Earth is a layered planet. You can see the layers in the cross-sectional Earth view in the lower-right window. We stopped the action 8 minutes after the Northridge earthquake, just after the P waves passed into Earth's inner core, and just after the S waves arrived at the mantle-core boundary. At each of these major boundaries in the Earth, some seismic energy passes through as refracted waves, and other seismic energy bounces back toward the surface as reflected waves. Notice that no S waves can pass into Earth's liquid outer core. S waves move with shearing motions, but liquids can't be sheared.
