Earth 119- interlude d: the earth"s interior- revisited: seismic layering, the magnetic field, and gravity. Wave front: the boundary between the region through which a wave has passed and the region through which it has not yet passed. Seismic ray: the changing position of an imaginary point on a wave front as the front moves through rock. Travel time: the time it takes for a wave to travel from the focus to a seismograph station along a given ray. Refraction: the bending of a ray as it passes through a boundary between two different materials. Moho: the seismic-velocity discontinuity that defines the boundary between the earth"s crust and mantle. Low-velocity zone (lvz): the asthenosphere underlying oceanic lithosphere in which seismic waves travel more slowly, probably because rock has partially melted. Seismic-velocity discontinuities: a boundary in the earth at which seismic-velocity changes abruptly. Upper mantle: the uppermost section of the mantle, reaching down to a depth of 400 kilometers.