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Scientists know that the outer core of the earth is liquid by studying the seismic waves produced from earthquakes. There are two different types of seismic waves produced in earthquakes: the primary waves or P waves and secondary waves or S waves.Seismic monitoring stations all over the world are capable of picking up both P- and S-waves, but S-waves do not travel through liquid (they are attenuaed though), while P-waves not only do travel through liquid, they are refracted!
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