Uncovering the geologic structure beneath Earth’s surface is important for a variety of reasons, including identifying natural resources and studying earthquake hazards. Scientists image Earth’s subsurface by observing seismic waves that travel through our planet.
When an earthquake occurs, several types of waves radiate from the event’s source. P waves and S waves move through the interior, and Love and Rayleigh waves (known as “surface waves” or “normal modes”) move across Earth’s surface. The speeds at which these waves travel and how each type is scattered in the subsurface can reveal a wealth of different information about underground structure.
To study seismic waves, scientists generally must wait for earthquakes, which are unpredictable, or create their own using explosions. In the past couple of decades, however, researchers have developed additional ways to study the subsurface noninvasively by using ambient noise, the constant hum of sounds made by people and natural processes, like mining activities or waves crashing on a beach.
—XXXX, Science Writer