Traveling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.

mantle

P waves travel fastest and are the first to arrive from the earthquake. In S or shear waves, rock oscillates perpendicular to the direction of wave propagation. In rock, S waves generally travel about 60% the speed of P waves, and the S wave always arrives after the P wave.

A P wave, or compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

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. The energy is thus less easily transmitted through the medium, and S-waves are slower.

There are three basic types of seismic waves – P-waves, S-waves and surface waves. P-waves and S-waves are sometimes collectively called body waves.

There are two broad classes of seismic waves: body waves and surface waves. Body waves travel within the body of Earth. They include P, or primary, waves and S, or secondary, waves.

A seismograph, or seismometer, is an instrument used to detect and record seismic waves. They travel through the interior of the Earth and can be measured with sensitive detectors called seismographs. Scientists have seismographs set up all over the world to track the movement of the Earth’s crust.

The P wave is designated the primary preliminary wave because it is the first to arrive at a seismic station after an earthquake. It travels at a speed usually less than 6 kilometers per second in the Earth’s crust and jumps to 13 kilometers per second through the core.

tsunami detection buoys

A seismogram is a record of the ground motions caused by seismic waves from an earthquake. A seismograph or seismometer is the measuring instrument that creates the seismogram. Almost all seismometers are based on the principle of inertia i.e. a suspended mass tends to remain still when the ground moves.

S-waves only move through solids. Surface waves travel along the ground, outward from an earthquake’s epicenter. Surface waves are the slowest of all seismic waves, traveling at 2.5 km (1.5 miles) per second.

Tsunamis are detected and measured by coastal tide gages and by tsunami buoys in the deep ocean. The tide gages measure the tsunami wave directly. In the deep ocean, sensors on the ocean floor detect the pressure signature of tsunami waves as they pass by.

1958 Lituya Bay earthquake and megatsunami

The authors of the tsunami scale proposed a tentative, rough correlation with tsunami wave heights, which are also noted below. Damage grades are 1, slight damage; 2, moderate damage; 3, heavy damage; 4, destruction; 5, total collapse.

Lituya Bay

The tsunami may be perceived as nothing more than a gentle rise and fall of the sea surface. For the same reason of low amplitude and very long periods in the deep ocean, tsunami waves cannot be seen nor detected from the air. From the sky, tsunami waves cannot be distinguished from ordinary ocean waves.