PSN-L Email List Message

Subject: Re: exact frequencies for arrival of different seismic waves ??
From: Dave Nelson davenn@...............
Date: Fri, 27 Jan 2006 15:30:47 +1100

Hi Geoff
               there are NO exact frequencies ...  the frequency of the 
different waves arriving at the sensor are dependant on many factors.
  1) type and depth of rupture
  2) distance between the sensor and the rupture
  3) sensor location  relative to the ruprture
  to name a few

   take  2) ..  distance between your sensor and the rupture.....
as distance increases  you loose the higher freq component of the signal
because of attenuation/absorption   just as you do with sound waves over 
distance  eg.   think of a rumble of thunder in a storm ....
if you are close to the lightning strike   you hear the sharp crack of the 
as distance increases you only get to hear the low freq rumblings

or  another pratical example ....    think of that loud music being played 
by your noisy neighbour  what do you hear ?? ... the thump thump of the 
bass and drums   the hi freq has all but gone ( they are all there at the 

when refering to sensor location I am referring to compass bearing to the 
from the sensor ( or visa versa)   eg on a strike slip fault ... if you are 
in the compressional quadrant of the outgoing waves you are likely to see a 
far different waveform  than if you are in the dilation quadrant.  see my 
brief discussion of this on my www page 

if you look at diag B  you can see the 4 quadrants  2 compressional and 2 
dilational   and the sensor location within those quadrants will have a 
determination on the amplitude and freq of the received waveform

this all applies to teleseisms as well .... note how they lack much in the 
way of P and S wave signal  MOST of it is the lower freq surface waves

basically  then,   the closer the quake,  the more hi freq component your 
sensor will detect

hence why professional stations use broadband sensors  ... record 
EVERYTHING,  then filter accordingly to retrieve the wanted data

Dave N

At 20:29 26/01/2006 -0700, you wrote:
>Can anyone tell me ?
>Are there exact frequencies for the first
>arrival time of P and S waves ?
>I have only seen ranges of frequencies.
>If I could pin down exact frequencies I could
>make a summation of maybe three narrow band pass
>filters with a relatively high Q to look for
>each of these waves.
>I suspect the ground may behave in a
>predictable fashion with the leading edge
>of a wave train.
>Has anyone ever researched this in any depth
>with test equipment that can show reality ?
>I know for a fact that the government has
>incredible test equipment because I have seen
>some of it but not that which is designed
>for geological work.
>I have seen charts showing ranges of freqs
>but here I am only interested in exact
>frequencies or slew rates of the first
>time of arrival and not the train that follows.
>If you are looking for seismic waves
>created by nuclear explosions, what
>would you be looking for ?


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