The lines represent P-waves radiating out from the earthquake. The different colors of the lines represent the velocity of the waves: Note that the lines turn blue as they cross the Moho, because they speed up.
A flat line in the seismograph means that the ground is stable. When the P waves arrive from an earthquakes, there are large squiggles in the seismograph.
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This animation simulates what Andrija Mohorovičić observed in 1909.
Click on Earthquake to start the simulation. Roll your mouse over
or tap the animation for more explanation.