Figure 9.3 Hitting a large block of rock with a heavy hammer will create seismic waves within the rock. You can think of a compression wave as a “push” wave - it’s called a P-wave (although the “P” stands for “primary” because P-waves arrive first at seismic stations). If you give it a sharp push so the coils are compressed, the compression propagates (travels) along the length of the spring and back (Figure 9.4). This is known as a compression wave, and it can be illustrated by holding a loose spring (like a Slinky) that is attached to something (or someone) at the other end. That compression will transfer to the neighbouring part of the rock, and so on through to the far side of the rock, from where it will bounce back to the top - all in a fraction of a second. At the point where the hammer strikes it, a small part of the rock will be compressed by a fraction of a millimetre. Imagine hitting a large block of strong rock (e.g., granite) with a heavy sledgehammer (Figure 9.3). The types of waves that are useful for understanding Earth’s interior are called body waves, meaning that, unlike the surface waves on the ocean, they are transmitted through Earth materials. Before going any deeper into Earth, however, we need to take a look at the properties of seismic waves.
Of course, seismic techniques have been most widely applied to the detection and study of earthquakes, but there are many other applications, and arguably seismic waves provide the most important information that we have concerning Earth’s interior. These vibrations are caused by various events, including earthquakes, extraterrestrial impacts, explosions, storm waves hitting the shore, and tidal effects. Seismology is the study of vibrations within Earth. 9.1 Understanding Earth through Seismology
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