## PSN-L Email List Message

From: ChrisAtUpw@.......
Date: Tue, 9 Jan 2007 21:51:02 EST

```In a message dated 10/01/2007, tchannel@.............. writes:

Hi Folks,  These questions may have to do  with art and experience, as much=20
as science.  Can one explain how to read  a Seismogram, if that is the corre=
ct=20
word?  That is the enlarged section  of the recorded trace.  I normally can=20
find the P and S. I also  understand the different directional movements, as=
=20
illustrated in  pictures I have seen.=20

Hi Ted,
=20
I suggest that you download and print a copy of the  seismic time travel=
=20
graph from _http://neic.usgs.gov/neis/travel_times/ttgraph.html_=20
(http://neic.usgs.gov/neis/travel_times/ttgraph.html) =20
=20
When an earthquake occurs, two types of wave are  generated. The faster=20
compression P waves have their displacement in the  direction of propagation=
..=20
The slower S shear waves have their displacement  perpendicular to their=20
direction of propagation. Local frequency components up  to a few hundred Hz=
may be=20
recorded, but these =E2=80=98high frequency=E2=80=99 components are  rapidly=
absorbed as the=20
signals travel through the  Earth.=20
When these waves reach the surface of  the Earth, they generate slower=20
surface waves, which propagate radially  outwards. The Love waves have the h=
igher=20
velocity and their displacement is  perpendicular to the direction of=20
propagation in the plane of the surface. The  Rayleigh waves are slower and=20=
have a=20
retrograde vertical motion in the direction  of propagation.
The wave propagation is constrained  by the transmission velocities in the=20
various Earth layers, the crust, the  mantle and the cores.  I suggest that=20=
you=20
_http://psn.quake.net/info/analysis.pdf_ (http://psn.quake.net/info/analysis=
..pdf)    Due to layering and the=20
curvature of the Earth, the seismic responses are  divided into local (0 to=20
1.4 deg), near regional (1.4 to 6 deg.), regional (6 to  16 deg.) and=20
teleseismic responses (16 to 180 deg.).  The angle is that  between the sour=
ce and the=20
observer at the centre of the Earth. See p 12.   A 1 degree angle correspond=
s=20
to 111.3 km, 69.2 miles or 60 nautical mile at the  surface.
The P and S waves have frequencies between  0.5 and maybe 40 Hz, with the S=20
waves having a bit less than =C2=BD the period of the  associated P waves. P=
waves=20
from regional and teleseismic events may have  frequencies between 1 and 3 H=
z.=20
The L and R surface waves may have periods up to  several hundred seconds=20
depending on the physical dimensions of the seismic  slip, but for smaller q=
uakes=20
they are more typically between 10 and 30 seconds.  The amplitude of Rayleig=
h=20
waves of about 20 sec period are often measured to  determine the magnitude=20=
of=20
a quake. For Local and Near Regional quakes, the  amplitudes of the P or S=20
waves may be used to determine the quake  magnitude.
Visual seismic displays are described at=20
_http://www.geol.binghamton.edu/faculty/jones/_ (http://www.geol.binghamton.=
edu/faculty/jones/)=20
While the  surface waves radiate outwards in a fairly simple and predictable=
=20
manner, the  waves within the earth generate new wave pairs at every=20
refraction and  reflection. The response to a large quake depends on the dep=
th of the=20
quake as  well as the angle between it and the observer and can get very =20
complicated.

1.  Are the large waves that follow the  S, surface waves?

Yes, but see the time travel graph

2.  If so can one discern Loves from  Rayleigh's, within that section?

The Love waves have the higher velocity and are  horizontally polarised.
The Rayleigh waves are a bit slower and are  vertically polarised.

3.  Do the Loves and Rayleigh's arrive at  different times, as with P and S?=
=20
and or, do the look  different?
=20
I am sure the more you work with these recordings  the better you can=20
interpret them.
=20
A related question: I just recorded a very  nice image of the 5.7m 01/09/200=
7=20
Southeastern Alaska 2259km from Boise Idaho,  using my Vertical Spring=20
sensor.  The best recording, so far, using this  new sensor. I was please wi=
th the=20
image; a clear P, a smaller but clear S,  followed by a large series of wave=
s,=20
I assume,  to be surface waves. But  here I can't tell a Love from a Rayleig=
h.

Note that the surface waves are polarised. A  vertical sensor will pick=20
up mostly Rayleigh waves. A Lehman will pick up mostly  Love waves

I  did not know a vertical spring sensor would pick up surface waves, eve=
n=20
though  the Rayleigh's could move the vertical spring.  Is it that the =20
vertical is less likely to pick up these waves at greater distances, because=
of=20
their longer periods?

A sensor will still react to motions at less than  it's resonant period,=
=20
but the amplitude of the response decreases with the  square of the ratio of=
=20
the resonant period to the wave period. So if your  vertical sensor has a=20
period of 2 secs, a typical Rayleigh wave of 20 seconds  will show about 1/1=
00 the=20
'normal' sensitivity.
=20
Regards,
=20
Chris Chapman

In a message dated 10/01/2007, tchannel@.............. writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
Hi Folks,  These questions may have=20=
to do=20
with art and experience, as much as science.  Can one explain how to=20=
a Seismogram, if that is the correct word?  That is the enlarged sect=
ion=20
of the recorded trace.  I normally can find the P and S. I also=20
understand the different directional movements, as illustrated in=20
pictures I have seen.
Hi Ted,

the=20
seismic time travel graph from http://neic.usg=
s.gov/neis/travel_times/ttgraph.html

When an earthquake occurs, two types of wave ar=
e=20
generated. The faster compression P waves have their displacement in the=20
direction of propagation. The slower S shear waves have their displacement=20
perpendicular to their direction of propagation. Local frequency components=20=
up=20
to a few hundred Hz may be recorded, but these =E2=80=98high frequency=E2=
=80=99 components are=20
rapidly absorbed as the signals travel through the=20
Earth.     When these waves reach the surface o=
f=20
the Earth, they generate slower surface waves, which propagate radially=20
outwards. The Love waves have the higher velocity and their displacement is=20
perpendicular to the direction of propagation in the plane of the surface. T=
he=20
Rayleigh waves are slower and have a retrograde vertical motion in the direc=
tion=20
of propagation.    The wave propagation is constrain=
ed=20
by the transmission velocities in the various Earth layers, the crust, the=20
mantle and the cores.  I suggest that you download the seismograph trai=
ning=20
manual from http://psn.quake.net/info/an=
alysis.pdf =20
Due to layering and the curvature of the Earth, the seismic responses are=20
divided into local (0 to 1.4 deg), near regional (1.4 to 6 deg.), regional (=
6 to=20
16 deg.) and teleseismic responses (16 to 180 deg.).  The angle is that=
=20
between the source and the observer at the centre of the Earth. See p 12.&nb=
sp;=20
A 1 degree angle corresponds to 111.3 km, 69.2 miles or 60 nautical mile at=20=
the=20
surface.    The P and S waves have frequencies betwe=
en=20
0.5 and maybe 40 Hz, with the S waves having a bit less than =C2=BD the peri=
od of the=20
associated P waves. P waves from regional and teleseismic events may have=20
frequencies between 1 and 3 Hz. The L and R surface waves may have periods u=
p to=20
several hundred seconds depending on the physical dimensions of the seismic=20
slip, but for smaller quakes they are more typically between 10 and 30 secon=
ds.=20
The amplitude of Rayleigh waves of about 20 sec period are often measured to=
=20
determine the magnitude of a quake. For Local and Near Regional quakes, the=20
amplitudes of the P or S waves may be used to determine the quake=20
magnitude. Visual seismic displays are described at http://www.geol.bingh=
amton.edu/faculty/jones/
program SeismicEruptionSetup.exe    While the=20
surface waves radiate outwards in a fairly simple and predictable manner, th=
e=20
waves within the earth generate new wave pairs at every refraction and=20
reflection. The response to a large quake depends on the depth of the quake=20=
as=20
well as the angle between it and the observer and can get very=20
complicated.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
1.  Are the large waves that fo=
llow the=20
S, surface waves?
Yes, but see the time travel graph
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
2.  If so can one discern Loves from=
=20
Rayleigh's, within that section?
The Love waves have the higher velocity and are=
=20
horizontally polarised.
The Rayleigh waves are a bit slower and are=20
vertically polarised.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
3.  Do the Loves and Rayleigh's arri=
ve at=20
different times, as with P and S? and or, do the look=20
different?

I am sure the more you work with these re=
cordings=20
the better you can interpret them.

A related question: I just recorded=20=
a very=20
nice image of the 5.7m 01/09/2007 Southeastern Alaska 2259km from Boise Id=
aho,=20
using my Vertical Spring sensor.  The best recording, so far, using t=
his=20
new sensor. I was please with the image; a clear P, a smaller but clear S,=
=20
followed by a large series of waves, I assume,  to be surface waves.=20=
But=20
here I can't tell a Love from a Rayleigh.
Note that the surface waves are polarised. A=20
vertical sensor will pick up mostly Rayleigh waves. A Lehman will pick up mo=
stly=20
Love waves

I=20
did not know a vertical spring sensor would pick up surface waves, even th=
ough=20
the Rayleigh's could move the vertical spring.  Is it that the=20
vertical is less likely to pick up these waves at greater distances, becau=
se=20
of their longer periods?

A sensor will still react to motions at less th=
an=20
it's resonant period, but the amplitude of the response decreases with the=20
square of the ratio of the resonant period to the wave period. So if your=20
vertical sensor has a period of 2 secs, a typical Rayleigh wave of 20 second=
s=20
will show about 1/100 the 'normal' sensitivity.

Regards,

Chris Chapman
```