Seismic Tomography

 

Activity

Preview the animations by pressing the play button on them. If you want to view a better version, click the links labeled "Direct Link to Animation".

If you cannot view the YouTube videos below, click the links next to them labeled "Direct Link to Animation" to view the videos using Quicktime Player.

Introduction

Seismic tomography is an imaging technique that uses seismic waves generated by earthquakes and explosions to create computer-generated, three-dimensional images of Earth's interior. This is how seismologists infer the different layers in the Earth. How is this done? The time it takes for a seismic wave to arrive at a seismic station from an earthquake can be used to calculate the speed along the wave's ray path. By using first arrival times of P waves recorded by seismic stations all over the world, scientists are able to define slower or faster regions deep in the Earth. Those that come sooner travel faster. Those that come later are slowed down by something along the way. Human CAT scans (see below) are often used as an analogy. (More about seismic tomography below.)

In this animation we simplify things and make an Earth of uniform density (isotropic; constant velocity sphere) with a slow zone that we image as a magma chamber for simplicity. For reference, the Earth figures below show the difference between our simple animation and a body with changes in rock type and temperature that cause the seismic waves to refract and bend when transmitted between different rock compositions.

Direct Link to Seismic Tomography (Small 2.8mb)

Direct Link to Seismic Tomography (Larger 3.1mb)

Direct Link to Seismic Tomography - Spanish (3mb)

A CAT (computed axial tomography) scan animation is included because seismic tomography is often compared to CAT scans. Both techniques have an energy source (seismic tomography uses the energy generated from earthquakes; CAT scans use x-ray energy) and a receiver (seismic tomography uses seismograph stations; CAT scans use comtuters) that records the data. For more, see CAT scans & seismic tomography below.

Direct Link to Cat Scan (Small 2.1mb)

Direct Link to Cat Scan (Larger 2.1mb)

Direct Link to Cat Scan - Spanish (2.1mb)

This figure to the left shows the difference between our simple animation and a body with changes in rock type and temperature that cause the seismic waves to refract and bend when transmitted between different rock layers. [From Seismic Waves and Earth's Interior.] (For actual rates of travel compared to rates within a uniform crust, see P-wave travel times (PDF).)

More

The bulk character of the rock, such as composition, density, elastic moduli, mineralogy and phase (ex. the presence of melt).

Elastic waves may propagate through the earth in a manner which depends on the material properties of the earth.

The elasticity of the material provides the restoring force of the wave. When they occur in the Earth as the result of an earthquake or other disturbance, elastic waves are usually called seismic waves.

Typically, seismic waves travel faster through cooler, more rigid material than through its hotter, less rigid equivalent. But it is never so simple.

Various material properties control the speed and absorption of seismic waves. Careful study of the travel times and the amplitudes can be used to infer the existence of features within the planet.

Seismic waves travel at speeds of several kilometers per second in the Earth, with the speed of compressional waves (P waves), being about 1.7 times faster than that of shear waves (S wave). Seismic wave speeds are different in different kinds of rock., Speed increases with pressure (which is very nearly a function of depth alone) and decreases with an increase in temperature. What variables yield the best resolution to infer details at depth?

1) A high-density array of earthquake stations around the area being studied
2) Many earthquakes recorded by every station
3) Earthquake signals coming to stations from different parts of the world

Complications of This Technique

Seismic tomography is a rapidly evolving discipline. Observed anomalies within the Earth are open to discussion and controversy, making it a dynamic science. Because of this, creating an animation is a moving target. Our aim is merely to give a generalized picture of the underlying principles.

Two main complications of this techniquie are: 1) seismic waves don't move in a straight trajectory away from an earthquake, but refract, or bend in response to changes in density; and 2) seismic waves can bounce off of sharp boundaries such as the boundary between the core and mantle, and between the crust and the atmosphere. (Caveats.)

Several aspects of graphical presentation may make it difficult to interpret three-dimensional models in terms of Earth structure and processes:

A model typically includes a huge amount of information, only a small fraction of which is shown, usually in the form of maps at various depths or vertical cross sections. The visual impression of the results given may be very sensitive to the precise position of the section. Wave speeds are usually displayed with colors, blue representing high wave speeds and red representing low ones. It is natural to associate red colors with higher temperatures. However, many factors affect the wave speed, including composition, crystal orientation, mineralogy and phase (especially the presence of melt). Red anomalies may not really be hot, nor blue ones cold.

The eye's sensitivity to color varies greatly across the spectrum, so inevitably some features are prominent while other, equally strong ones, are nearly invisible. For example, the transition from blue to yellow is much more noticeable than that from orange to red.

If the color scale saturates, anomalies may look the same when they actually differ in strength by an order of magnitude. For example, lower-mantle anomalies are much weaker than upper- mantle ones, but this is obscured in figures where the color scale saturates at the maximum lower-mantle anomaly Because of these factors, the appearance of tomographic images may be highly variable, depending on graphical design choices made by seismologists. [from Seismology: The hunt for plumes]

CAT Scans & Seismic Tomography

For lack of a perfect analogy, seismic tomography is often compared to a CAT scan (Computed Axial Tomography). The CAT scan uses computers to generate a three-dimensional image from a lot of flat X-ray pictures. The basic idea of a CAT scan is this: The X-ray beam is the energy source, which sends its signal (electromagnetic radiation) to the receiver (computer), which then captures and stores the data. Images are collected from hundreds of angles and the computer analyzes the information to produce a three-dimensional image. X-rays are absorbed unequally by different materials, and computer-aided tomography consists of studying the attenuation (reduction in intensity and amplitude) of X-rays that pass through the body.

The weakness in this analogy is that the observed quantity in a CAT scan is not a travel time, but rather the amount of X-ray absorption (attenuation). Seismic tomography uses the same principles, with the difference that the travel-times of the signals, rather than their attenuation, are observed. In terms of wave behavior, ultrasound imaging is more analogous to P-wave tomography, where compressive waves are reflected and refracted off materials of different composition and density. We use the CAT scan because it is commonly referred to without fully understanding how it works, plus it does provide an energy-receiver analogy.

Seismic tomography is much more difficult than X-ray tomography, because the ray paths are curved and initially unknown, and in some cases the locations of the sources are poorly known.

Compared to CAT scans, how does seismic tomography work?

Seismic tomography uses seismograms from thousands of local and global earthquakes to measure the speed of sound waves through the earth. With seismic tomography, the energy source is the earthquake. The earthquake sends its signal (seismic waves) to the receiver (seismograph), which records the data. Unlike an X-ray beam that shoots in one direction, the earthquake sends seismic waves in all directions, so instead of having to move the energy source around the Earth, scientists use multiple seismometers. One seismometer can only tell you that there has been an earthquake (as one X-ray can only tell you that you have bones). Many closely spaced seismometers yield three dimensional information about features below Earth's surface.

To determine how features seen at Earth's surface correlate with structural and compositional differences deep within the planet, seismologists need denser networks of seismic stations so that they are recording seismic waves that propagate through finer and finer slices of the earth beneath them. It's important to understand that tomographic techniques give a very coarse look at the subsurface.

Ongoing seismic-tomography experiments in the U.S. today

USArray, a component of EarthScope, is a fifteen-year program to place a dense network of permanent and portable seismographs across the continental United States. . The thousands of seismograph locations on public and private lands across the continental United States record local, regional, and distant (teleseismic) earthquakes. Hundreds of earthquakes occur throughout the world every day. By analyzing the seismograms of these earthquakes, scientists can learn about Earth structure and dynamics, and the physical processes controlling earthquakes and volcanoes. (LEARN MORE: USARRAY)

Resources

 

To save these animations to your computer. Right click one of the links titled "Direct Link to ..." and choose "Save Link As" or "Download Linked File" depending on your browser.

Animations By Jenda Johnson
Please send any comments or questions to