In a message dated 20/06/2007, Paulc@........ writes:
I want to try to apply some of the basic ideas from Radio Wave propagation
for earthquake propagation.
Boy, this is not making sense.....
Hi Paul,
Sound wave propagation in the Earth is more complex than radio waves in
the Atmosphere.
You have the two direct sound waves, the P or pressure waves for which
the motion is in the direction of propagation and the S or shear waves for
which the motion is perpendicular to the direction of propagation. These
propagate from the quake in all directions, but S waves cannot travel through the
Earth's liquid core.
Whenever these are reflected or refracted by a layer which gives a sharp
change in the refractive index (velocity), new pairs of waves types are
generated.
Then you have the Rayleigh surface waves for which the motion is
vertical and the Love waves for which the motion is in the surface plane, both at
right angles to the direction of propagation. These are generated by
interactions with the P and S waves. Their amplitude can depend on the surface rocks
and on resonance effects - like shaking a jelly.
The 2D surface waves decrease in amplitude with distance at a lower rate
than 3D body waves.
The Earth's massive surface plates are in continual very slow motion.
The rock deforms and strain energy is stored, until the local fracture stress
is reached, when you get the sudden massive movement which we call an
earthquake. You get a wide range of frequencies generated at the fracture site, but
above about 2 Hz the sound waves are progressively absorbed, so what a distant
observer sees depends on the distance, both vertically and horizontally. The
relative amplitudes of the body and surface waves depend on the physical
depth of the fracture. Since the surface waves are generated by the body waves,
a very deep earthquake is likely to have low amplitude surface waves.
The magnitude of a quake is related to the length of the fracture. The
time taken for a quake to 'happen' is also related to the physical length.
High magnitude quakes are long and 'slow'.
Very large quakes generate many complex eigenmode (natural vibration
mode) oscillations of the whole earth, which can be observed at very low
frequencies. The lowest frequency is about 0.0003 Hz. These may continue for several
days.
Surface waves from large quakes can travel several times around the
Earth. Internal body waves may also be reflected on the far side of the Earth.
Go to _http://web.ics.purdue.edu/~braile/edumod/svintro/svintro.htmand_
(http://web.ics.purdue.edu/~braile/edumod/svintro/svintro.htmand) download
and play Seisvole for several recorded examples of differing amplitude and
depth.
Regards,
Chris Chapman
In a message dated 20/06/2007, Paulc@........ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I want=20
to try to apply some of the basic ideas from Radio Wave propagation
for=
=20
earthquake propagation.
Boy, this is not making=20
sense.....
Hi Paul,
Sound wave propagation in the Earth is more com=
plex=20
than radio waves in the Atmosphere.
You have the two direct sound waves, the P=
or=20
pressure waves for which the motion is in the direction of propagation and t=
he S=20
or shear waves for which the motion is perpendicular to the direction of=20
propagation. These propagate from the quake in all directions, but S wa=
ves=20
cannot travel through the Earth's liquid core.
Whenever these are reflected or refracted =
by a=20
layer which gives a sharp change in the refractive index (velocity=
),=20
new pairs of waves types are generated.
Then you have the Rayleigh surface waves for wh=
ich=20
the motion is vertical and the Love waves for which the motion is in th=
e=20
surface plane, both at right angles to the direction of propagation. Th=
ese=20
are generated by interactions with the P and S waves. Their amplitude can de=
pend=20
on the surface rocks and on resonance effects - like shaking a jelly.
The 2D surface waves decrease in amplitude with=
=20
distance at a lower rate than 3D body waves.
The Earth's massive surface plates are in conti=
nual=20
very slow motion. The rock deforms and strain energy is stored, until the lo=
cal=20
fracture stress is reached, when you get the sudden massive movement wh=
ich=20
we call an earthquake. You get a wide range of frequencies generated at the=20
fracture site, but above about 2 Hz the sound waves are progressively absorb=
ed,=20
so what a distant observer sees depends on the distance, both vertically and=
=20
horizontally. The relative amplitudes of the body and surface waves depend o=
n=20
the physical depth of the fracture. Since the surface waves are generated by=
the=20
body waves, a very deep earthquake is likely to have low amplitude surface=20
waves.
The magnitude of a quake is related to the leng=
th=20
of the fracture. The time taken for a quake to 'happen' is also related to t=
he=20
physical length. High magnitude quakes are long and 'slow'.
Very large quakes generate many complex eigenmo=
de=20
(natural vibration mode) oscillations of the whole earth, which can be obser=
ved=20
at very low frequencies. The lowest frequency is about 0.0003 Hz. These=
may=20
continue for several days.
Surface waves from large quakes can travel seve=
ral=20
times around the Earth. Internal body waves may also be reflected=20=
on=20
the far side of the Earth.
Go to htt=
p://web.ics.purdue.edu/~braile/edumod/svintro/svintro.htmand=20
download and play Seisvole for several recorded examples of differing amplit=
ude=20
and depth.
Regards,
Chris Chapman