Representation of western US strain-rate field in terms of the amplitudes and directions of the principal strain-rate axes (thick and thin line segments denoting a principal contractile or tensile strain-rate axis, respectively) and rotational strain rate (indicated by colour shading). There is no restriction on the standard deviations of the velocity gradient values. Grey lines indicate outlines of planes upon which Juan de Fuca–North America and Pacific–North America forces are imposed.