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Shadow zone
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.〔(【引用サイトリンク】title=Earthquake Glossary - shadow zone )〕〔(【引用サイトリンク】title=PX266 Geophysics - Extra Material - Seismic shadow zones )〕 P waves that have been converted to s-waves on leaving the outer core may be detected beyond 104 degrees.
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 (), density () and rigidity (). P-wave velocity is equal to whereas S-wave velocity is equal to 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.
==See also==
*Seismic wave
*Ray tracing (physics)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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