In which plane must fault slip be measured?

• A. Perpendicular to the fault plane
• B. In the plane of the fault
• C. Along the strike line of the fault
• D. In a vertical plane through the fault

Answer: (B)

Fault slip is the relative motion of blocks along the fault surface, so the slip vector lies in the fault plane. Measuring in that plane captures the true displacement parallel to the fault, regardless of whether the motion is primarily along strike, along dip, or oblique. Measuring perpendicular to the fault would quantify opening or separation across the fault, not the sliding motion along it, which is why slip must be taken in the plane of the fault. The exact direction can vary, but the movement always occurs within the fault plane.