The complex distribution of aftershock focal mechanisms of the 1968 Tokachi-Oki earthquake is consistent with a best-fitting uniform stress tensor in which the minimum principal stress acts approximately perpendicular to the main-shock thrust plane. This implies low resolved shear stresses on the interplate thrust fault as well as low frictional fault strength, which can be explained by elevated pore pressure within a hydraulically sealed fault zone. After great subduction-zone earthquakes, episodic fluid flow may relieve the high fault-zone pore pressures via hydraulic fracturing along the main-shock thrust plane.

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