Strike, Dip, & Rake (Focal Mechanism)


Fault strike is the direction of a line created by the intersection of a fault plane and a horizontal surface, 0° to 360°, relative to North. Strike is always defined such that a fault dips to the right side of the trace when moving along the trace in the strike direction. The hanging-wall block of a fault is therefore always to the right, and the footwall block on the left. This is important because rake (which gives the slip direction) is defined as the movement of the hanging wall relative to the footwall block.


Fault dip is the angle between the fault and a horizontal plane, 0° to 90°.


Rake is the direction a hanging wall block moves during rupture, as measured on the plane of the fault. It is measured relative to fault strike, ±180°. For an observer standing on a fault and looking in the strike direction, a rake of 0° means the hanging wall, or the right side of a vertical fault, moved away from the observer in the strike direction (left lateral motion). A rake of ±180° means the hanging wall moved toward the observer (right lateral motion). For any rake>0, the hanging wall moved up, indicating thrust or reverse motion on the fault; for any rake<0° the hanging wall moved down, indicating normal motion on the fault.

Dip = 90°   Rake = 0°    ::  left-lateral strike-slip
Dip = 90°   Rake = 180°  ::  right lateral strike slip
Dip = 45°   Rake = 90°   ::  reverse fault
Dip = 45°   Rake = -90°  ::  normal fault
See also the Wikipedia and USGS entries on strike and dip.