A geologist is studying seismic waves produced by an earthquake. The geologist notices that some monitoring stations stop receiving wave signals when the waves reach a layer of liquid rock deep underground. Explain, using your knowledge of P-waves and S-waves, why some monitoring stations stop receiving signals while others continue to receive signals.

S‑waves are transverse waves; their particle motion is perpendicular to the direction of travel. Because of this, they cannot propagate through a fluid or liquid, which cannot support shear stresses. When the seismic waves reach a liquid rock layer, the S‑waves are blocked and cannot continue beyond that layer. Monitoring stations that lie in a position where the only path from the source to the station involves the liquid layer therefore stop receiving the S‑wave signal.

P‑waves are longitudinal waves; their particle motion is parallel to the direction of travel. They can propagate through solids, liquids and gases because they only require compression and rarefaction, not shear. Thus P‑waves can pass through the liquid rock layer and reach stations that are beyond it. Those stations continue to receive the P‑wave signal even though the S‑wave has been stopped.

In summary, stations that lose the signal are those that rely on S‑waves passing through the liquid layer, while stations that keep receiving the signal are those that are reached by P‑waves, which can travel through the liquid.