<Dl> <Dd> ∇ ⋅ u = 0 (\ displaystyle \ nabla \ cdot \ mathbf (u) = 0) </Dd> </Dl> <Dd> ∇ ⋅ u = 0 (\ displaystyle \ nabla \ cdot \ mathbf (u) = 0) </Dd> <P> Its name, S for secondary, comes from the fact that it is the second direct arrival on an earthquake seismogram, after the compressional primary wave, or P - wave, because S - waves travel slower in rock . Unlike the P - wave, the S - wave cannot travel through the molten outer core of the Earth, and this causes a shadow zone for S - waves opposite to where they originate . They can still appear in the solid inner core: when a P - wave strikes the boundary of molten and solid cores, S - waves will then propagate in the solid medium . And when the S - waves hit the boundary again they will in turn create P - waves . This property allows seismologists to determine the nature of the inner core . </P> <P> The prediction of S - waves came out of theory in the 1800s . Starting with the stress - strain relationship for an isotropic solid in Einstein notation: </P>

S waves cannot travel through which part of the earth