Fault rake ($\lambda$)

The fault rake specifies the slip direction within a fault plane. With an along-strike unit vector $\mathbf{s}$ and a down-dip unit vector $\mathbf{d}$, one explicit convention is $\mathbf{b}/b=\cos \lambda \mathbf{s}-\sin \lambda \mathbf{d}$. Positive $90^{\circ }$ then denotes up-dip slip. A convention must accompany a signed fault rake.