Editing P wave (section)

==Seismic waves in the Earth==
{{See also|Core-mantle boundary|Mohorovičić discontinuity|Low-velocity zone|Lehmann discontinuity}}
[[File:Speeds of seismic waves.svg|thumb|upright=1.4|Velocity of seismic waves in the Earth versus depth.<ref name=Helffrich>{{cite journal |journal=Nature |year=2002 |volume=412 |issue=2 August |pages=501–7 |title=The Earth's Mantle |author=GR Helffrich & BJ Wood |url=http://www.phys.uu.nl/~sommer/master/Structure%20and%20Evolution/articles%20for%20presentation/9.pdf | doi=10.1038/35087500 |pmid=11484043 |s2cid=4304379 }}</ref> The negligible S wave velocity in the outer core occurs because it is liquid, while in the solid inner core the S wave velocity is non-zero.]]

Primary and secondary waves are body waves that travel within the Earth. The motion and behavior of both P and S waves in the Earth are monitored to probe the interior [[structure of the Earth]]. Discontinuities in velocity as a function of depth are indicative of changes in phase or composition. Differences in arrival times of waves originating in a seismic event like an earthquake as a result of waves taking different paths allow mapping of the Earth's inner structure.<ref name="Negendank">
{{cite book |author=Rubinstein |first=Justin L. |title=New Frontiers in Integrated Solid Earth Sciences |last2=Shelly |first2=D. R. |last3=Ellsworth |first3=W. L. |publisher=Springer |year=2009 |isbn=978-90-481-2736-8 |editor=Cloetingh |editor-first=S. |page=287 ''ff'' |chapter=Non-volcanic tremor: A window into the roots of fault zones |quote=The analysis of seismic waves provides a direct high-resolution means for studying the internal structure of the Earth... |editor-last2=Negendank |editor-first2=Jorg |chapter-url=https://books.google.com/books?id=7AIPPoWf3KIC&pg=PA287}}
</ref><ref name="Fowler">
{{cite book |author=Fowler |first=C. M. R. |title=The solid earth: an introduction to global geophysics |publisher=Cambridge University Press |year=2005 |isbn=978-0-521-58409-8 |edition=2nd |page=100 |chapter=§4.1 Waves through the Earth |quote=Seismology is the study of the passage of elastic waves through the Earth. It is arguably the most powerful method available for studying the structure of the interior of the Earth, especially the crust and mantle. |chapter-url=https://books.google.com/books?id=PifkAotvTroC&pg=PA100}}
</ref>

===P wave shadow zone===
[[Image:Earthquake wave shadow zone.svg|thumb|upright=1.4|P wave shadow zone (from [[USGS]])]]
Almost all the information available on the structure of the Earth's deep interior is derived from observations of the travel times, [[Reflection (physics)|reflection]]s, [[refraction]]s and phase transitions of seismic body waves, or [[normal modes]]. P waves travel through the fluid layers of the [[Internal structure of Earth|Earth's interior]], and yet they are refracted slightly when they pass through the transition between the semisolid [[mantle (geology)|mantle]] and the liquid [[Internal structure of Earth#Core|outer core]]. As a result, there is a P wave "[[shadow zone]]" between 103° and 142°<ref>Lowrie, William. ''The Fundamentals of Geophysics''. Cambridge University Press, 1997, p. 149.</ref> from the earthquake's focus, where the initial P waves are not registered on seismometers. In contrast, S waves do not travel through liquids.

===As an earthquake warning===

Advance earthquake warning is possible by detecting the nondestructive primary waves that travel more quickly through the Earth's crust than do the destructive [[S wave|secondary]] and [[Rayleigh wave]]s.

The amount of warning depends on the delay between the arrival of the P wave and other destructive waves, generally on the order of seconds up to about 60 to 90 seconds for deep, distant, large quakes such as the [[2011 Tohoku earthquake and tsunami|2011 Tohoku earthquake]]. The effectiveness of a warning depends on accurate detection of the P waves and rejection of [[ground vibrations]] caused by local activity (such as trucks or construction). [[Earthquake early warning]] systems can be automated to allow for immediate safety actions, such as issuing alerts, stopping elevators at the nearest floors, and switching off utilities.