Why does energy move in waveforms

Earthquake waves and their effects

If the rock breaks at a tectonic fault under the pressure of the pent-up tension, the released energy is given off in the form of waves - the earth shakes. The frequently perceived sequence of shaking, rolling and rocking in a quake is due to waveforms following each other at different speeds.

P and S waves

In the case of the fastest propagating waves - the P waves or primary waves - the rock particles vibrate - similar to water waves in a pond - in their direction of propagation, the rock is alternately compressed and stretched. In contrast to water or air, rock can also vibrate transversely. With this oscillation, the so-called transversal or S-waves, the soil particles move back and forth across the direction of propagation of the waves. The rock is deformed and shaken horizontally or vertically.

Both waveforms oscillate in a frequency range between 0.1 and 30 Hertz. While the P waves can propagate equally in liquids and solid matter, the S wave can only propagate in solid, shearable rock and is therefore "swallowed" by the liquid areas of the earth's interior. The P waves travel at 6 to 13 km / sec, almost twice as fast as the S waves, which travel between 3.5 and 7.4 kilometers per second. The distance from the quake focus or the focus depth can be calculated from the time interval between the two wave types in a seismogram.

Surface waves

When the S and P waves reach the surface or the boundary layer of a geological structure, they are reflected or converted into other wave forms, the so-called surface waves. In these, the energy is only conducted along or near the surface; deeper underground, the rock movement is usually only minimal.

There are two main types of surface waves: Love waves, named after the English physicist Augustus E.H. Love, deform the rock in a horizontal direction. Owing to their often large amplitudes, these lateral vibrations of the ground are among the most destructive waves of an earthquake, as they can cause enormous damage, especially on buildings.

The second type of surface waves, first described by Lord Rayleigh in 1885 and named after him, generates the rolling movements of the subsurface during an earthquake, which are often described. During a Rayleigh wave, the rock particles move elliptically on a vertical plane.

First up and down, then shaking sideways

Since all these waves have a slightly different transit time, an earthquake actually consists of a sequence of different ground movements. The first P waves to arrive generate an up and down movement of the floor, but usually do not cause any major damage. Some time later the violent lateral shaking of the horizontal and vertical S-waves follows, which lasts a little longer than the P-waves.

Shortly afterwards, the Love waves arrive, followed by the Rayleigh waves. The trembling and rolling movements of these surface waves last a relatively long time and form the main part of an earthquake. The end of an earthquake is usually a mixture of the different wave types, which arrive with a delay due to multiple refractions and complex rock structures.

Localization of the quake focus

The position of the epicenter, possibly even the hypocenter, can be determined from the different transit times of P and S waves by means of triangulation. Since a P-wave is about twice as fast as an S-wave, the time interval with which the waves arrive at the seismograph increases with the distance they have traveled.

If an earthquake is registered by three seismological stations that are in different directions from the starting point of the earthquake, the point at which the three determined distance radii intersect corresponds to the epicenter of the earthquake. If one also wants to determine the hypocenter, either a measurement at a fourth station is necessary or three measuring stations must register additional P and S waves. By networking seismic stations via cables or with the help of standardized time stamps in the seismograms, even distant earthquakes can be localized more precisely.

5th February 2019

- Nadja Podbregar