He reasoned that the waves that traveled farther were faster because they bent down deep enough to get into different rocks where they could travel much faster (those of the mantle) before being bent upward back into the crust (Figure 3.6).Īn analogy would be hiking in the wilderness and deciding that instead of taking a long winding trail, you’ll take a shortcut through a swamp. He noticed that sometimes, seismic waves were detected at seismic stations (measuring locations) farther from an earthquake before they were detected at stations closer to the earthquake. In the early 1900s, Croatian seismologist Andrija Mohorovičić (pronounced Moho-ro-vi-chich) made one of the first seismology-related discoveries about Earth’s interior. Discoveries with Seismic Waves The Moho: Where Crust Meets Mantle Over all, seismic rays tend to take curved paths through the Earth because refraction bends their path until they’re reflected and directed upward again, as in Figure 3.5. Pressure increases with depth within the Earth, so broadly speaking, seismic waves can gofaster deeper within the Earth. Seismic velocities are higher in more rigid layers, and higher pressures tend to make layers more rigid. In Figure 3.5, this bending causes the ray to go at a shallower and shallower angle on the way down, and then at progressively steeper angles on the way up. If the wave can travel faster in the new layer, it will be bent slightly toward the slower layers. If the wave travels at a different speed in the new layer, its path will be bent, or refracted, as it crosses into the new layer. But some waves will travel through the layer. When seismic waves encounter a different rock layer, some might bounce off the layer, or reflect. Dashed arrows on the right show the direction the ray would have travelled without refraction. Seismic ray paths are refracted (bent) when they enter a rock layer with a different seismic velocity. The paths of seismic waves can be represented as rays. Figure 3.5 Seismic waves and seismic rays. Seismic waves travel in all directions from their source, but it’s more convenient to imagine the path traced by one point on the wave front, and represent that path as a seismic ray (heavy arrows, Figure 3.5). This is handy because observing where P-waves travel, and S-waves do not, allows us to identify regions within Earth that are melted. This is similar to the way ultrasound is used to image the human body.Īnother feature of seismic waves is that some, called P-waves, can travel rapidly though both liquids and solids, but others, called S-waves, can only travel though solids, and are slower than P-waves. By measuring how long it takes for seismic waves to travel from their source to a recording station, and applying knowledge of how they interact with different materials, we can figure out where Earth’s layers are, and what they’re like. Seismic waves travel through different materials at different speeds. You may have heard of seismology in the context of detecting and studying earthquakes, but vibrations can also come from extraterrestrial impacts, explosions, storm waves hitting the shore, and tides. Seismology is the study of vibrations within Earth.
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