PSN-L Email List Message

Subject: Re: Relative Mag Calc question
From: Stephen & Kathy skmort@............
Date: Mon, 26 Mar 2007 00:46:27 -0700

Hi John,  thanks,  the counts per micro meter was my quest??    so, I 
plugged in the guess-ta-calculated 29 for my system response, along with 
the other data and came up with 7.1Mb for the first Vanuatu event,  and 
6.9Mb for the second Vanuatu event!    I'll be looking to see how I 
compare to a USGS update for the second event,,  for now they are 
calling it a 6.0????   My system has about a 4.5 second natural period = 
..2222 hz, so the surface wave component was only about 10 count, which I 
subtracted from the peak count for the calculation!    For the Japan 
quake, I guess-ta-mated and subtracted the Vanuatu component and came up 
with 6.8Mb,,,   (famous last words,,  I think I understand a little 
better now,, ha!!)
Also, it looks like my first problem concept  (log of count ratio)  is a 
reasonable quick estimate to compare events with the same location and 
  Thanks again,

John or Jan Lahr wrote:
> Stephen,
> The is a magnitude calculator built in to the latest version of 
> AmaSeis.  Once you have the event within the viewer window, enter the 
> latitude, longitude, and depth from the USGS web site.  Then measure 
> the zero to peak amplitude and the period of the maximum of the P 
> phase group.  Click on Control/Compute Magnitude, fill in the 
> information, click Compute and you're done.
> The Calibration.txt file that comes with AmaSeis gives the 
> relationship between amplitude and counts for various frequencies.
> I computed an mb of 7.4 for the Vanuatu event from my AS-1.  The 
> second event will have less amplitude by about a factor of 10^.5  =  
> 3.16 because it is 0.5 magnitude units smaller.  I don't think I can 
> see this smaller event, as it's lost in the coda of the larger one.
> There is some information posted on calculating magnitude here:
> Cheers,
> John

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