A seismic
shadow zone is an area of the
Earth's surface where
seismographs cannot detect an
earthquake after its
seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquake's
focus. The
primary seismic waves are
refracted by the liquid outer core of the Earth and are not detected between 104° and 140° (between approximately ) from the
epicenter. The
secondary seismic waves cannot pass through the liquid outer core and are not detected more than 104° (approximately ) from the
epicenter. P waves that have been converted to s-waves on leaving the outer core may be detected beyond 140 degrees.