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
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