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Complete the crack normal to an orthonormal basis $({\mathbf{e}}_1,{\mathbf{e}}_2,\mathbf{n})$ with the first two vectors in the crack plane. The seismic moment tensor is diagonal in this basis:Each term gives an elastic force dipole, $f_i=-D{e}_i{e}_j{\partial }_j\delta$: two equal and opposite forces in direction $\mathbf{e}$, separated along $\mathbf{e}$, with moment $D$. Hence there are two identical in-plane dipoles of strength $\lambda V$ and one normal dipole of strength $(\lambda +2\mu )V$. For positive opening and ordinary positive Lamé parameters, all are extensional and the normal one is larger by $2\mu V$.

More generally, stable isotropic linear elasticity requires $\mu >0$ and $3\lambda +2\mu >0$. These imply $\lambda +2\mu >|\lambda |$, so the normal dipole has greater magnitude even if an auxetic material has negative $\lambda$ and compressive in-plane dipoles. The point source has zero net force, because it differentiates a delta function, and zero net torque, because its moment tensor is symmetric. Its normal moment distinguishes opening from a pure shear dislocation.