Basic Geophysics The Structure of the Earth

In the early 20th century, seismologists wanted to find out more about the interior structure of the earth and recorded the arrival times of seismic waves on a travel- time graph. Initially, the arrival times of the various wave types were measured using seismometers. Researchers entered the arrival time of the wave after the earthquake on the vertical axis and the distance to the earthquake on the horizontal axis. Primarily, they only saw a cloud of dots on the diagram. However, the more earthquakes they plotted, the more clearly individual lines became apparent over time. What was the reason for this relationship between the arrival times of the individual earthquake waves and their distance? Hello and welcome. In this video, we will address the following questions: What is the structure of the earth's interior and how do we know this? Can we use seismological methods to “cut open” the earth and have a look inside? Early 20th-century seismologists already postulated that the curves on their travel- time graphs must be due to the structure of the earth. Furthermore, the graphs for all seismometers around the world more or less matched up, which meant that the underground structures were similar everywhere around the globe. Using the travel-time graphs, the scientists then calculated how the propagation velocities of the seismic waves changed with depth and how the shape of the curves observed came to be. For P-waves, the propagation velocity over depth is plotted here with a black line, and for S-waves with a gray line. The scientists saw that the change in seismic velocities of the P- and S-waves were particularly significant at certain depths. From this observation, the seismologists concluded that a change in the material composition must exist at these depths - that means that the earth's structure consists of different concentric layers in which seismic waves travel at different velocities. Today, we know that the earth has a radius of almost six and a half thousand km. If we compare the cross-section of the earth with a cut orange, we see that both are composed of various layers. The earth's crust corresponds to this thin outermost layer making up less than one percent of the radius, which I can remove here using a vegetable peeler. In the schematic diagram of the earth's interior, it is shown in brown. Under the oceans, it goes down to a depth of approx. 10 km, and under the continents to a depth of up to 30 km on average. Under the earth's mountains, it can reach a thickness of up to 60 km. The earth's crust consists of solid rock and forms the upper portion of the continental plates. Since it is relatively thin, the existing pressures and seismic velocities in the crust are lower than in the deeper layers. In the earth's crust, the P-wave velocity is between 5.8 and 6.5 km/s, and that of S-waves below 4 km/ s. The thickness of the actual orange peel corresponds approximately to that of the earth's upper mantle. It goes down to a depth of around 400 to 600 km. This is around five to ten percent of the earth's radius. In the diagram, it is shown in red. Below it is the lower mantle in bright orange. The deeper one goes into the earth, the denser are the rocks that are found there. This leads to an increase in the seismic velocities in the upper and lower mantle. P-waves reach velocities of almost 14 km/s, S-waves around 7 km/s. The boundary between the lower mantle and the outer core is located at a depth of approx. 2,900 km. From here on, the comparison with the orange no longer applies, and we need to use some other comparison. The radius of the earth's core is slightly more than half of that of the entire planet. In the image, it is represented in bright yellow. This corresponds approximately to this ping-pong ball. The inner core has a radius of around 1,200 km and accounts for as much of the earth's overall size as the innermost white part of the orange. At the core-mantle boundary, the P-wave velocity decreases and the velocity of S-waves drops to zero. This is because the outer core consists primarily of molten iron due to the high temperatures, which has no restoring forces in case of shearing. Hence, shear waves cannot travel through it. Inside the outer core of the earth, the P-wave velocity increases again up to the boundary with the inner core. Due to the high pressure in the inner core, the iron once again exists in a solid state, such that in addition to P- waves, S-waves can also travel through this layer. The velocity of both wave types remains constant here: for P-waves approx. 11 km/s and for S-waves around 4 km/s. At the boundaries of these major layers, seismic waves are refracted and reflected on their journey through the earth. Each point in the travel-time graph belongs to a seismic wave on its path through the earth. Each line or branch describes a combination of reflections and refractions at certain layer boundaries in the earth and has its own name. One example here is PKIKP. The PKIKP wave is a P-wave which, from the earthquake, first travels through the mantle and then through the outer core - the K here stands for the German "Kern". The wave is then reflected at the inner core - I for inner - and emerges back out on the other side of the earth through the outer core and mantle before reaching the recording seismometer. It occurs in various distance ranges, which result in the branch in the travel-time graph. Today, scientists can better determine the structure of the earth and resolve three-dimensional structures tomographically. This is made possible by the many seismic networks distributed across the earth which record waves from both big and small earthquakes. One result from such a calculation with several hundred earthquake records worldwide is, for example, this image of the earth's mantle. You see the variation in the S- wave velocity in the mantle. Red and yellow show slower velocities, while blue indicates higher velocities. Although the earth is made of layers, it contains heterogeneous structures which deviate from the layered structure by several percent. These variations in seismic velocities within the earth are, in many cases, caused by changes in temperature. Hence, the elastic parameters of the earth which determine the seismic velocity are temperature- dependent. Higher temperatures result in slower wave velocities, while lower ones result in higher wave velocities. High temperatures in the earth's core result from radioactive processes and the presence of residual heat from the formation of the earth. They lead to rock material in the mantle and outer core rising and being set in motion. During this process, convection cells are also created in the mantle, which transport material from the underground subduction zones and the mid-ocean spreading centers back to the surface. The animation shows the result of a model calculation, i.e. a possible pattern of movement based on current knowledge of the earth's material composition and temperature distribution. Reddish colors stand for warm, rising material, while bluish-green colors represent cold, sinking material. The movements here take place at velocities of up to ten centimeters per year. This corresponds approximately to the scale of the tectonic plate movements on the earth's surface which are driven by these very convection cells. In this video, I showed you that scientists can use travel-time graphs to calculate that the earth's structure is more or less made up of various concentric layers. You have learned about the characteristics of these layers. Let me summarize the most important properties once again: The earth consists of a crust, which is thinner under the oceans and thicker under the mountains. Beneath it is the mantle, in which rock material moves in convection currents. The earth's core is composed primarily of iron with an outer molten layer that also experiences convection currents, through which shear waves are unable to travel, and a solid inner core. These findings were made possible by evaluating the arrival of seismic waves at seismometers which record earthquakes around the globe. Today, three-dimensional heterogeneities within the layers can be resolved with the help of dense seismometer networks both globally and locally, constantly expanding our knowledge of the earth.

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