Seismic tomography

Earth has a hidden inside.

We use shakes to see it. These shakes come from earthquakes. They can also come from man-made blasts.

Shakes move through the ground. They change as they go. This happens because of what is in the way. Some parts of Earth change how fast the shakes move.

Scientists look at these changes. They use a computer to make a map. This map shows the inside of our world.

It is like a special scan for the Earth. It helps us see deep down. Can you imagine seeing inside the Earth?

Scientists want to see deep inside the Earth. They cannot dig that far. Instead, they use a way called seismic tomography.

This method uses seismic waves. These are shakes that move through the ground. They come from earthquakes or man-made blasts. As these waves travel, they change. They might speed up or slow down. This happens because of the material in the way. Some parts of the Earth are hot or have different chemicals. These things change how fast the waves move.

To make a map, scientists use an inverse problem. This is a set of steps to find an answer. They start with a guess of what Earth looks like. Then, they compare that guess to the real wave data. They change the model bit by bit. They do this until the guess matches the data.

It is like a CT scan for a person. A CT scan uses X-rays to make a 3D image. Seismic tomography uses waves to make a 3D model.

This helps us see big things like plumes. Plumes are large areas of hot material deep down.

Scientists want to see deep inside our planet. They cannot dig deep enough to see the core. Instead, they use a special way called seismic tomography.

This method uses seismic waves to create 3D models of the subsurface. These waves are shakes that move through the ground. They usually come from earthquakes or man-made explosions. By watching these waves, we can learn about the Earth's structure. This helps us understand the layers beneath our feet.

Seismic waves change as they travel through different materials. They might speed up or slow down along the way. This happens because of changes in temperature or chemicals. These changes cause the waves to bend or reflect. Scientists use different types of waves like P waves and S waves. They also use surface waves called Rayleigh and Love waves. Each wave type has its own benefits for making images.

To build a model, scientists solve what is called an inverse problem. They start with an initial guess of what the Earth looks like. Then, they compare this guess to the real data from seismometers. They change the model in small steps to make it better. This process continues until the model matches the observed data. It is very much like a medical CT scan. A CT scan uses X-rays to make a 3D image of a person.

People have been studying these waves for a long time. In the early 1900s, scientists used them to find the outer core. Modern tomography really began in the 1970s. This was possible because of new global seismic networks. These networks were first built to watch for nuclear tests. In 1976, researchers used local earthquakes to map Southern California. By 1984, the term "tomography" was first used in seismology. This was a big step for the science.

Seismic tomography helps us see huge things deep underground. It can show us subducted slabs or large plumes.

A plume is a large area of hot material. These images help us see the mantle and the crust. Some methods look at the whole Earth at once. Other methods look at small areas using special tools. This helps us understand how our planet works from the surface to the core.

Seismic tomography is a sophisticated imaging technique used to map the subsurface of the Earth. It allows geoscientists to visualize structures deep underground, ranging from the upper few meters of the crust to the planet's core. By using seismic waves, researchers can create detailed 3D models of the Earth's interior. These models are usually seismic velocity models, which show how fast waves travel through different regions. These variations in speed can reveal important information about the thermal, structural, or chemical makeup of the subsurface.

The process relies on the way seismic waves interact with different materials. These waves are typically generated by natural earthquakes or man-made sources like explosions. As waves travel through the Earth, their properties are modified by the materials they encounter. For example, changes in temperature or chemical composition can change the velocity of the waves. These velocity changes cause the waves to undergo reflection or refraction, which means they bounce or bend. To create an image, scientists solve what is called an inverse problem. They begin with an initial Earth model and compare its predictions to actual seismic data recorded by seismometers. The model is then modified repeatedly until the predicted data matches the observed data as closely as possible.

Different types of seismic waves provide different types of information for these models. P waves, or primary waves, are used in most local and global models because they provide high-resolution images of the mantle. When there is not enough earthquake activity or seismograph coverage, scientists use S waves, or secondary waves. S waves can also be used alongside P waves in differential arrival time models. Additionally, surface waves like Rayleigh and Love waves can be used to image the crust and upper mantle. While these surface waves have lower frequencies and provide lower resolution, they are useful when body waves like P and S waves are unavailable.

Seismic tomography can be applied at various scales, from local to global. Local tomography often uses temporary seismic arrays to image the crust and upper mantle. Some local methods, like reflection tomography, use artificial sources to resolve small-scale features at crustal depths. Other methods, such as teleseismic tomography, use waves from distant earthquakes that travel upward to a local array. On a much larger scale, regional and global tomography focuses on massive features. These models often use long wavelengths to image large structures like subducted slabs or superplumes. Because they look at such large areas, these global models have a coarser resolution, often measuring hundreds of kilometers.

The history of this science is tied to both geological discovery and global politics. In the early 20th century, seismologists used travel time variations to discover the Moho and the depth of the outer core. However, modern tomography did not emerge until the 1970s. This was driven by the expansion of global seismic networks, such as the World-Wide Standardized Seismograph Network. These networks were originally created to monitor underground nuclear tests. The availability of standardized datasets, combined with growing computing power, allowed scientists to solve large inverse problems. In 1976, a major study used local earthquakes to map the 3D velocity structure beneath Southern California. By 1984, the term "tomography" was officially applied to seismology, borrowing the name from medical X-ray tomography.

Technological advancements have significantly increased the accuracy of these images. Early methods often treated seismic waves as simple 1D rays, a method known as ray theory. While ray theory fits travel-time data well, it is a simplification. Newer methods, such as finite-frequency methods, attempt to account for the wider paths that waves actually take. Even more advanced is waveform tomography, or full waveform tomography. This method abandons ray theory entirely to model the full complexity of seismic wave propagation. While these methods were once too computationally expensive for global studies, modern numerical modeling has made them more common.

Understanding these seismic models is vital for grasping the complex systems of our planet. For instance, attenuation tomography helps distinguish between thermal and chemical variations. It does this by measuring attenuation, which is the energy loss that occurs as waves travel through the Earth. Because attenuation is highly sensitive to temperature, it provides a clue as to why a certain area might be slowing down waves. This helps scientists move beyond simple velocity models to understand the actual physical state of the mantle and crust. By combining different data types through adjoint inversions, researchers continue to build a clearer picture of the world beneath our feet.