What differentiates seismic waves in earthquakes?



by LORENZO CIOTTI

What differentiates seismic waves in earthquakes?
© Dimas Ardian / Stringer Getty Images

The study of the propagation of seismic waves inside the Earth has allowed seismologists, geophysicists and geologists to trace the internal structure of the Earth with good precision; this represents, together with the study of earthquakes, one of the main objectives of modern seismology.

By analyzing a seismogram it is easy to trace the type of seismic source that generated it and know all the characteristics necessary to identify it precisely. This allows us to easily distinguish an earthquake generated by a tectonic movement, a landslide, a volcano or an explosion.

A great push for the study of seismograms occurred in the years of the Cold War, in the context of military research.

But what are seismic waves?

Seismic waves are waves that propagate across the globe and are generated by an earthquake, volcanic activity or artificially by man through an explosion or another form of energization of the ground.

Volume Waves are those waves that propagate from the seismic source, through the volume of the medium involved, in all directions. P waves are compressional waves, also called longitudinal waves or prime waves. These are similar to acoustic waves and correspond to compressions and rarefactions of the medium in which they travel.

As they pass, the particles of the material they pass through perform an oscillatory motion in the direction of propagation of the wave. They are the fastest of the waves generated by an earthquake and, therefore, the first that are felt by a seismic station, hence the name P wave.

S waves are transverse waves that cause oscillations in the material they pass through perpendicular to their direction of movement. propagation. It can be imagined as waves propagating along a string of finite length, which is made to oscillate by moving its two ends.

Earthquake© Buddhika Weerasinghe / Stringer Getty Images News

An important characteristic of these waves is that they cannot propagate in fluid media. Surface waves are created due to the intersection of body waves with a physical discontinuity surface, the most studied of which is the free surface of the earth, the separation surface between the earth's crust and the earth's atmosphere.

These waves propagate guided along the surface and their energy decays exponentially with depth. Love waves are surface waves, also generated by the meeting of S waves with the free surface of the ground, but they are generated only in media in which the speed of the S waves increases with depth (therefore we are in the presence of a non-homogeneous medium ) and therefore they are always dispersed waves.

Love Waves make the ground vibrate on the horizontal plane in a direction orthogonal to the direction of propagation of the wave.