Amplification of S Waves

[matle]

Hi there,

Just trying to get a firmer grip on the rationale behind seismic classification of ground types.

EN1998 (& equivalent American codes) group grounds based on the shear wave velocity...this is because softer grounds amplify S-waves ...is this just because there is more movement of the ground particles and the movement caused is greater? Rather than for a stiffer ground where the particles movement is a lot more confined.

The general guidance is a <5m of soft ground sat atop stiff ground will tend to behave like stiff ground...the explanation of this will undoubtedly follow on from the above but is this because that sort of depth is insufficient for the S-wave to be 'stretched out' into a greater movement/slower speed?

Explanation/pointers to useful links online etc appreciated!

matle

 

[dgillette]

If I remember correctly, the magnification can be explained in terms of conservation of energy. In stiff material, it takes less strain (and therefore less displacement, velocity, and acceleration) to store a given amount of energy as elastic potential, as in a spring. When the upward-propagating wave hits the softer material, there has to be more strain, displacement, etc. to contain the same amount of elastic strain energy. (It's not actually the same amount of energy, because there would be a return wave going back down the rock from the contact, in addition to what propagates up through the soil, but a magnification factor of 2 or more is common for ground motions on bedrock outcroppings and on soil surfaces from the same source.) The 1985 Michoacan, Mexico earthquake is the classic example. Mexico City (in a bowl of soft sediments) was devastated, but areas of stiffer ground between there and the epicenter had little damage.

http://en.wikipedia.org/wiki/1985_Mexico_City_earthquake