The Asthenosphere, located just below the lithosphere, is a “Low Velocity Zone” because the rocks there are near their melting point and are slightly plastic. This semi-fluid state causes seismic waves, especially S-waves, to slow down as they enter this layer. ANSWER: (C) Asthenosphere
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The LVZ exists between depths of about 100 to 250 km. The presence of a tiny percentage of melt (liquid) between the crystal grains of the peridotite rock makes the asthenosphere less rigid. Since seismic wave speed is directly related to the rigidity of the material, they slow down. This discovery was vital because it explained how tectonic plates (the rigid lithosphere) are able to slide across the Earth’s surface—they are literally “lubricated” by the more pliable, low-velocity asthenosphere layer below.