how the behavior of seismic body waves changes at the different boundaries within the Earth?
Jul 21, 2010
in
Earthquake Questions
how and why the behavior of seismic body waves changes at the different boundaries within
the Earth (refraction, reflection).
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One comment
Super Eurypterid on July 21, 2010 at 3:19 pm
You can write a book on this question. In fact, many have been written.
Essentially the behavior of seismic waves at boundaries are dictated by the impedance change at that boundary. The impedance of a layer is the Density * Velocity It is the difference of these two conditions that determine weather a seismic wave reflects, refracts, or is transmitted deeper into the earth.
Using Snell’s law you can determine the angle of reflection and transmission. This will tell you what percentage of energy travels deeper in the earth and what percentage is reflected back to the surface and at what direction. There are some special cases through, then the angle of transmission becomes 90 degrees the energy refracts along the surface.
This becomes a little mode complicated, because at every reflection point in the earth, not only does the seismic wave reflect and transmits through it, the wave also goes through a mode conversion. For example, if you have a P- wave ( compression wave ) and this strikes a major impedance boundary, you will get a P- wave the is transmitted ( at some angle ), a P-wave that is reflected ( at the incoming angle ), but you will also get a S-wave ( shear wave ) mode conversion so you will get a S-wave rreflected wave ( at a third angle ) and a S-wave that is transmitted ( at a fourth angle )
There seems to be a good app showing the mode conversion here : http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/modeconversion.htm