method is however, applicable for estimating the amount of displacement of the river terraces located on the middle course of a river.