What is P Wave?
Short answer: A P wave is the fastest seismic body wave. It compresses and expands the ground in the direction of travel, passes through solids and liquids, and is the first arrival on a record.
| P wave | S wave | |
|---|---|---|
| Full name | Primary, compressional | Secondary, shear |
| Particle motion | Parallel to travel | Perpendicular to travel |
| Speed | Fastest; arrives first | Slower; arrives second |
| Liquids | Travels through liquids | Does not travel through liquids |
Questions worth quoting
Why is the P wave first?
It has the highest velocity of the seismic waves, so it reaches the geophone or hydrophone before the S wave and the surface waves.
Why can a P wave cross water when an S wave cannot?
A P wave is a pressure wave, and water can be compressed. An S wave needs a solid that can resist shear, so it dies out in liquid.
Related: What are seismic waves? · What is a hydrophone?
P wave is body waves that propagate through the planet. P-waves travel 60% faster than S-waves on average because the interior of the Earth does not react the same way to both of them.
P-waves are compression waves that apply a force in the direction of propagation. As the interior of the Earth is almost incompressible, P-waves transmit their energy quite easily through the medium and thus travel quickly.
Why do P wave travel faster than S wave?
When an earthquake occurs, it releases energy in the form of seismic waves that radiate from the earthquake source in all directions. The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities. The fastest wave, and therefore the first to arrive at a given location, is called the P waves( Primary Wave ).
The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling. The S wave( Secondary Wave ) is slower than the P-wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling. Surface waves follow the P-waves and S waves.
