CoachThe first motion of a P wave at a seismometer is the direction, up or down, of the first movement of the ground on the vertical record. The focal mechanism of an earthquake is the orientation of the fault plane and the direction of slip on it, as found from its seismic waves. The two perpendicular planes through the focus that separate the pushes from the pulls are its nodal planes; one is the fault plane, and the other, perpendicular to the slip, is the auxiliary plane. The P axis and the T axis are the directions at from both nodal planes in the middle of the pull quadrants and of the push quadrants:
