PSN-L Email List Message

Subject: Re: Seismometer Siting
From: "Geoffrey" gmvoeth@...........
Date: Mon, 28 Sep 2009 09:27:37 -0700

The question I have is thus:
(plz ignore whatever you do not like)

Do you need a P wave first to tell
the following is an S wave ?

What tells you what when either may look the same.

A vertical polarized S wave ?
A horizontal P wave at the surface ?
A vertical P wave at the surface ?

It seems to me you need one to ID the other
or they differ significantly in some other attribute.
Like frequency.

What you guys say here does not yet make sense to me.

Direction of motion tells me nothing for only a single wave
since I can not see in three dimensions with a single sensor.

I am not looking for the same things as yourselves.
Only first time of arrival.

If I could have a single signal no matter what the
direction that is OK for me.

But I would need according to my understanding
three different sensors sampled at the same identical time instant
to recreate through Mathematics the single magnitude
I am looking for in the display.
I just cant afford ($$$) that kind of luxury
(Never have been able and never will be able).

I really like the idea of Forrest MIMSIII for a horizontal
sensor which is planar and not linear in design.
One pendulum and one vertical and possibly
summing the two together would cheaply let an amateur
see all three dimensions ?
I have not tried this yet but have seen very good
non-dimensional horizontal signals from his design.
It will respond 360 degrees with a single sensor in
the horizontal plane. But that's not right for
most professionals. Just peoples like myself.
LOL, talk about frustrations.


ps, People are nastier than you can ever imagine and a PhD can
make one even worse than nasty. sort of like hitler was to whomever.

----- Original Message ----- 
From: "Brett Nordgren" 
Sent: Monday, September 28, 2009 8:21 AM
Subject: Re: Seismometer Siting

> Geoffry,
> It relates to the direction of the ground oscillations caused by different 
> phases.  P waves, more or less, vibrate at right angles to S waves.  So, 
> *for a particular quake*, if you point your sensor in the direction of the 
> arriving P waves in order to see them the best, you would then have to turn 
> it by 90 degrees to pick up the S waves most strongly, something like 
> aiming a directional radio antenna.
> With two or three-axis sensors the pointing can be done mathematically with 
> a computer, working on the saved data, rather than by actually rotating the 
> sensor, which allows you to easily estimate the direction of the source of 
> the quake.
> Brett
> At 07:37 AM 9/28/2009 -0700, you wrote:
>>It seems silly to me that you can differentiate between
>>various types of disturbances in any way except through
>>frequencies. Even P waves can present themselves
>>with motion or deformation in almost any direction.
>>We live at a phase boundary between air and earth
>>where waves will meet a melodramatic velocity change.
>>Where P can become S and vice versa.
>>----- Original Message ----- From: "GPayton" 
>>To: "PSN Network List" 
>>Sent: Sunday, September 27, 2009 11:14 AM
>>Subject: Seismometer Siting
>>>Over and over, I have read that normally a seismometer is sited for N-S, 
>>>E-W, and Vertical sensing.
>>>However, I just read a paragraph in An Introduction to Seismology, 
>>>Earthquakes and Earth Structure about a seismometer in Hawaii that "was 
>>>oriented to receive S phase signals."  How is that accomplished?
>>>I could venture a guess, but probably would be wrong.
>>>Jerry Payton
>>Public Seismic Network Mailing List (PSN-L)
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> Watch our wiggles
> or watch some very very good wiggles
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