表層堆積層の地殻応力への影響 : 飛騨から富山能登にかけての断層タイプの変化への応用

概要

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The influence of a surface sedimentary layer on crustal stress was formulated, assuming the isostasy of the crust. A sedimentary layer weakens vertical stress because it has a lower density than crystalline rocks in the crust. If the vertical stress is sufficiently reduced by the low-density material, it can change the directions of principal stress axes and lead to regional variations of fault type. The strike-slip fault dominates the Hida area, Central Japan, while the thrust fault is dominant in the Toyama and Noto area, which is north of the Hida area. Thick sedimentary layers are well developed in the Toyama and Noto area. The influence of these sedimentary layers was evaluated using the P-wave velocity structure model constructed by Iidaka et al. (2003). Density is estimated to be 2,000－2,200kg/m3 based on the P-wave velocity and the depth of MOHO. The porosity of the surface layer is estimated to be ～30％, with pores filled with water. The estimated low density can reduce the vertical stress sufficiently to make it the minimum principal stress. This might explain the change of fault type from Hida to Toyama and Noto.
2008-12-22