Wave Propagation

Wave Propagation. Part of the Exploring the Earth Using Seismology poster. Earthquakes are a constant reminder that we are living on the rigid crust of a cooling planet. By studying rocks near the surface of the Earth, geologists can infer the Earth’s interior structure to about 100 km depth. But what lies below? By analyzing the ground motion created by large earthquakes around the world, seismologists can explore the Earth's interior to its very center.
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The Earth<br />
The schematic on the left shows the basic structure of Earth's interior. Energy released by earthquakes creates seismic waves which travel through the Earth and are reflected and refracted at boundaries that separate regions of different materials. Shown here are the paths for seismic waves from the 1994 Northridge earthquake that were recorded at seismic stations around the world. Seismic station locations are marked as triangles and some are labeled with their station codes. Seismograms for these seismic stations are shown on the right.
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The Seismogram Section<br />
Below, each horizontal trace shows the arrival of seismic waves from the Northridge earthquake. The traces are the actual ground motion recorded at the seismic stations shown on the Earth. Some traces are labeled with the location of the seismic station at which they were recorded. The direct ray paths for P, S and Surface waves are shown in green. Seismologists compare the arrival times and amplitudes of seismic waves from many stations to infer the seismic velocity and hence the structure of Earth’s deep interior.


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