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The focal mechanism of the 1968 Tokachi-Oki earthquake (MJ = 7.9) was thrust faulting but that of the largest aftershock (MJ = 7.5) was normal faulting. The tectonic implication of the reversal of the focal mechanisms of the two events has not been clarified yet. In order to investigate this, we ought to take into account the relative location of the two fault planes. In the present study, the length and the direction of rupture propagation for the largest aftershock is derived from the azimuth dependence of the duration of observed strong ground motion. If we assume that the rupture initiation point is the hypocenter location determined by the Japan Meteorological Agency (JMA), the location of the fault plane is considerably further north of the tsunami source area as estimated previously, and at the northern end of the aftershock area within 24 hours after the main shock. The fault plane of the largest aftershock is deeper than that of the main shock and the two fault planes are nearly parallel to each other. The thrust faulting of the main shock and the normal faulting of the largest aftershock indicate that the part of the Pacific plate between the two fault planes moved relatively further northwestward due to the two events than the deeper part beneath the fault plane of the largest aftershock.

Fault extent of the largest aftershock of the 1968 Tokachi-Oki, Japan, earthquake and an interpretation of the normal faulting focal mechanism