Earthquakes produce shockwaves called seismic waves. These waves can be detected using seismographs. Some seismic waves are surface waves, while others can travel through the Earth.
The S wave shadow zone is the area of the Earth’s surface where S waves are not detected following an earthquake. This shadow zone has led geologists to a model of the Earth with a solid mantle and a liquid core. From the diagram, we can see the region where S waves are not detected. Geologists have been able to calculate the size of the liquid core from these measurements.