## PSN-L Email List Message

Subject: Re: Burning Questions
From: ChrisAtUpw@.......
Date: Fri, 4 Jan 2008 11:41:22 EST

```In a message dated 04/01/2008, gpayton880@....... writes:

While everyone has their "Thinking Caps" on from Ted's excellent  questions,
I have a couple that have been smoldering for some time:

1)    Months ago I posted an event and I received an email  commenting on it.
He said, "It was very good, but I might try  improving my P-wave."  HOW does
one "improve" one phase over  another?  It seems that the P-wave is always
less  stronger.

Hi Jerry,

I suggest that you ask him? Raw data files for the  relevant time
interval are normally submitted.

It is usual to extract the digital trace and to  then apply filters to it
to make the waves more visible while doing your own  analysis. You might set
the HP and LP filters both to 1 second when searching  for teleseismic P
waves, for example.

2)    Much has been written about the length of a pendulum  needing to be
long to be effective for teleseismic detection.  However,  the commercial devices
are quite compact and obviously have short  pendulums.  Can someone explain
how they accomplish what they do with  short pendulums?

They use very low noise capacitative detectors to  get the very high
resolution, to maybe well below 0.1 nano metre. The period may  then be extended
greatly by electronic feedback, or by digital feedback, or  both. They may use
small pendulums with a natural period of say 0.5 second or  more, but these
are totally controlled by the force feedback. Because direct  position and not
velocity is being measured, you get a fall off in the signal  below resonance
of only 1/f, not 1/f^2. You can extend a 'natural' 1 second  system to over
1,000 seconds, but the electronics required to do this may be  quite expensive.

When I receive an earthquake, the earth and my  house are moved by the event.
The various phases move the ground in  different directions. This question
has to do  with the movement of the earth like that of a teeter-totter.  Tilt.
like  that measured by a level.

1.  How much does the earth move?  I do  understand the earth would move in
many different directions, and move more,  if the earthquake was larger, and or
closer. But somewhere I think I read some numbers indicating how much the
earth  would be expected to move.

If you go to _http://jclahr.com/science/psn/magnitude.html_
(http://jclahr.com/science/psn/magnitude.html)  you  will find several graphs and tables.
Remember that surface waves are often the  largest in amplitude and that their
amplitude is greatly effected by the local  ground type. Waterlogged alluvial
ground may behave very like a jelly.

I am asking this question, to help me understand,  the approximate tilt from
an earthquake.   If I hung a one meter  pendulum, and a major earthquake
occurred 1000km away, the earth here, would  tilt.  If I were looking at the
pendulum at the moment the S wave  arrived, assuming the sensor was pointing in the
correct direction, the  pendulum would appear to tilt, but unless the event
was large enough I could  not see it with my eyes.

You need to remember the difference in response of  a pendulum to both
sideways motion and to direct tilts. S waves will show  lateral motion,  but P
and Rayleigh waves may show direct tilt effects as  well as motion.

Regards,

Chris Chapman

In a message dated 04/01/2008, gpayton880@....... writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
While everyone has their "Thinking Caps" on from Ted's excellent=20
questions, I have a couple that have been smoldering for some time:

1)    Months ago I posted an event and I received an e=
mail=20
commenting on it.  He said, "It was very good, but I might=20=
try=20
improving my P-wave."  HOW does one "improve" one phase over=20
another?  It seems that the P-wave is always less=20
stronger.
Hi Jerry,

I suggest that you ask him? Raw data files for=20=
the=20
relevant time interval are normally submitted.

It is usual to extract the digital trace and to=
=20
then apply filters to it to make the waves more visible while doing your own=
=20
analysis. You might set the HP and LP filters both to 1 second when searchin=
g=20
for teleseismic P waves, for example.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
2)    Much has been written about the length of a pend=
ulum=20
needing to be long to be effective for teleseismic detection.  Howeve=
r,=20
the commercial devices are quite compact and obviously have short=20
pendulums.  Can someone explain how they accomplish what they do with=
=20
short pendulums?

They use very low noise capacitative detectors=20=
to=20
get the very high resolution, to maybe well below 0.1 nano metre. The period=
may=20
then be extended greatly by electronic feedback, or by digital feedback, or=20
both. They may use small pendulums with a natural period of say 0.5 second o=
r=20
more, but these are totally controlled by the force feedback. Because direct=
=20
position and not velocity is being measured, you get a fall off in the signa=
l=20
below resonance of only 1/f, not 1/f^2. You can extend a 'natural' 1 second=20
system to over 1,000 seconds, but the electronics required to do this may be=
=20

quite expensive.

<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
When I receive an earthquake, the earth a=
nd my=20
house are moved by the event.  The various phases move the ground in=20
different directions. This question has=
to do=20
with the movement of the earth like that of a teeter-totter.  Tilt. l=
ike=20
that measured by a level.

1.  How much does the earth move?&nb=
sp; I do=20
understand the earth would move in many different directions, and move mor=
e,=20
if the earthquake was larger, and or closer. But somewhere I think I read some numbers indicating how much the=
earth=20
would be expected to move.
If you go to http://jclahr.com/scie=
nce/psn/magnitude.html you=20
will find several graphs and tables. Remember that surface waves are often t=
he=20
largest in amplitude and that their amplitude is greatly effected by the loc=
al=20
ground type. Waterlogged alluvial ground may behave very like a jelly.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
I am asking this question, to help me und=
erstand,=20
the approximate tilt from an earthquake.   If I hung a one meter=
=20
pendulum, and a major earthquake occurred 1000km away, the earth here, wou=
ld=20
tilt.  If I were looking at the pendulum at the moment the S wav=
e=20
arrived, assuming the sensor was pointing in the correct direction, the=20
pendulum would appear to tilt, but unless the event was large enough I cou=
ld=20
not see it with my eyes.
You need to remember the difference in response=
of=20
a pendulum to both sideways motion and to direct tilts. S waves will show=20
lateral motion,  but P and Rayleigh waves may show direct tilt effects=20=
as=20
well as motion.

Regards,

Chris Chapman
```