PSN-L Email List Message

Subject: Re: Rayleigh waves
From: chrisatupw@.......
Date: Mon, 21 Dec 2009 13:45:57 EST


 
In a message dated 21/12/2009, PETERS_RD@.......... writes:

Thus I  expect that microseisms should be confined to the crust, behaving 
in a manner  similar to electromagnetic guided waves.  Since the crust as a 
waveguide  cannot support waves whose wavelength is greater than the crustal 
thickness,  this results in a low frequency cutoff.  In other words, the 
longest  period limit should be established (assuming crust average thickness 
of 15 km)  at about 6 s, assuming P-wave velocity of 3 km/s.  Is it just a  
coincidence that micro's with a period longer than 15 s are not (to my  
knowledge) observed?  



Hi Randall,
 
    We not infrequently see microseisms with periods of  10 to 12 seconds 
in the UK, when there are storms in the north Atlantic, as  well as the 
normal ones at about one half this period, 5 to 7  seconds.
 
    Is the 6 sec microseism period not determined by  the period of the 
deep ocean waves? Or are we considering on shore waves, which  seem to be 
different?
 
    How do the other component amplitudes measured on  the 20 ft pier and 
on the surface plinth compare? The P, S and Love waves? The  USGS time travel 
graph assumes about 3.7 km / sec for Rayleigh waves.
    Have the microseism velocities been measured  separately, please? Wide 
variations seem to have been reported in the  past.
 
    Do you have a reference for the comparison of  microseism amplitudes 
detected at the surface and in a borehole, please?
    
    Regards,
 
    Chris





In a message dated 21/12/2009, PETERS_RD@.......... writes:
Thus I=20 expect that microseisms should be confined to the crust, behaving in a= manner=20 similar to electromagnetic guided waves.  Since the crust as a wave= guide=20 cannot support waves whose wavelength is greater than the crustal thickn= ess,=20 this results in a low frequency cutoff.  In other words, the longes= t=20 period limit should be established (assuming crust average thickness of= 15 km)=20 at about 6 s, assuming P-wave velocity of 3 km/s.  Is it just a=20 coincidence that micro's with a period longer than 15 s are not (to my= =20 knowledge) observed? 
Hi Randall,
 
    We not infrequently see microseisms with peri= ods of=20 10 to 12 seconds in the UK, when there are storms in the north Atlant= ic, as=20 well as the normal ones at about one half this period, 5 to 7=20 seconds.
 
    Is the 6 sec microseism period not determined= by=20 the period of the deep ocean waves? Or are we considering on shore waves,= which=20 seem to be different?
 
    How do the other component amplitudes measure= d on=20 the 20 ft pier and on the surface plinth compare? The P, S and Love waves?= The=20 USGS time travel graph assumes about 3.7 km / sec for Rayleigh waves.
    Have the microseism velocities been measured= =20 separately, please? Wide variations seem to have been reported in the=20 past.
 
    Do you have a reference for the comparis= on of=20 microseism amplitudes detected at the surface and in a borehole, please?
    
    Regards,
 
    Chris

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