The Earth is in constant motion, including through space, on its surface and throughout its core. Though movement on a continental level is so slow as to be imperceptible, it creates shock waves and reverberations that, on occasion, can be felt by those living on the surface. While most seismic activity is faint enough that only sensitive instruments can detect it, some waves become strong enough to move the ground beneath your feet in an earthquake. The first detectable shock wave preceding an earthquake is known as a preliminary wave, or P-wave, and it has a number of attributes that make it useful in furthering our knowledge of the planet.
Characteristics of a P-wave
P-waves are a type of seismic wave, meaning that it is a wave of energy resulting from activity within the Earth. A P-wave has an origination point, known as the epicenter in seismology, from which it moves uniformly outward. It is a compression wave that, if moving through air, becomes a sound wave, and its velocity is greater than any other waves associated with the activity.
P-waves and seismology
The detection of P-waves by seismometers is a good indication that an earthquake is about to ensue. The waves may be caused by the movement of continental plates or by volcanic activity. Earthquakes result when these plates grind together or, on occasion, when molten material from Earth's mantle strives to come through a weak point in the crust.
Earthquake early warning systems
Every early warning system is built on the ability to detect P-waves. The key is to ensure that the instruments can differentiate between movements within the Earth as compared to waves that are the result of activity above ground. These early warning systems allow recognition of the precursor waves, which then triggers an automatic warning for people within the earthquake area to take action. This is especially essential for individuals in high-risk jobs that can be severely affected by an earthquake, such as construction or surgery.
What P-waves can tell us about Earth's core
It is impossible-at least with current technology-to actually observe the Earth's interior. Obviously, humans can't survive below the crust. Even though certain technological advancements have allowed observation of the depths of the sea and even around volcanic vents, there is still nothing that can withstand the incredible temperatures and pressure of the interior. Instead, the composition of the Earth can be assumed based on observation of the behavior of waves sent through its layers.
Assuming that the speed of a wave is known in Earth's open atmosphere and how various states of matter will affect it, it's possible to know both the depth and approximate composition of each layer in the mantle and the core. The P-waves slow or speed up according to the density and pressure of the material through which it is passing. Using seismometers, humans can follow the progress of a given wave and document its behaviors. From this, scientists can form theories about what lies beneath the crust.