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?=
DIV>
Regards,
Chris
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