Re: [Phys-l] Disapointed. Was: Re: different types of waves have different speeds

[Bernard Cleyet <bernardcleyet@redshift.com>]

Thanks -- Gary!

Coincidentally!    That's the answer  I used in reply to the teacher's question.  
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These are s and p waves.    For example one can gauge distance from the epicenter of an earthquake by their separation.  The speeds of mechanical waves depend on the materials' elasticities.  The shear and Young's moduli are quite different, in all materials, AFAIK.


bc  experienced no separation from the Loma Prieta quake

p.s. FYI:

Seismic Waves

And:

Poisson's ratio, waves and deformation
The Poisson's ratio of a material influences the speed of propagation and reflection of stress waves. In geological applications, the ratio of compressional to shear wave speed is important in inferring the nature of the rock deep in the Earth. This wave speed ratio depends on Poisson's ratio. Poisson's ratio also affects the decay of stress with distance according to Saint Venant's principle, and the distribution of stress around holes and cracks. 

What is Poisson's ratio?

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This engenders another point  Evidently in semi-infinite (3-D)  media primary and secondary waves are described by the wave equation taught in AP (I presume) and frosh physics, i.e. second order in x & t.  While in the one D case it's beam bending and results in fourth order in x.  

I haven't (yet) found the derivation for the seismic wave equations, but several sites give their speeds.  The following gives the speeds and intuitizes why the compression wave involves both the bulk and the shear moduli.

http://appliedgeophysics.lbl.gov/seismic/seismic_21.pdf


Here maybe: (pages missing)


http://books.google.com/books?id=oRP5fZYjhXMC&pg=PA147&lpg=PA147&dq=telford+wave&source=bl&ots=C80mLmXwpd&sig=2QcjK1ZNrgpe6RXxIQ26JmCKW4k&hl=en&ei=bIqMS_fwLomCsgPUyr3pAw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CBkQ6AEwBQ#v=onepage&q=telford%20wave&f=false


bc


On 2010, Feb 28, , at 20:12, Gary Karshner wrote:

> There is a nice example - Earth Quake Waves
> They come in two types S-waves (Shear waves) 
> which are transverse and P-waves (pressure 
> waves), they travel at different speeds and their 
> separation is used to determine the distance to 
> the quake. Interestingly the shear waves won't 
> travel through the core of the earth, which is 
> why we say the core of the earth is a fluid. 
> Fluids won't support shear waves. The speed 
> difference is because it is they depended the 
> separate moduli - Shear and Youngs.
> Gary


much cut