[1] We conducted true triaxial compression tests to examine the effect of the intermediate principal stress (s 2 ) on brittle failure and deformability of the siltstone overlying the Chelungpu fault, Taiwan. For constant minimum stress (s 3 ) a pattern of increased strength (s 1 ) as a function of s 2 , unpredicted by common strength criteria, was observed. Strength data are well fitted by a Mogi-modified Nadai criterion relating the octahedral shear stress at failure to the mean normal stress on the induced shear fracture or fault. The induced fault strikes subparallel to s 2 and dips steeply in the direction of s 3 . Significantly, fault dip angle steadily increases as s 2 rises for unchanged s 3 . SEM inspection of tested specimens reveals only sporadic localized intergranular microcracks associated with the creation of the fault. The modulus of elasticity and the onset of dilatancy exhibit a similar behavior to that of s 1 , as they increase with s 2 when subjected to a constant s 3 .

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