Thanks, again, John for your valuable and interesting comments here and on other recent threads.

You state: "Another reason for stronger motions at the surface is the low impedance of the material there. A shear wave will wiggle the surface much more than the rock deep in a mine because it has a lower modulus." With my imperfect understanding, I rather assumed that it WAS the constrained nature of materials at depth that kept surface waves from being able to cause strong motion there. As my ignorance extends to a lack of full understanding of the terms "impedance" and "modulus", as they are used in seismology, I'm still not clear on what the "two explanations" are, and how they differ . . .

Mike W.
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