1. A surface within a steady-state wavefield that does not involve motion. 2. The first motion from an earthquake may be either a push or pull, depending on the orientation of the station with respect to the epicenter and the direction of motion along the earthquake fault. A nodal plane separates the region where the first motion is a push from the region where it is a pull. There are two such planes for first-energy P-waves, one of which is the fault plane.