Zona de sombra sísmica: Diferenzas entre revisións
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[[Ficheiro:Earthquake wave shadow zone.svg|miniatura|200px|Zona de sombra sísmica (do [[USGS]])]]
Unha '''zona de sombra sísmica''' é unha área da superficie da [[Terra]] na cal os [[sismógrafo]]s só poden detectar un [[terremoto]]<ref>{{Cite web|url=http://www.astronomynotes.com/solarsys/s8.htm|title=Planet Interiors}}</ref> despois de que as súas [[onda sísmica|ondas sísmicas]] atravesaron a Tera. Cando se produce un terremtoo, as ondas sísmicas irradian de forma esférica desde o seu [[hipocentro|foco]]. As [[onda P|ondas sísmicas primarias]] (P) orixinadas polo terremoto son [[Refracción|refractadas]] fortemente polo [[núcleo externo]] fundido da Terra, o que fai que non se detecten entre os 104° e 140° (aproximadamente entre os 11 570 e os 15 570 km) medidos desde o [[epicentro]] do terremoto. Aínda que as ondas P sofren unha forte caída da súa velocidade e refracción, poden propagarse dentro do núcleo externo. Ao contrario, as [[onda S|ondas sísmicas secundarias]] (S) non poden pasar a través do núcleo externo líquido da Terra e non se detectan a máis de 104° desde (aproximadamente aos 11 570 km) desde o epicentro.<ref name="USGS Glossary">{{cite web|url=https://earthquake.usgs.gov/learn/glossary/?termID=170&alpha=S |title=Earthquake Glossary - shadow zone |publisher=[[USGS]] |accessdate=May 8, 2011}}</ref><ref name="UoW">{{cite web|url=http://www2.warwick.ac.uk/fac/sci/physics/teach/module_home/px266/diag/shad/ |title=PX266 Geophysics - Extra Material - Seismic shadow zones |publisher=[[University of Warwick]] |accessdate=May 8, 2011}}</ref>
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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: [[Compressibility|incompressibility]] (<math>k</math>), [[density]] (<math>p</math>) and [[Stiffness|rigidity]] (<math>u</math>). P-wave velocity is equal to <math>\sqrt{(k+\tfrac{4}{3}u)/p}</math> whereas S-wave velocity is equal to <math>\sqrt{(u/p)}</math> 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.<ref>{{Cite document|last1=Armstrong |first1=D. |last2=Mugglestone |first2=F. |last3=Richards |first3=R. |last4=Stratton |first4=F. |title=OCR AS and A2 Geology |publisher=[[Pearson Education]] |pages=14 |year=2008}}</ref> Analysis of the seismology of various recorded earthquakes and their shadow zones led [[geologist]] [[Richard Dixon Oldham|Richard Oldham]] to deduce in 1906 the liquid nature of the Earth's outer core.<ref>{{cite journal |last=Bragg |first=William |year=1936 |title=Tribute to Deceased Fellows of the Royal Society |journal=Science |publisher=American Association for the Advancement of Science |volume=84 |issue=2190 |pages=539–46 |issn=0036-8075 | doi = 10.1126/science.84.2190.539 | bibcode = 1936Sci....84..539B |pmid=17834950}}</ref>
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