One row of an interaction matrix. A 40 × 20 km fault dipping 60° and breaking the free surface, cut into 256 triangles, in a half space with μ = λ = 30 GPa. One patch is the source: it slips 1 m, and every patch is coloured by the stress change that slip causes there. Red promotes slip, blue opposes it, and the scale is symmetric-log about zero. Drag to orbit; click any patch to move the source.

The source patch is always blue, and that is not a choice of colours — a patch that slips relieves the shear stress that was driving it, so the diagonal of this matrix is negative everywhere. Here it runs from −14.5 to −12.4 MPa, and the build refuses to write the payload if any entry comes out positive. Coulomb is the shear change resolved on the receiver's own strike direction plus 0.6 times the normal change; the shear and normal views are those two terms alone. The normal term is much the smaller — a factor of 18 at the peak and 47 in r.m.s. — so on this geometry Coulomb is shear plus a correction. See Concepts for why the free surface costs nothing to impose here, Running for the call that builds this matrix, and Examples for the field it radiates into the half space.