= Double-couple fault source
{title2=$M=\mu(b\otimes n+n\otimes b),\quad b\cdot n=0$}
Tangential fault slip $\mathbf b$ in an <isotropic> elastic material has the displayed moment density. It is symmetric and trace-free, with <eigenvalues> $\mu|b|,-\mu|b|,0$. The two off-diagonal force dipoles form equal couples with opposite <torques>, producing zero resultant force and <torque>. For a uniform patch of area $A$, the scalar seismic moment is $M_0=\mu A|b|$. <Traction> continuity removes the single-force layer in the <elastodynamic surface-jump representation>.
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