## PSN-L Email List Message

Subject: Re: Relative Mag Calc question
From: Stephen & Kathy skmort@............
Date: Mon, 26 Mar 2007 09:14:46 -0700

```Thanks John,  yes, I have noticed about a 3 to 4 tenth difference from
quakes that are south of me like Toms Place to those west of me like the
Geysers, (Geyser quakes being higher to create a similar trace),  thus
the relative part of the question!  Since I was relating my signal to
the Mw value of the 7.2 event,  (1.4 micro meters of ground motion from
the USGS distance calculator),  and had the unique opportunity to
compare to another event at the same location and depth,  I was curious
to see if the rest of my math was reasonable, or if it was just a
coincidence??   So,  I see today that the math either works, or it is a
great coincidence,,  ha,,   as they have now updated the second Vanuatu
event to a 6.9Mw!
As always, you have been a great help and inspiration,
thanks,  Stephen

John or Jan Lahr wrote:
> 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
> 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
>
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

```