Relationship between fault growth mechanism and permeability variations with depth of siliceous mudstones in northern Hokkaido, Japan

Ishii, Eiichi   ; Funaki, Hironori ; Tokiwa, Tetsuya; Ota, Kunio 

In order to assess influences of a remote mean stress on a principal mode of failure near fault tips due to fault slip in a lithologically homogeneous, fractured rock mass, the growth mechanisms of strike-slip faults on outcrop-scale in the siliceous mudstones of northern Hokkaido, Japan, have been studied. A multifaceted approach combining geological characterizations of fractures, rock mechanical characterizations, and theoretical analyses have been done, which suggested (1) the principal mode of failure depend on not only the rock strengths, but also the remote mean stresses, (2) during and/or after uplift and erosion the faults grew mainly by linking with adjacent faults via many splay cracks which are formed by tensile failure above roughly 400 m depth, while, below the depth, the faults predominantly grew by shear failure. Such growth mechanisms are consistent with the fact that highly permeable sections are restricted to depths of less than 400 m.