= Solution
A <fault plane> represents the surface across which the two sides of a fault slip. A real fault need not be perfectly planar; these angles can describe a local planar segment. Its <fault strike> is the azimuth, measured clockwise from north, of its horizontal intersection. The line itself has a $180^\circ$ ambiguity; choose the direction for which the fault dips to the right. Its <fault dip>, $\delta$, is the angle below horizontal measured perpendicular to that <fault strike>, from $0^\circ$ for horizontal to $90^\circ$ for vertical.
The <fault rake>, $\lambda$, is the angle of the relative slip <vector> within the <fault plane>. State the sign convention: if $\mathbf s$ is the chosen strike direction and $\mathbf d$ points down dip, use
$$
\boxed{\mathbf b=b(\cos\lambda\,\mathbf s-\sin\lambda\,\mathbf d).}
$$
Thus $\lambda=0$ is slip along $\mathbf s$, $+90^\circ$ is up-dip slip and $-90^\circ$ is down-dip slip. This distinguishes the direction in which the fault is tilted from the direction in which it actually moves.
The <hypocentre> is the point where rupture begins within the <Earth>. The <epicentre> is the point on the surface directly above it. The <hypocentre> need not coincide with the centroid of the final rupture. \b[Orientation uses strike and dip; motion additionally needs rake; <epicentre> and <hypocentre> locate the rupture's start.]<Earthquake Hazards Program Glossary by United States Geological Survey>[Entries “fault plane”, “strike”, “dip”, “hypocenter” and “epicenter”]{ref}
\Image[/past-exam-of-the-physics-of-the-earth-as-a-planet-course-of-the-university-of-cambridge/2015/paper-1-fault-geometry.png]
{title=Fault orientation, source location and an explicitly signed rake convention}
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