1. A region of the subsurface from which seismic reflections cannot be detected because their ray-paths do not emerge to the surface.

2. A zone over the Earth’s surface in which P-waves and/or S-waves generated by an earthquake are detected only weakly, or are absent, because of refraction within the various layers deep within the Earth, and especially in association with the Earth’s core. Seismic signals refracted from the mantle are usually poorly discernible at a range of up to about 10°. At a range of 103–142° from the earthquake P- and S-waves are observed only very weakly; between 110° and 142° some very weak P-waves can be measured and these are thought to have been refracted from the inner core. Between 142° and 180° from an earthquake no S-waves are recorded because the waves cannot travel through the liquid outer core.