Well, when rocks break, at the point of rupture stress, different types of waves are sent out. The two of concern here are the P and the S waves. The P waves travel through solids , liquids and gases. The S waves travel only through solids. Because liquids and gases offer no resistance to shear, no S waves propagate through a liquid or gas. Hence, because the earth has a shadow zone, a region where no S waves appear, we can infer the presence of a liquid outer core.