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SPAZIO CLIL Exploring the Earth’s interior with seismic waves T he push-pull motions of sound waves in a solid are called compressional waves to distinguish them from the side-to-side motions of shear waves. It’s harder to compress solids than to shear them, so compressional waves always travel faster than shear waves. Compressional waves are always the P (primary) arrivals, and shear waves are the S (secondary) arrivals. Another important property of seismic waves is that their shear wave speeds must be zero, because gases and liquids have no resistance to shear. Shear waves cannot propagate through any fluid: air, water, or the liquid iron in Earth’s outer core. From seismograms, geologists can calculate the speed of a P or S wave by dividing the distance traveled by the travel time. The measurements of these wave speeds can then be used to infer which materials the waves encountered along their paths. For example, P and S waves travel about 17 percent more rapidly through rock typical of oceanic crust (gabbro) than through typical upper continental crust (granite), and they travel about 33 percent more rapidly through the upper mantle (peridotite). Figure 1 In this experiment, two beams of laser light enter a bowl of water from the top. Both beams are reflected from a mirror on the bottom of the bowl. One is then reflected at the water-air interface and passes through the bowl to make a bright spot on the table. Most of the energy from the other beam is bent downward (refracted) as it passes from the water to the air, although a small amount is reflected to form a second spot on the table. You can also trace the paths of other beams reflected by the interfaces. (Susan Schwartzenberg/The Exploratorium) Fantini, Monesi, Piazzini - Elementi The concepts of travel times and wave paths sound simple enough, but complications arise when waves pass through more than one type of material. At the boundary between two different materials, some of the waves bounce off (that is, they are reflected) and others are transmitted into the second material, just as light is partly reflected and partly transmitted when it strikes a windowpane. The waves that cross the boundary between two materials are bent, or refracted, as their velocity changes from that in the first material to that in the second. Figure 1 shows a laser light beam whose path bends as it goes from air into water, much as a P or an S wave bends as it travels from one material to another. By studying how fast seismic waves travel and how they are refracted and reflected at Earth’s internal boundaries, seismologists have been able to measure the layering of Earth’s crust, mantle, and core with great accuracy. Paths of Seismic Waves in the Earth If Earth were made of a single material with constant properties from the surface to the center, P and S waves would travel from the focus of an earthquake to a distant seismograph along straight lines through the interior. When the first global networks of seismographs were installed about a century ago, however, seismologists discovered that the structure of Earth’s interior was much more complicated. The first observations of long-distance seismic waves showed that the paths of the P and S waves curved upward through the mantle, as illustrated in figure 2. From the travel times and the amount of upward bending, seismologists were able to demonstrate that P waves traveled much faster through rocks at great depths than they did through rocks found on the surface. This was hardly surprising, because rocks subjected to great pressures in Earth’s interior are squeezed into tighter crystal structures. The atoms in these tighter structures are more resistant to further compression, which causes P waves to travel through them more quickly. Seismologists were very surprised, however, by what they found at progressively greater distances from an earthquake focus. After the P waves and S waves had traveled beyond about 11 000 km from the earthquake focus, they suddenly disappeared! Like airplane pilots and ship captains, seismologists prefer to measure distances traveled on Earth’s surface in angular degrees, from di Scienze della Terra • Italo Bovolenta editore - 2013 1 SPAZIO CLIL 0° at the earthquake focus to 180° at a point on the opposite side of Earth. Each degree measures 111 km at the surface, so 11 000 km corresponds to an angular distance of 103°. When they looked at seismograms recorded beyond this distance, they did not see the distinct P and S arrivals that were so clear on seismograms recorded at shorter distances. Then, beyond about 16 000 km from the focus (143°), the P waves suddenly reappeared as big arrivals, but they were much delayed compared to their expected travel times. The S waves never reappeared. These observations were first put together in 1906 by the British seismologist R.D. Oldham, and they provided the first evidence that Earth has a liquid outer core. No S waves can travel through the outer core because it is liquid, and there is thus an S-wave shadow zone beyond 103° from the earthquake focus (see figure 2 B). The propagation of P waves is more complicated (see figure 2 A). At 103°, their paths just miss the core, whereas waves that would have traveled to greater distances encounter the core-mantle boundary. At the core-mantle boundary, the P-wave speed drops by almost a factor of two. Therefore, the waves are refracted downward into the core and emerge at greater distances after the delay caused by their detour through the core. This refraction effect forms a P-wave shadow zone at angular distances between 103° and 143°. Focus Focus km 0 16 km km 00 00 00 0 11 0 11 Inner core P-wave shadow zone P-wave shadow zone 103° Outer core 103° S-wave shadow zone Mantle 143° Figure 2 (A), the pattern of P-wave paths through Earth’s interior. The dashed blue lines show the progress of wave fronts through the interior, at 2 minute intervals. Distances are measured in angular distance from the earthquake focus. The P-wave shadow zone extends from 103° to 143°. P waves cannot reach the surface within this Fantini, Monesi, Piazzini - Elementi zone because of the way they are bent when they enter and leave the core. (B), the pattern of S-wave paths through Earth’s interior. The larger S-wave shadow zone extends from 103° to 180°. Although S waves strike the core, they cannot travel through its fluid outer region and therefore never emerge beyond 105° from the focus. di Scienze della Terra • Italo Bovolenta editore - 2013 2