The Path of P and S Waves Through the Earth

Seismic waves are essentially energy waves that travel through and around the Earth, usually caused by earthquakes, volcanic eruptions or man-made explosions. These waves are broadly classified into two types: Body Waves and Surface Waves.
Body Waves are further divided into P waves (Primary waves) and S waves (Secondary waves).
P Waves: These are the first seismic waves to be detected during an earthquake, hence the name “Primary” waves. Also known as longitudinal or compressional waves, they have the ability to move through solids, liquids, and gases. This means they travel not only through the Earth’s crust but also its mantle and core.
P Waves’ path: They follow a curved path through the Earth, this is due to the varying composition and density at different depths within the Earth.
P Waves typically cause particles to move in the same direction in which the waves are moving, creating compressions and rarefactions similar to sound waves.
S Waves : These waves arrive after the P waves, which is why they are called “Secondary” waves. They are slower than P waves and are also known as transverse or shear waves.
S Waves’ path: Unlike P waves, S waves cannot pass through liquids or gases. This means they can only travel through the solid parts of the Earth’s structure. Consequently, they are not detected in the shadow zones, areas on Earth’s surface which are about more than 103 degrees from the epicentre of an earthquake.
S Waves cause particles to move perpendicular to their direction of travel, producing a side-to-side motion.
Both P and S waves cause vibrations in the Earth which can be detected by instruments called seismographs. By analysing P and S wave arrival times, the location of the earthquake epicentre can be calculated.
The difference in arrival times and the path of P & S waves aids in the understanding of Earth’s internal structure. Scientists have noted the absence of S waves and the bending of P wave paths, suggesting that Earth’s outer core is likely liquid.
The knowledge about the paths of these waves is also used in detecting and measuring the strength of explosions, including nuclear tests, so it has a crucial role in maintaining global security.