PSN-L Email List Message

Subject: Re: Relative Mag Calc question
From: John or Jan Lahr JohnJan@........
Date: Sun, 25 Mar 2007 17:59:18 -0700


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:


At 03:42 PM 3/25/2007, you wrote:
>Calc. Problem #1
>On my AS-1, it shows a P to P count of 147.48 with a period of 3.6 
>seconds for the P wave of the 7.2 Vanuatu quake on 3 March 07!  I 
>divide by 3.6 to get a one second count of approx. 41.
>Later, another event from Vanuatu gives me a count of 90.58 at 4.5 
>seconds,, which converts to a count of about 20.
>Assuming that the converted one second counts are proportional to 
>ground motion, and that the second event produced about half,  I 
>found the log of .5 = about -.3,,   summed to 7.2 = approx. 6.9 for 
>an approx. mag for the second event!!  distance and depth are about 
>the same, is the distance to great for this kind of relative calculation?
>I'm about 6080 miles from the two events!
>Calc. Problem #2
>Using the USGS quake distance calculator I find that my estimated 
>one second ground motion for the 7.2 Vanuatu quake would be 1.4 micro meters!
>On my AS-1,  225 counts = one volt!   Can I assume 41 / 1.4 = approx 
>29 count per micro meter per second = .13 volts per micro meter per 
>second sensitivity?????   (or convert to proper terminology????) and 
>or does 2pi enter in somewhere???   I see the 2pi factor in the USGS 
>calculator from micro meter to micro meter per sec?  (.13 * 2pi = 
>.82 v / micro meter / s)
>Any tutoring on my overly simple process, and or assumptions are welcome!!!
>Please make it as simple as possible, thanks.
>  Stephen
>  PSN Station #55
>  38.828N  120.979W


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