《《Seismic Tomography—Using earthquakes to image Earth’s interior》.pdf

Seismic Tomography—Using earthquakes to image Earth’s interior Background to accompany the animations videos on: IRIS’ Animations Link to Vocabulary (Page 4) 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. If the Earth were of uniform composition and density seismic rays would travel in straight lines as shown in Figure 1. But our planet is broadly layered causing seismic rays to be refracted and reflected across boundaries. Layers can be inferred by recording energy from earthquakes (seismic waves) of known locations, not too unlike doctors using CT scans (next page) to “see” into hidden parts of the human body. How is this done with earthquakes? 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 arrival times of different Figure 1: Image capture from animation depicts 15 seismic stations seismic waves scientists are able to define slower recording earthquakes from 4 earthquakes. An anomaly (red) is or faster regions deep in the Earth. Those that come inferred by using data collected from stations that indicate slower sooner travel faster. Later waves are slowed down by wave speeds. something along the way. Various material properties control the speed and absorption of seismic waves. Careful study of the travel times and amplitudes can be used to infer the existence of features within the What variables yield the best resolution that allow