抄録
The shape of aftershock area is investigated and numerically calculated by using energy releasing rate g at the tip of the elliptical crack. When g is independent of the eccentric angle φ of ellipse, that is ∂g/∂φ=0, energy state of the elastic medium is thought to be most stable. If aftershock phenomena are considered to be quassi-static process, it is easy to calculate g.
Using these assumptions, it is concluded that the crack acted by shear stress any direction approximately becomes the elliptical crack of which the ratio between long radius and short one is about 4:3. However it gradually approaches the circular crack as the tension perpendicular to the crack face increases. These results apply to comparatively small earthquakes whose boundary conditions are considered to be infinite, isotropic and homogeneous.
