EVIDENCE FOR A WEAK INTERPLATE THRUST-FAULT ALONG THE NORTHERN JAPAN SUBDUCTION ZONE AND IMPLICATIONS FOR THE MECHANICS OF THRUST FAULTING AND FLUID EXPULSION

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