P waves are like sound waves---matter in one place pushes against adjacent matter compressing it. The result is a series of alternating stretched and compressed rock propagating in the same direction as the compression.
Because of their greater speed, the P waves are the first to reach any point on the Earth's surface. The first P-wave onset starts from the spot where an earthquake originates.
It can often be identified by taking a pulse and observing that the heartbeats don't occur at regular intervals, but a conclusive indication of AF is the absence of P waves on an electrocardiogram ....
or flutter ...
 See also: Time, Mass, Degree, Earth, Magnetic Field
  
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