Focal mechanism of the Iwate-Oki earthquake of June 12, 1968.

The focal mechanism of the Iwate-Oki earthquake (Ms=7.2) of June 12, 1968, is studied on the basis of P-wave first motion and Love and Rayleigh wave radiation patterns. This earthquake represents a low-angle thrust fault with a left-lateral strike-slip component. The direction of slip vector is N55°W, which is parallel to that of the neighboring Tokachi-Oki earthquake (Ms=7.9). The source parameters determined are: dip angle=30° and dip direction=N119°W for the fault plane; dip angle=76° and dip direction= N125°E for the auxiliary plane; seismic moment=5.1×1026 dyne·cm; fault dimension=80×30km2; slip dislocation=48cm; stress drop=12bar; strain drop=0.27×10-4; released strain energy=0.70×1022erg. This earthquake was accompanied with small tsunamis. The sea bottom crustal deformation calculated from the seismic fault parameters fits the tsunami data very well. The generation mechanism of tsunami of the Iwate-Oki earthquake resembles that of the Tokachi-Oki earthquake of 1968.