PSN-L Email List Message

Subject: Re: Large Response. Corrected URL
From: Bob Hancock icarus@.........
Date: Wed, 5 Aug 2009 19:59:03 -0700

Hi Geoff -

I cannot speak about high gain, but to understand the signals you  
received, you also need to understand focal mechanism.  The focal  
mechanism of the M 6.9 Gulf of Mexico event was a strike slip.  See  
links below:

Following is a very simplistic explanation of focal mechanism for a  
strike slip event.  NOTE - As the focal mechanism changes to other  
than strike slip, the following will not apply.

The radiation pattern of a strike slip event is 4 lobed with two lobs  
compressional and two lobs dilation.

When you look at the sphere, you see it is divided into 4 quadrants,  
and it is like you are looking at the bottom inside of the beech ball  
that had been cut in half.  The dividers are called fault planes.   
When rocks break under pressure, they tend to break on a 45 degree  
line from the compression, although in reality it could easily vary  
between 30 & 60 degrees depending upon several factors.  The problem  
is determining which fault plane the rocks actually broke on as the  
physics of the fracture will be the same on either fault plane.  When  
focal mechanisms are listed, they list two fault planes labeled NP1,  
the most likely occurrence, and NP2, the alternate occurrence.

The two colored quadrants (most often red or black) represent the  
compressional quadrants when the initial movement of the P wave on the  
vertical channel is up.  The non-colored or open quadrants are the  
dilation quadrants where the initial movement of the P wave on the  
vertical channel will move down.

Understanding the focal mechanism will allow you to understand where  
the peak amplitudes and null points of the four basic waves, P, S,  
Love, & Rayleigh will occur.

The peak of the P wave and Rayleigh waves whether compression or  
dilation will be midway between the fault planes, and they become  
weakest at the fault planes.  The peak amplitude on the S waves and  
Love waves whether compression or dilation occur on the fault planes  
and they are weakest in the middle between the fault planes.

Now, back to our M 6.9 event.  Both of our stations are very close to  
this event and we were both in the dilation quadrant for the P wave on  
the vertical channel.  I looked at both my data and that from IU/TUC  
and they were the same.  On each station, the P waves were lost in the  
trailing coda from the preceding M 5.8 event of similar location.   
This has nothing to do with filtering, but everything to do with the  
closeness of the second event following the first event, and the lack  
of time for the earth to settle down after the first event.

I hope this helps.

Bob Hancock

On Aug 5, 2009, at 8:02 AM, Geoffrey wrote:

> Hello Bob & Brett ;
> If you were looking only for first arrival times
> Id think High gain is the way to go.
> You must see the noise because the emerging
> signal can sometimes be a part of the noise.
> Many signals i get from this area seems
> to attenuate the P wave in a way it arises
> slowly out of the grass and in such a case
> where exactly is the first arrival time.
> Most not likely the time you actually see.
> If somehow you could use a FFT to see
> the first arrival time I think that may be
> the way to go but since you need many
> samples to use the fft it may not be possible
> to see the exact arrival.
> Does anyone know of processing routines
> that will pull a signal out of the grass other
> than a FFT routine ??

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