Looking into the Deep Earth

Deep Earth Explorers song by Matthew Kemp
(performed at museum exhibit opening in March 2020)
Why are we so interested in the Deep Earth?

There are many big questions about how our planet functions. Our team believes that the Earth is an incredible place. As far as we know it is the only planet in space which can support complex life, and that we as humans can currently live. When it comes to the Earth and its large scale processes there is still a lot to be understood. We are fascinated by the Deep Earth, and how it is changing with time.  We create images of the deep structures using earthquake wave data collected around the globe. We carefully piece together, and understand more about our planet’s exciting formation and history; how it works today; and what our planet might become. 

The Earth is more than a set of layers, material is moving and changing with time 

The Earth is often shown as a set of simple layers: the solid iron inner core in the centre, then the liquid iron outer core, surrounded by the solid rock (silicate) mantle, and the thin cold crust on top that we all live on.  The Earth is very hot in the centre (~5000 degrees C), even though the whole planet has been slowly cooling down ever since it was first formed about 4.5 billion years ago. The cooling of the Earth and heat loss into space provides energy that drives lots of dynamic processes within the layers of the Earth, which constantly move and change with time.

Material moves around the Earth through a process known as convection. This is similar to how material in a lava lamp is heated at the bottom and rises up when it is hot, before cooling and sinking downwards again.

How can we understand more about the Earth?

Humans have never managed to drill beneath the crust. The deepest humankind has ever gone is around 12km in the Kola Superdeep Borehole in Russia. To put that in perspective, that is only 12km out of 6371km to the centre of the Earth, or less than 1%. It is unlikely that humans will ever go significantly deeper than this because even at 12km it is so hot and under so much pressure that it is very hard to stop the bore hole collapsing.

Seismic waves

Our team uses a different method to study the Deep Earth. We record and study earthquake wave data that has travelled through Earth’s interior. This is known as seismology. Seismology has allowed scientists to uncover many details inside the Earth’s interior. 

Earthquakes are destructive events on the surface, but we can also use them as a tool to learn about the Earth

Despite earthquakes having the potential to cause significant damage on the surface, our team actually uses them as a tool to learn more about the Deep Earth. Earthquake waves are recorded live on sensitive earthquake recording devices known as seismometers. The team collects these earthquake waves to make an image of the Deep Earth. 

When an earthquake happens, energy travels outwards in all directions as waves, similar to a ripple spreading across the surface of a pond after a stone is dropped. The earthquake waves travel outwards across the surface of the Earth, but also down through the Earth. The further away a wave is recorded, the deeper inside the Earth it has travelled.  Seismometers here in the UK can measure waves that arrive from big earthquakes that happen in Japan, South America or Alaska.

There are three main types of earthquake waves:

  1. Surface waves travel along the outside of the Earth.
  2. P waves are longitudinal or compressional, moving by stretching and squashing rock in  the direction of travel. They are a type of seismic wave known as body waves.
  3. S waves are transverse or shear, moving by shearing rock side-to-side,  perpendicular to the direction of travel. They are also a type of seismic body waves.

Sensitive devices called seismometers measure the tiny vibrations caused by earthquakes that happen all over the world. They record the motion of the ground which we match up with different seismic waves from the earthquakes.

P and S waves arrive before the surface waves on a seismometer recording. These smaller arrivals give a short warning time before the arrival of the very destructive surface waves.

A seismogram showing the up and down movement of the ground observed near Cambridge from an Earthquake in Alaska

Creating mantle images using seismic waves

We map out where the waves travel fast and slow, or bounce off different material, just like a doctor uses X-rays to “see” inside your body. However, unlike a doctor, we cannot cut open the Earth to see if our interpretation is correct. It can be difficult to understand exactly how material has been moving around over geological time scales. When we take a snapshot of the Earth the mantle appears static to us. This is because the mantle is moving extremely slowly. However, over the longer-term, this slow movement has consequences for the Earth. In contrast the outer core moves relatively fast, and generates powerful electric currents. 

Image shows a cut-out of the globe. The insides shows variations in seismic wave velocity as mapped by Auer et al. (2014).
An “x-ray” of the Earth made using earthquakes, showing variations in seismic wave speed. Red is slow, and is thought to be hotter material, blue is fast and is thought to be cold material. Surface shows geology and oceanic plate age – from Thorsten Becker’s webpage.

When seismic waves hit a sharp boundary, e.g. the change from the solid rock of the mantle to the liquid iron of the outer core, they can bounce off (reflect), change direction (refract), or even change type from P to S or S to P (convert). When the waves interact with these sharp boundaries we can create an image of the different layers and structures within the Earth. Earthquake waves travel slower through hot material, and faster through cold material, so the speed they travel to reach a seismometer, can show us hot and cold places within the mantle. 

We need results from other experimental scientists, who put rocks under very high pressures and temperatures, like those found deep inside the Earth to understand how rocks behave in those conditions. We also work with scientists who make computer models of how hot material moves and flows with time, to try to understand how the snapshot we can “see” today, would change – what it was like in the past, and what it might do in the future.

How we research the Deep Earth

Our comic strip, Life as a Researcher, from our online exhibition Deep Earth Explorers gives an example of what a research project life cycle might look like.

A lot of our work involves reading papers published in academic journals, conducting our own research, and communicating our findings to other scientists and the public.

Seismology is a very observation-driven field of science. As seismologists, we’re also data scientists, Earth scientists, physicists, mineralogists… To find our observations of the Deep Earth, we typically look at seismic data, which can be found on online repositories collected by organisations such as IRIS, the Incorporated Research Institutions for Seismology.

Now that we have data, we want to be able to process it. Our team primarily uses Python, a high-level programming language that is powerful and intuitive to use. However, we also use FORTRAN, GMT, bash, C++, MATLAB…

Over the years, we have built (and continue to build) a number of scripts that allow us to process the data in a number of interesting and informative ways, and given the very collaborative nature of scientific research, researchers also often build tools and release them for public use.

We use a wide range of tools available to us to help us identify features in the seismic wave-forms. Some of those with websites are listed below:

  • BurnMan: A Python toolkit that computes physical properties for potential Earth and planetary compositions. Very flexible for many applications. Co-created by Dr Cottaar as part of a CIDER workshop in 2012.
  • ObsPy: This Python library can handle seismic data downloading and processing. Loads of features!
  • AxiSEM: Spectral element method to compute  global seismic synthetics.​ Uses axisymmetric velocity model for computational efficiency.
  • Instaseis: This uses AxiSEM-produced databases to produce​ ‘instantaneous’ seismograms for 1D Earth models.
  • Mineos: Normal mode computations for 1D models to compute summed synthetic seismograms.
  • MSAT: Matlab Seismic Anisotropic Toolkit. Pretty much anything you want to do to an elastic tensor, you can do with this.

More geophysics software at Computational Infrastructure for Geodynamics.