How earthquakes show us the inside of the Earth

Earth cross section with body wave linesIt is pretty amazing how much we know about the inside of the Earth, given that we can only directly observe a small part of it.

When we learn about the layers of the Earth, we learn that the crust is very thin, but that is only on the scale of the rest of the Earth. Relative to the scale of the tools we use to study the Earth, it is very thick, and conditions become terribly harsh quite close to the surface.

The deepest mines are only a few km deep, and the deepest hole ever drilled is about 12 km deep. Efforts to drill where the crust is thin have all had to stop when conditions became too hot for drilling equipment. No boreholes have ever come close to the depth of the mantle.

Due to our lack of access to the Earth’s interior, scientists must rely on indirect observations to learn about what is below the surface. One way they do this is by studying the movement of pressure waves as they travel through the interior of the Earth. This is called Seismology.

Video Overview

Unlike the surface wave*s we see moving across the surface of bodies of water, the waves Seismologist study move through material rather than along surfaces. These types of waves are called body waves. Large explosions, storm activity, meteorite impacts, and earthquakes all cause body waves to move through the Earth. The waves caused by earthquakes are the most widely studied.

When an earthquake occurs somewhere inside the Earth, two types of body waves form: P and S waves, where P stands for primary and S stands for secondary. These names relate to the speed the waves travel. Primary waves travel faster than Secondary waves and are therefore the first ones to arrive at any point distant from the epicenter of the earthquake. This difference in travel speed allows seismologists to determine the location of an underground earthquake.

P waves are longitudinal wave*s in which the particles move back and forth in the same orientation as the wave’s propagation. They are also called push waves because particle motion pushes energy along in the same orientation as the direction the wave moves.

S waves, also called shear waves, are transverse wave*s in which particle motion is at right angles to the direction the wave propagates. This is similar to the movement of a wave along a length of rope that has just been snapped.

Particle motion in P and S waves
Particle motion is parallel to the direction of wave propagation for P waves and perpendicular to the direction of propagation for S waves

Tracking the movement of these waves provide seismologists with information about the composition of the Earth’s interior. The Longitudinal motion of P waves can pass through solids, liquids, and gases, while liquids and gases prevent the shearing motion of S waves.

Hotter areas cause waves to travel more slowly, revealing the presence of hot spots. Molten regions cause P waves to slow down and completely stop S waves. Partially molten areas such as the asthenosphere weaken but don’t completely stop the S waves.

Interactions between Earth and waves
Characteristics of the material the waves pass through effect the waves' behavior in predictable ways.

Both S and P waves travel faster through more dense material. Since density* increases with depth, waves speed up as they move deeper into the Earth. The change in density also causes the waves to follow curved paths as they move through the Earth.

Wave speed up and refract as they go deeper
As waves travel into more dense material, they speed up and curve.

The curving of these waves is similar to the refraction* that occurs when a light ray passes through an interface between two media (like air to glass), but rather than an abrupt change in direction the waves undergo a gradual change in direction as the density changes gradually with depth.

Red light refracting through a prims
Light refracts as it passes from one medium to another

The fact that these waves travel at different speeds at different depths results in some surprising behavior. When an earthquake occurs near the surface, body waves move out in all directions. Waves that start out moving deeper into the Earth encounter higher density material and start moving faster. At the same time, they undergo gradual refraction that can eventually reach a criticalangle, resulting in them turning back towards the surface.

Other waves from the same source remain near the surface and therefore move at a slower constant speed. The faster deeper waves the deeper waves arrive at a point far from the earthquake sooner than the shallow waves.

Waves that move deeper travel faster through the more dense material
Waves that go deeper travel faster and arrive at distant location sooner

P and S waves do refract more abruptly when they pass through the major transition zones separating the layers of the Earth.

Understanding how the composition, phase, temperature, and density of the material waves pass through affects their speed, direction, and refraction patterns have allowed scientists to infer a great deal about the Earth’s interior.

The distinct interfaces between layers - such as the transition between the mantle and the outer core - are called seismic discontinuities. This term reflects the fact that it is seismic data that identified these regions.

The first discontinuity waves encounter on their journey into the Earth is the Mohorovičić discontinuity. This discontinuity is named after the Croatian scientist who discovered it. It is also commonly referred to as the Moho for short. It is the boundary between the crust and the mantle. The Moho shows up in seismic data as a distinct change in wave speed due to a change in density of the rock across this boundary.

The next discontinuity is the partially molten asthenosphere. This shows up as another change in velocity and a weakening of S waves.

As the waves move deeper into the Earth they encounter another discontinuity 670 km below the surface due to a change in the composition of the minerals that make up the upper and lower mantle. The density of the mantle increases here, resulting in a speeding up of the waves.

The next discontinuity is the core-mantle boundary. At this boundary, the sheer S waves disappear completely. The disappearance of S waves shows that the outer core is liquid. P waves refract significantly at the core-mantle boundary providing more information about the changes in composition here.

The disappearance of the S waves and the refraction of the P waves create shadow zones where no waves are detected. Consistent with the liquid outer core preventing the propagation of shear waves, no S waves are detected past 103 degrees from the origin of the waves. The refraction patterns of P wave creates a gap in P wave detection between 103 and 143 degrees.

Beyond 143 degrees the speed of travel and the refraction patterns of P waves is consistent with there being another seismic discontinuity between a molten outer and solid inner core.

Cross section of Earth showing path of waves
The refraction patterns of P waves and the blocking of S waves by the liquid outer core create shadow zones where no waves are detected afer an earthquake

By combining information about the speed and paths of seismic waves as they travel through the Earth’s interior with

  • Laboratory experiments about how different minerals behave at different pressures
  • Observations about how the Earth’s magnetic and gravitational fields vary over space
  • measurements of how much heat escapes from the Earth’s interior
  • Analysis of minerals from inside the Earth brought to the surface by tectonic activity and
  • The study of asteroids formed from the same material that combined to form the early Earth.
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