## PSN-L Email List Message

From: "tchannel" tchannel@..............
Date: Wed, 10 Jan 2007 10:22:35 -0700

```Hi Chris,  Thanks again!   Extremely well written and very useful reply. =
Linda my wife, an English major, worked as an report specialist and =
editor for 31 years, working with scientific publications.......She will =
Cheers, Ted
----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Tuesday, January 09, 2007 7:51 PM

In a message dated 10/01/2007, tchannel@.............. writes:
Hi Folks,  These questions may have to do with art and experience, =
as much as science.  Can one explain how to read a Seismogram, if that =
is the correct word?  That is the enlarged section of the recorded =
trace.  I normally can find the P and S. I also understand the different =
directional movements, as illustrated in pictures I have seen.=20
Hi Ted,

I suggest that you download and print a copy of the seismic time =
travel graph from http://neic.usgs.gov/neis/travel_times/ttgraph.html=20

When an earthquake occurs, two types of wave are generated. The =
faster compression P waves have their displacement in the direction of =
propagation. The slower S shear waves have their displacement =
perpendicular to their direction of propagation. Local frequency =
components up to a few hundred Hz may be recorded, but these =
=E2=80=98high frequency=E2=80=99 components are rapidly absorbed as the =
signals travel through the Earth.=20
When these waves reach the surface of the Earth, they generate =
slower surface waves, which propagate radially outwards. The Love waves =
have the higher velocity and their displacement is perpendicular to the =
direction of propagation in the plane of the surface. The Rayleigh waves =
are slower and have a retrograde vertical motion in the direction of =
propagation.
The wave propagation is constrained by the transmission velocities =
in the various Earth layers, the crust, the mantle and the cores.  I =
http://psn.quake.net/info/analysis.pdf  Due to layering and the =
curvature of the Earth, the seismic responses are divided into local (0 =
to 1.4 deg), near regional (1.4 to 6 deg.), regional (6 to 16 deg.) and =
teleseismic responses (16 to 180 deg.).  The angle is that between the =
source and the observer at the centre of the Earth. See p 12.  A 1 =
degree angle corresponds 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 waves having a bit less than =C2=BD the period of the =
associated P waves. P waves from regional and teleseismic events may =
have frequencies between 1 and 3 Hz. The L and R surface waves may have =
periods up to several hundred seconds depending on the physical =
dimensions of the seismic slip, but for smaller quakes they are more =
typically between 10 and 30 seconds. The amplitude of Rayleigh waves of =
about 20 sec period are often measured to determine the magnitude of a =
quake. For Local and Near Regional quakes, the amplitudes of the P or S =
waves may be used to determine the quake magnitude.
Visual seismic displays are described at =
http://www.geol.binghamton.edu/faculty/jones/
SeismicEruptionSetup.exe
While the surface waves radiate outwards in a fairly simple and =
predictable manner, the waves within the earth generate new wave pairs =
at every refraction and reflection. The response to a large quake =
depends on the depth of the quake as well as the angle between it and =
the observer and can get very 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? and or, do the look different?

I am sure the more you work with these recordings the better you can =
interpret them.

A related question: I just recorded a very nice image of the 5.7m =
01/09/2007 Southeastern Alaska 2259km from Boise Idaho, using my =
Vertical Spring sensor.  The best recording, so far, using this new =
sensor. I was please with the image; a clear P, a smaller but clear S, =
followed by a large series of waves, I assume,  to be surface waves. But =
here I can't tell a Love from a Rayleigh.
Note that the surface waves are polarised. A vertical sensor will =
pick 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, even though the Rayleigh's could move the vertical spring.  Is it =
that the vertical is less likely to pick up these waves at greater =
distances, because of their longer periods?
A sensor will still react to motions at less than it's resonant =
period, but the amplitude of the response decreases with the square of =
the ratio of the resonant period to the wave period. So if your vertical =
sensor has a period of 2 secs, a typical Rayleigh wave of 20 seconds =
will show about 1/100 the 'normal' sensitivity.

Regards,

=EF=BB=BF

Hi Chris,  Thanks again!   Extremely well written =
and very=20
useful reply.  Linda my wife, an English major, worked as an report =

specialist and editor for 31 years, working with scientific=20
publications.......She will also enjoy reading this.
Cheers, Ted

----- Original Message -----
From:=20
ChrisAtUpw@.......
To: psn-l@..............
Sent: Tuesday, January 09, 2007 =
7:51=20
PM
Seismogram

In a message dated 10/01/2007, tchannel@..............=20
writes:

Hi Folks,  These questions may =
have to do=20
with art and experience, as much as science.  Can one explain =
how to=20
read a Seismogram, if that is the correct word?  That is the =
enlarged=20
section 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,

copy of=20
the seismic time travel graph from http://neic.=
usgs.gov/neis/travel_times/ttgraph.html

When an earthquake occurs, two types of =
wave are=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 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 of=20
the Earth, they generate slower surface waves, which propagate =
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. The=20
Rayleigh waves are slower and have a retrograde vertical motion in the =

direction of propagation.    The wave =
propagation is=20
constrained by the transmission velocities in the various Earth =
layers, the=20
crust, the mantle and the cores.  I suggest that you download the =

seismograph training manual from http://psn.quake.net/info=
/analysis.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=20
to 16 deg.) and teleseismic responses (16 to 180 deg.).  The =
angle is=20
that between the source and the observer at the centre of the Earth. =
See p=20
12.  A 1 degree angle corresponds to 111.3 km, 69.2 miles or 60 =
nautical=20
mile at the surface.    The P and S waves have =

frequencies between 0.5 and maybe 40 Hz, with the S waves having a bit =
less=20
than =C2=BD the period of the associated P waves. P waves from =
regional and=20
teleseismic events may have frequencies between 1 and 3 Hz. The L and =
R=20
surface waves may have periods up to several hundred seconds depending =
on the=20
physical dimensions of the seismic slip, but for smaller quakes they =
are more=20
typically between 10 and 30 seconds. The amplitude of Rayleigh waves =
20 sec period are often measured to determine the magnitude of a =
quake. For=20
Local and Near Regional quakes, the amplitudes of the P or S waves may =
be used=20
to determine the quake magnitude. Visual seismic displays are =

described at http://www.geol.bi=
nghamton.edu/faculty/jones/
program SeismicEruptionSetup.exe    While =
the=20
surface waves radiate outwards in a fairly simple and predictable =
manner, the=20
waves within the earth generate new wave pairs at every refraction and =

reflection. The response to a large quake depends on the depth of the =
quake as=20
well as the angle between it and the observer and can get very=20
complicated.

1.  Are the large waves =
that follow=20
the S, surface waves?
Yes, but see the time travel graph

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.

3.  Do the Loves and =
Rayleigh's arrive 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=20
recordings the better you can interpret them.

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

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

A sensor will still react to motions at =
less than=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 =
seconds=20
will show about 1/100 the 'normal' sensitivity.

Regards,

Chris=20
Chapman
```