<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> A seismic shadow zone is an area of the Earth's surface where seismographs can only barely 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 11,570 and 15,570 km or 7,190 and 9,670 mi) from the epicenter . The secondary seismic waves cannot pass through the liquid outer core and are not detected more than 104 ° (approximately 11,570 km or 7,190 mi) from the epicenter . P waves that have been converted to s - waves on leaving the outer core may be detected beyond 140 degrees . </P> <P> The reason for this is that the velocity for P - waves and S - waves is governed by both the different properties in the material which they travel through and the different mathematical relationships they share in each case . The three properties are: incompressibility (k (\ displaystyle k)), density (p (\ displaystyle p)) and rigidity (u (\ displaystyle u)). P - wave velocity is equal to (k + 4 3 u) / p (\ displaystyle (\ sqrt ((k+ (\ tfrac (4) (3)) u) / p))) whereas S - wave velocity is equal to (u / p) (\ displaystyle (\ sqrt ((u / p)))) and so S - wave velocity is entirely dependent on the rigidity of the material it travels through . Liquids, however, have zero rigidity, hence always making the S - wave velocity overall zero and as such S - waves lose all velocity when travelling through a liquid . P - waves, however, are only partially dependent on rigidity and as such still maintain some velocity (if greatly reduced) when travelling through a liquid . Analysis of the seismology of various recorded earthquakes and their shadow zones, led geologist Richard Oldham to deduce in 1906 the liquid nature of the Earth's outer core . </P>

The s-wave shadow zone is evidence of which boundary or property of earths interior
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