Re: pinhole p waves

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From: pauld@exploratorium.edu
Date: Thu Jan 17 2002 - 13:14:24 PST 

Message-Id: <200201172114.g0HLEJi28606@isaac.exploratorium.edu>
From: pauld@exploratorium.edu
Subject: Re: pinhole p waves
Date: Thu, 17 Jan 2002 13:14:20 US/Pacific

Hi Kevin

I don't think they mean the asthenosphere here, otherwise known as the low
velocity zone thought to be low velocity because of the presence of regions
that are nearly liquid.

The upper mantle is a region hundreds of kilometers thick (600 km ?)
I can infer that the velocity there is higher than in the crust because seismic
waves go down into the upper mantle and then refract back up to the surface.
This requires a higher velocity at depth.

Indeed in a material of the same composition and pressure and density the P
waves will be faster in the solid than in the liquid. The shear modulus (which
is zero for liquids) adds a term to the velocity of P waves.

Hope this helps.

Paul D

> One of the questions on a review handout about seismic waves asks "Can
> you infer why P waves travel faster through the upper mantle than they
> do through the crust?" This seems contrary to what I remember about
> sound waves traveling faster through solids than liquids. I have a
> feeling I'm missing something, can someone shed some light?
> (P.S. I'm assuming they mean the liquid-like asthenesphere layer when
> they say 'upper mantle', but I may be wrong)
>
> Kevin Kinsella
> Science Explorer
> Woodside Elementary School
> 650.851.1571 x256
>
>
>
>
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