A new three-dimensional method of fault reactivation analysis

A 3-D method to evaluate the reactivation potential of fault planes is proposed. The method can be applied to cohesive or noncohesive faults whatever their orientation and without any conditions on the regional stress field. It allows computation of the effective stress ratio σ 3 ′ / σ 1 ′ required to reactivate any fault plane and to determine whether the plane is favorably oriented, unfavorably oriented or severely misoriented with respect to the ambient stress field. The method also includes a graphical sorting tool that involves plotting poles of fault planes on stereoplots for which the boundaries separating the three domains corresponding to favorable orientations, unfavorable orientations and severe misorientations cases are shown. The delineation of these domains is based on the value of the σ 3 ′ / σ 1 ′ ratio that depends on the orientation of the fault plane with respect to the principal stress axis orientations, the stress shape ratio (Φ = (σ2 − σ3)/(σ1 − σ3)), the coefficient of static friction μs of the fault, and the fault cohesion C0. The method is applied on 145 focal mechanisms of the 2011 March 11th Tohoku-Oki (Japan) earthquake sequence. This application delineates, along or in the vicinity of the Pacific-Okhotsk plate interface, three types of domains characterized by favorable orientations, unfavorable orientations or severe misorientations of mainshock/aftershock fault planes. Aftershock focal mechanisms that plot in the ‘severe misorientation’ domains are interpreted to have occurred because of pore fluid pressures exceeding the regional minimum principal stress at those locations. The distribution of these ‘severe misorientation’ domains partly overlaps the asperities or the low-velocity anomalies mapped on the plate interface off NE Japan. The proposed 3-D fault reactivation analysis appears complementary to geophysical investigations.