PSN-L Email List Message
Subject: Re: Relative Mag Calc question
From: John or Jan Lahr JohnJan@........
Date: Mon, 26 Mar 2007 08:29:32 -0700
Sounds like you're doing well. Don't be surprised if there is quite
a variation between your computed magnitude and the USGS value. For
large quakes they will be quoting moment magnitude (Mw), not
mb. There are always variations between the various scales (Mw, mb,
Ms, etc). Also for any given mb or Ms magnitude reported, the value
given for the event will be an average over all of the stations
reporting. The station to station values can vary by as much as 0.5
units; some of this is due to the "radiation pattern," which causes
some directions to receive more energy than others. To "back
calculate" to find your station sensitivity, it would be best to do
this for a large number of earthquakes and then average the
numbers. Even if your calibration is correct, you can expect to find
variations of a few tenths of a unit from the reported magnitude .
To find the USGS estimates for the various magnitudes that have been
computed for recent events, go to:
ftp://hazards.cr.usgs.gov/weekly/qedevents.txt
U.S. DEPARTMENT OF THE
INTERIOR NO. 7-085
GEOLOGICAL
SURVEY
MAR 26, 2007
NEIC/WDCS-D QUICK EPICENTER
DETERMINATIONS
UTC TIME LAT LONG DEP GS MAGS Q SD GAP STA F-E
REGION AND COMMENTS
HRMNSEC MB Msz USED
MAR 25
004002.1 20.666S 169.426E 35G A 0.9 39 192 VANUATU. MW 7.2 (GS).
004157.3 37.281N 136.602E 5G A 0.9 33 115 NEAR THE WEST
COAST OF HONSHU, JAPAN. MW 6.7
(GS).
For some reason, mb and Ms are not given for these recent large
events. Perhaps this is a new policy.
Cheers,
John
At 12:46 AM 3/26/2007, you wrote:
>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 depth????
> Thanks again,
> Stephen
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