PSN-L Email List Message

Subject: Re: Natural Microseismic Noise
From: ChrisAtUpw@.......
Date: Sun, 17 Sep 2006 02:26:10 EDT


In a message dated 2006/09/16, gmvoeth@........... writes:

> Subj:Natural Microseismic Noise 
> 
> I have noticed since early this morning the appearance of seizable 
> microseismic noise that seems related to the weather disturbance in or near the Gulf 
> Of California.
> This similar thing has happened several times before over the last 10 years 
> or so.
> Just thought I would comment on this. This station is located in Arizona at 
> GVA.
> I am relatively sure that this type of noise will appear each time a major 
> storm
> hits the Gulf Of California.
> If I am wrong, please send me a comment.
> Regards;
> geoff

Hi Geoff,

       Storms and frontal weather systems, particularly cold fronts, increase 
the seismic background noise, often quiite considerably. It may tend to be 
more obvious on longer period instruments like 30 sec Lehmans. You can also get 
very significant wind noise, not only by the direct response of nearby trees 
and buildings, but also due to mountain wave systems, some of which 'travel' 
rather than staying nearly still. You may also be able to see the 'clear air 
turbulence' which effects aircraft. If you have a conical mountain directly 
upwind, you may see von Karman 'shedding vortices' as the air flow oscillates 
around the mountain. Associated clouds can sometimes be seen on satellite photos. 
       These signals have tended to be rejected as 'just noise' by 
seismologists, but may be worthy of study in their own right. The Pacific Coast 
microseisms may vary in period quite a bit, as may the microseisms associated with 
hurricanes on the east coast. You may get more than one generating centre and the 
oscillations may show 'beat frequency interference'.

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/09/16, gmvoeth@........... writes:

Subj:Natural Microseismic No= ise

I have noticed since early this morning the appearance of seizable microseis= mic noise that seems related to the weather disturbance in or near the Gulf=20= Of California.
This similar thing has happened several times before over the last 10 years=20= or so.
Just thought I would comment on this. This station is located in Arizona at=20= GVA.
I am relatively sure that this type of noise will appear each time a major s= torm
hits the Gulf Of California.
If I am wrong, please send me a comment.
Regards;
geoff


Hi Geoff,

       Storms and frontal weather systems, par= ticularly cold fronts, increase the seismic background noise, often quiite c= onsiderably. It may tend to be more obvious on longer period instruments lik= e 30 sec Lehmans. You can also get very significant wind noise, not only by=20= the direct response of nearby trees and buildings, but also due to mountain=20= wave systems, some of which 'travel' rather than staying nearly still. You m= ay also be able to see the 'clear air turbulence' which effects aircraft. If= you have a conical mountain directly upwind, you may see von Karman 'sheddi= ng vortices' as the air flow oscillates around the mountain. Associated clou= ds can sometimes be seen on satellite photos.
       These signals have tended to be rejecte= d as 'just noise' by seismologists, but may be worthy of study in their own=20= right. The Pacific Coast microseisms may vary in period quite a bit, as may=20= the microseisms associated with hurricanes on the east coast. You may get mo= re than one generating centre and the oscillations may show 'beat frequency=20= interference'.

       Regards,

       Chris Chapman

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