Which wave is not able to travel through liquids?

S waves, or secondary waves, are shear waves that propagate through solids but cannot travel through liquids. This inability is due to their mode of motion, which involves moving perpendicular to the direction of wave propagation, creating shear stress. Liquids, lacking the necessary rigidity to support this type of movement, cannot transmit S waves effectively.

In contrast, P waves, or primary waves, are compressional waves that can travel through both solids and liquids because they involve the compression and expansion of material in the same direction as the wave propagation, which is possible in both solid and liquid states. Surface waves travel along the Earth's surface and also require a solid medium to propagate effectively.

Therefore, the identifying characteristic of S waves is pivotal in understanding seismic wave behavior, highlighting why they do not propagate through liquids, solidifying the answer to this question.