reply to post by JustMike
does it tend to be the case that they eventually resolve the "M" designation into something more precise? And if so, does the final number --
eg a 5.9 (plus a defined type such as Mw) seem to match up with what they gave originally as simply an "M"?
Taking Potsdam first, "M" I believe is often mb as that is the normal scale value calculated from the first seconds of the P wave. When they do a
tensor calculation this is then converted to an Mw scale. The page of information
If we take the Mag 4.8 in the Molucca Sea....
usgs,2012-12-28 05:21:07, 0.526, 126.178, 4.8, 21.0, Molucca Sea
gfzp,2012-12-28 05:21:08, 0.600, 126.260, 4.5, 10.0, Northern Molucca Sea
emsc,2012-12-28 05:21:11, 0.552, 126.290, 4.8, 47.0, Molucca Sea
...the USGS magnitude is stated to be mb, but the EMSC and Potsdam magnitudes are "M". If you look at the EMSC scientific data (linked above) they
state 4.9 mb - so what is 4.8???
Potsdam have 4.5 but if you look in the EMSC data you will find various GTZ (Potsdam) entries some with magnitudes and some without. From their
connections they pull 4.1 mb form Indonesia but 5.1 mb from Australia.
Their 4.5 is not even the average of the tw0 (which is the way the USGS lists work). It may be that their are others which are not shown here which
give an average of 4.5
If you look at the section for seismologists
on the EMSC site however you get the
2012-12-28 05:21:11.5 0.56 N 126.30 E 47 M 4.8 A MOLUCCA SEA MIX
2012-12-28 05:21:08.9 0.64 N 126.26 E 10 mb 5.1 A MOLUCCA SEA SC3
2012-12-28 05:21:08.0 0.73 N 126.28 E 10 M 4.5 A MOLUCCA SEA GFZ
2012-12-28 05:21:07.9 0.53 N 126.18 E 21 M 4.8 M MOLUCCA SEA NEIC
2012-12-28 05:21:05.4 0.48 N 126.15 E 20 mb 4.9 M MOLUCCA SEA GSRC
Note that the first entry says MIX. Now we have the source of the EMSC "M" 4.8 and it is the average of the other 4 (4.825) rounded down. (But this is
not always the case!!!!)
The list above is of course a perfect demonstration of just how inexact a science the determination of earthquake magnitudes is. None of the latitudes
and longitudes are the same, there are three different depths - at least two of which look suspiciously like defaults and I suspect all may be - and a
mish mash of automatic and manual solutions.
Somehow, and I have no idea how, EMSC comes up with a depth of 47km from the selection above. Again there are probably some feeds we don't see.
If you like to be accurate about things, as I do, earthquake magnitudes are a source of immense stress
(or should that be stress release?)
ETA: I did not answer your question. Mostly the M quakes stay as M as they tend to be the lower magnitude quakes. M on Potsdam will be converted to Mw
when a tensor is calculated and there is no specific relationship above or below - but I suspect that most are in fact a version of mb when they are
M. mb does not always result in an equivalent Mw value as they are calculated on different elements of the data.
USGS recently confuses the issue even more by using Mi for final solutions and Mwp (Mw calculated on the P wave which is the equivalent of converting
mb to Mw) and Mwt and various others along with it's famous GS which I am convinced stands for G
I guess I should also add that many providers of information around the world still use the Richter scale (ML) and that is specific to the area. The
ML scale as originally devised for California by Richter is not the same as the Richter scale for Iran or Greece. Each area is calculated according to
it's geology. A 4.8ML in Chile may not be exactly the same as a 4.8ML in Iran or a 4.8ML in California.
I have to stop......................this is driving me potty!
edit on 28/12/2012 by PuterMan because: (no reason given)