Earthquake Magnitude Confusion

This issue has bothered me for about 4 years, maybe I'm just too picky, but to me it possibly causes confusion amongst the general public when they look at the Network Lists or read the news.
USGS are the worst, what the heck does (GS) magnitude mean?
and a new one this week, (UCMT)????

When you look at their Past 7 Days list or the 8-30 days Lists, they give a magnitude shown as MAG.
There is no indication of what scale it is.
The various scales used in seismology are listed here
earthquake.usgs.gov...

To find out what scale they are talking about you have to go into Phase Data ( by clicking on the blue link of the individual quake on the list) and then clicking on the Sciencific & Technical tab. Phase Data is on the list there.

Lets do one as an example

MAP, 6.3, 2010/05/25 10:09:06,35.342,-35.940,10.0,NORTHERN MID-ATLANTIC RIDGE

on the Phase data we can see
MAGNITUDE 6.3 (UCMT)

then lower down (12 lines) we have
mb = 5.4 (226) ML = 0.0 ( 0) mblg = 4.5 ( 7) md = 0.0 ( 0) MS = 4.8 ( 2) The numbers in brackets are the number of stations used to collate the readings.

Which is all fine and dandy, too bad there is no ML, but being in the middle of the ocean as this quake was its probably irrelevant in this case.

So the 6.3 quake was really just a 5.4mb in disguise?????

As a cross check lets have a look at EMSC and GEOFON, same event.
EMSC have it at 6.0 Mw

GEOFON do it as well as USGS, just giving a M= for magnitude. They have this event as a M= 5.6
In this case that event hasn't been checked by a human yet, but if you go to their Phase Data page (off that link page) they normally give mb, ML and mB readings so you can assume they are talking about mb's when they refer to M=.

Having experienced many 5.0 - 5.6 magnitude quakes and a few low 6 magnitude at less than 100km distance, I can assure you that there is a vast difference in the shaking between a 5.4 and a 6.3.

What are they (USGS) up to?
why post magnitudes at GS or UCMT or whatever which no one know what it means, why complicate things?, why not just post the magnitudes in mb which anyone with half a clue can understand and relate to?

Its almost like they are pumping up the magnitudes for some reason.









[edit on 25-5-2010 by muzzy]

reply to post by muzzy


Yes, it is very confusing... IMO They all should have a universal quake magnitude measurement as they have a universal time clock. I certainly would help to clarify information for who ever is trying to follow the quakes. Or They should at least list each calculation right out front without having to go through hoop to figure it out!!!

reply to post by muzzy


US means it was generated by the USGS and will almost certainly - no without a doubt - is Moment of Magnitude. This is differnt to Mb which is body moment.

Unless it is a local (US) quake, then ALL USGS data will be MMS which is designated by the letters Mw.

The Richter scale, or ML standing for magnitude local is no good for larger quakes because of instrument saturation.

Now I have to say those are the only two scales that I know about. I believe USGS does not use MMS (Mw) under 3.5 since it becomes inaccurate at that level.

Thanks for the replies

I have been looking back to the 1990's on USGS Global Search PDE files, downloaded as Expanded File Format with Headers and Spaces which does break the events down into Mw, Mb, Ms, Me and even ML in some cases.

Its probably not their fault, I think its me not being able to find the right resource to look at

The general 7 day and 8-30 day lists aren't all that detailed..

Understated/Overstated

I might keep a record of the anomolies that are showing up in the usgs reporting. I'll use this format. Links to the quake data is off the network name at the end of each line

IMO this one is

Understated

Magnitude, Type, Date(y/m/d) Time(hh:mm:ss), Latitude, Longitude, Depth km, Location, Source
5.9Mw, 2010/08/16 19:35:49, -20.768, -178.739, 597.1, Fiji Region, usgs
5.6, mb, usgs
7.0 ML, usgs
6.0, Mw? 2010/08/16 19:35:47, -20.73, -178.71, 577, Fiji Region, geofon
6.2, Mw, 2010/08/16 19:35:46.9, -20.81, -178.66, 574, Fiji Region, emsc
6.2 Unk, 2010/08/16 19:35:46.9, -20.5669, -178.797, 600, Fiji Region, geoaus
rasgs offline

usgs= United States Geological Survey
emsc= European- Mediterranean Sismological Centre
geofon= German Research Centre for Geosciences
rasgs= Russian Academy of Sciences Geological Survey
geoaus= Geoscience Australia

could just be an error?, but how on earth can a 7.0ML come out as a 5.9Mw? when for example the Dusky Sound, NZ quake of July 2009 was 7.2ML and converted to 7.6Mw



[edit on 16-8-2010 by muzzy]

reply to post by muzzy


Well lookie here

5 hours since I posted and the Magnitude has been changed to 6.2Mw and 5.7mb now

neic.usgs.gov...

I can see I'm going to have to screenshot the pages, the original phase data has changed at the top, the data below looks the same though

It doesn't bother me that they change it, but how many in the past didn't get checked over?

[edit on 17-8-2010 by muzzy]