Focal mechanism of the Tokachi-Oki earthquake of May 16, 1968: Contortion of the lithosphere at a junction of two trenches

Abstract

The focal mechanism of the Tokachi-Oki earthquake of 1968 (M_s∼ 8.0) and its aftershocks is studied on the basis of P-wave first motion, S-wave polarization angle, and long-period surface-wave data. The major objective is to understand the nature of the deformation of the oceanic lithosphere at a junction of two trenches. The main shock is interpreted as a low-angle thrust fault with a considerable strike-slip component, the oceanic side underthrusting beneath the continent. This type of faulting is common with other great earthquakes of the northwestern Pacific belt, and is considered to represent a major tectonic movement in this region. The largest aftershock (M_s≈ 7.5), that occurred about 10 hours after the main shock, suggests a faulting in which the slip direction is almost opposite to that of the main shock. Other aftershocks are grouped into either the main shock type or the largest aftershock type. A simple model is proposed to explain this unusual aftershock sequence. In this model a contortion of the underthrusting lithosphere at a junction of two trenches, the Kurile and the Japan trenches respectively, plays a key role. Because of this contortion of the lithosphere, the source region of the 1968 Tokachi-Oki earthquake interacts mechanically with a neighboring region where the 1952 Tokachi-Oki earthquake occured. This interaction causes aftershocks whose faulting is in a direction opposite to that of the main shock. The source parameters of the main shock are as follows: plane a (fault plane) dip angle = 20°, dip direction = S66°W; plane b dip angle = 78°, dip direction = S60°E; seismic moment = 2.8·10^(28)dyn·cm; slip dislocation = 4.1 mm; stress drop = 32 bar; strain drop = 0.71·10^(−4); strain energy release (residual strain is assumed to be zero) = 1.0·10^(24) erg. In these calculations, the fault dimension and the rigidity are assumed to be 100 × 150 km^2 and 4.5·10^(11) dyn/cm^2 respectively.

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