In a message dated 2006/10/24, carpediem1@......... writes:

> I believe that the intensity of light drops by the square of the distance 
> i.e., double the distance and the new value is the square root of the 
> distance. How does this apply to seismic waves which are basically sound waves? In 
> addition, how does this affect the amplitude and radiated seismic energy of the 
> waves?

Hi Bob,

       The transmission can get quite complicated. I suggest that you visit 
Larry Braile's website and download 'seismic waves' and other introductory 
programs. Some of the programs are quite large. See 
http://web.ics.purdue.edu/~braile/

       The initial Pressure and Shear waves are generated asymmetrically at 
the quake site, but travel outwards in all directions. They will experience 
square law effects and the higher frequency waves tend to be absorbed selectively 
with increasing distance. 
       Since the density of rocks tends to increase with depth, the ray paths 
are curved, not straight lines, due to changes in the refractive index. Add 
on the fact that we are considering a solid sphere and you get ring shaped 
areas where some signals are weak or non existant. 
       See http://neic.usgs.gov/neis/travel_times/index.html
       and  http://neic.usgs.gov/neis/travel_times/ttgraph.html   
       There are several layers at which signals can be reflected, or 
refracted to a different angle, mostly both. In general, a single signal like a P 
wave will give a mixture of P and S waves at these boundaries.
       Only the P waves can travel through the Earth's core. S waves cannot 
travel through a liquid.
       So, inside the Earth you get quite a complicated dynamic wave pattern 
building up with continuous absorption, some focusing, some refraction and 
also increased spreading.

       When the initial P and S waves travel upwards, they are partially 
reflected from the Earth's surface, but they also interact to generate Rayleigh 
vertical surface waves and Love shear surface waves, which then propagate 
outwards, roughly in a circle. Because they are surface waves, the fall off of 
intensity with distance is two dimensional and hence slower. Again, the higher 
initial frequencies tend to be absorbed more. 
       Surface waves travel with decreasing amplitude out to about 140 
degrees. For the next 20 degrees the amplitude in nearly constant, since while there 
is still absorption, the periphery of the wave circle around the globe is now 
shrinking significantly. Then they increase in amplitude as they approach 180 
degrees, due to the rapidly decreasing perimeter. 

       The intensity of the initial surface waves depends on the depth of the 
rupture below the surface, with deeper quakes tending to give less energetic 
surface waves. The earthquake intensity also depends on the length and the 
width (total area) of the rupture. The great quakes tend to occur at subduction 
zones when huge areas of land with diagonal faulting to considerable depths may 
be involved, not just a short near vertical surface crack. 

       With great quakes, like the 2004 one in Indonesia, the various signals 
can travel several times through and around the Earth, being reflected and 
refracted all along the way. They also make the whole Earth oscillate in several 
'natural eigen modes', a bit like a jelly, for several days.  

       Regards,

       Chris Chapman

       
In a me=
ssage dated 2006/10/24, carpediem1@......... writes:



I believe that the intensity of=
 light drops by the square of the distance i.e., double the distance and the=
 new value is the square root of the distance. How does this apply to seismi=
c waves which are basically sound waves? In addition, how does this affect t=
he amplitude and radiated seismic energy of the waves?




Hi Bob,



       The transmission can get quite complica=
ted. I suggest that you visit Larry Braile's website and download 'seismic w=
aves' and other introductory programs. Some of the programs are quite large.=
 See http://web.ics.purdue.edu/~braile/



       The initial Pressure and Shear waves ar=
e generated asymmetrically at the quake site, but travel outwards in all dir=
ections. They will experience square law effects and the higher frequency wa=
ves tend to be absorbed selectively with increasing distance. 

       Since the density of rocks tends to inc=
rease with depth, the ray paths are curved, not straight lines, due to chang=
es in the refractive index. Add on the fact that we are considering a solid=20=
sphere and you get ring shaped areas where some signals are weak or non exis=
tant. 

       See http://neic.usgs.gov/neis/travel_ti=
mes/index.html

       and  http://neic.usgs.gov/neis/tra=
vel_times/ttgraph.html   

       There are several layers at which signa=
ls can be reflected, or refracted to a different angle, mostly both. In gene=
ral, a single signal like a P wave will give a mixture of P and S waves at t=
hese boundaries.

       Only the P waves can travel through the=
 Earth's core. S waves cannot travel through a liquid.

       So, inside the Earth you get quite a co=
mplicated dynamic wave pattern building up with continuous absorption, some=20=
focusing, some refraction and also increased spreading.



       When the initial P and S waves travel u=
pwards, they are partially reflected from the Earth's surface, but they also=
 interact to generate Rayleigh vertical surface waves and Love shear surface=
 waves, which then propagate outwards, roughly in a circle. Because they are=
 surface waves, the fall off of intensity with distance is two dimensional a=
nd hence slower. Again, the higher initial frequencies tend to be absorbed m=
ore. 

       Surface waves travel with decreasing am=
plitude out to about 140 degrees. For the next 20 degrees the amplitude in n=
early constant, since while there is still absorption, the periphery of the=20=
wave circle around the globe is now shrinking significantly. Then they incre=
ase in amplitude as they approach 180 degrees, due to the rapidly decreasing=
 perimeter. 



       The intensity of the initial surface wa=
ves depends on the depth of the rupture below the surface, with deeper quake=
s tending to give less energetic surface waves. The earthquake intensity als=
o depends on the length and the width (total area) of the rupture. The great=
 quakes tend to occur at subduction zones when huge areas of land with diago=
nal faulting to considerable depths may be involved, not just a short near v=
ertical surface crack. 



       With great quakes, like the 2004 one in=
 Indonesia, the various signals can travel several times through and around=20=
the Earth, being reflected and refracted all along the way. They also make t=
he whole Earth oscillate in several 'natural eigen modes', a bit like a jell=
y, for several days.  



       Regards,



       Chris Chapman