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Basic Geophysics Body Waves

Shown here are the arrival times of the seismic waves of the Great Tohoku Earthquake in Japan on March 11, 2011, which also gave rise to a devastating tsunami. It was recorded at the BFO seismometer station in the Black Forest. You see the arrival times of various wave types here, here, and here. In addition, I have also converted these seismic waves into acoustic ones so you can hear what earthquakes sound like. Listen to the wave types arriving in succession. Hello and welcome. Which earthquake waves did we see and hear in the example? What are seismic body waves, what are their properties, and what can we use them for? I will be addressing these questions in this video. Seismic waves from an earthquake spread out in all directions, cause the earth to oscillate, and can be measured using seismometers. If they are strong enough, we can also feel the waves. But what exactly is oscillating here? To answer this question, let's first look at other types of waves. Light also travels like a wave. In the case of light, it is the electromagnetic field that oscillates and which is interpreted by our eyes as brightness and color. Light can travel through various materials, but also in a vacuum. One other example are acoustic waves, that means sound waves. These waves cannot exist in a vacuum. Instead, they require a propagation medium in which molecules are excited and made to oscillate. Hence, they are similar to seismic body waves. Body waves also cause the earth which in this case is the propagation medium to oscillate. Let me show you the seismogram from the beginning again. It records vertical ground motion, that means the rise and fall of the earth's surface. These oscillations were triggered by the Tohoku quake over 9,000 km away from the BFO seismic observatory, which is located in the Black Forest and operated by the Karlsruhe Institute of Technology and the University of Stuttgart. The seismogram contains records lasting over an hour. Strong earthquakes can set the earth in motion for this long (or even longer!). There are different types of seismic waves which are triggered by an earthquake and which travel at different velocities through the earth. The various wave types we saw and heard in the example are the following: P-waves. These are the first or primary waves to arrive hence the P. They oscillate parallel to the direction of propagation this is called longitudinally which means that they are ultimately acoustic waves. P-waves move through the interior of the earth, which is why they are called body waves. In the animation, you can clearly see how all movement only takes place in one direction. S-waves - the S stands for secondary - arrive later. They, too, are body waves, but, unlike P-waves, they are transverse in nature. This means that the direction of oscillation is orthogonal to the direction of propagation. They are also called shear waves. In the animation, the movement is shown schematically upwards and downwards. However, it also takes place to the right and the left. S-waves generally oscillate slower than P-waves. You can see this in the seismogram by the larger distances between oscillations. This means that S-waves have a lower frequency than P-waves. Surface waves are the last to arrive. They have larger amplitudes than both types of body waves (P- and S-waves) and travel along the earth's surface. In this video, I will be focusing exclusively on body waves. In the acoustic representation of the earthquake, you heard the waves as various tones, or frequencies. Seismic waves therefore occur at different frequencies, whereby higher frequencies equate to faster oscillations and hence smaller wavelengths. The wavelengths and frequency are linked via the propagation velocity. The rule is as follows: The wavelength lambda ist equal to the propagation velocity v divided by the frequency f. In the seismogram, for example, the different types of body waves have different oscillation frequencies. In the case of light waves, we perceive different wavelengths as colors. Acoustic waves of different frequencies are heard as pitches. Hence, by measuring the wave frequency in a seismogram and armed with the knowledge of propagation velocities through rock, we can calculate the extent of the seismic wavelengths in the earth. It follows that major global quakes generate waves that have a wavelength of up to 1,000 kilometers. This is important if we wish to use seismic waves to investigate structures in the earth. When we speak of the seismic propagation velocity we need to remember that P-waves have a different velocity than S-waves. In the seismogram, we saw that the P-waves arrived before the S-waves hence, the P-waves are faster. With the help of laboratory measurements, it has been shown that for many types of rock, the velocity of P-waves is approximately 1.7 times higher than that of S-waves. Typical P-wave velocities in the earth's crust are 2-4 km/s for sandstone and 4-7 km/s for granite. The velocity is determined to a great extent by the crystal structure of the rock. Along with depth, the propagation velocity typically increases and can reach up to 14 km/s in the lower mantle. Because S-waves oscillate in a transverse manner, they can only travel through media that are capable of shearing in other words, materials that can be distorted in a transverse direction. Hence, shear waves are unable to travel through media that are not capable of shearing, such as water or the liquid outer core of the earth. However, P-waves can. In order to pass clear through the earth once, P-waves require approx. 20 minutes. In our example, the P-wave required around 12 min to travel from Japan to the BFO seismometer in the Black Forest. Now imagine the other seismometers that are distributed across the earth. The Tohoku earthquake on March 11, 2011, was observed at all these devices and the P-Wave arrival time was measured, for example, in China and Alaska. With the help of many of these travel time data, the time of the earthquake - the origin time - and hence the distances to the seismometers can initially be determined. By superimposing the individual distances, the underground source of an earthquake - the hypocenter - is then localized. The point directly vertically above this source on the earth's surface is called the epicenter. The hypocenter of the Tohoku quake was at a depth of 29 km, around 100 km off the Japanese coast. In addition, seismic body waves are also used to determine other characteristics of earthquakes: Differences in the travel times of P- and S-waves, for example, are of particular significance for determining the exact depth. Furthermore, maximum values of ground motion for calculating the earthquake magnitude and the direction of the initial arrivals are used to determine the focal mechanism. Another important application is the investigation of the earth's structure, which is called tomography. This technique allows structures in the earth's interior, the sizes of which correspond approximately to those of the seismic wavelengths observed, to be investigated. In this video, you became acquainted with the properties of seismic body waves. These waves are radiated by an earthquake in all directions through the earth as longitudinal P- and transverse S-waves. Hence, P-waves oscillate in the direction of propagation and are a type of acoustic wave. S-waves oscillate perpendicular to the direction of propagation, which is why they cannot exist in liquid media. P-waves have a higher frequency than S-waves and hence have shorter wavelengths. P-waves are also faster than S-waves. As an approximation, a factor of about 1.7 can be assumed. With the help of body waves, earthquakes can be localized and many other characteristics of the earthquake's source investigated. Seismic body waves provide us with information on the structures underground and help us to better understand the processes in the earth.

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