From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Explanation of Magnitudes
Date: 26 Jan 93 15:12:58 GMT
Distribution: ca

In article <9301242123.FS.131@n6ume2.uucp>, dorothy@n6ume2.uucp (Dorothy Darby) writes:
> 1.  I know that Menlo uses Duration Magnitudes when posting their
>     reports, and the NEIC uses both MB (Body Wave) and MSZ (Vertical
>     surface wave)....What does Berkley use when reporting a magnitude
>     for an event?

If they are like most local array operators, most of the smaller quakes
will have duration magnitudes, but special quakes will also have ML
(Richter magnitude) computed as well.

> 2.  What is the difference between all of them?

Duration magnitude (also named coda magnitude) is computed by fitting
an exponential curve to the decay in amplitude of shaking.  Distance,
which causes the coda to spread in duration, is accounted for in the
computattion.  Then what varies is the size of the fault and the
displacement time function.  Works only for very local events (<200 km
usually) and at higher frequency (~1-20 Hz).

mb is a teleseismic P-wave magnitude computed from the amplitude of the
P-wave in a narrow band.  Teleseismic range is > 30 degrees distance
(about 3300 km).  Gutenberg and Richter originall proposed this magnitude
(they called "m") as measured anywhere in the phases P, PP or SH, and on
broad-band instruments, ususally around 6-12 second periods.  USGS
for a long time measured this in a narrow band around 1 second, and
required that the amplitude be measured within 5 seconds of onset.
This leads to very different mb's than the original definition.

MSZ (often just noted as MS - it is frequently computed on vertical) is
measured from the amplitude of long-period (20 second, usually) surface
waves and is also teleseismic.  It needs a big correction when used for
very deep events (especially deeper than 200 km).  Gutenberg's original
formula has been altered somewhat.  Also, the horizontal MS should be
different from MSZ, but the precise variation has not been studied
(Rayleigh waves tend to make MSZ bigger, but the Love waves tend to
make it smaller).  For the horizontal MS, there is not agreement on
whether the two horizontal directions should be added vectorially
or if they should be rectified and averaged.  Richter recommends vectorial
addition, but this is not generally practiced since there are too many
factors that could cause the addition to be performed slightly off-azimuth
(which can greatly affect the result).  Rectify and sum tends to be more
stable and independent of azimuth.

ML is Richter magnitude, which is computed from the amplitude of
regional (< 30 degrees) phases, as they would be recorded on an
0.8 second Wood-Anderson torsion seismometer.

The exact formulae and corrections used vary from region to region and
agency to agency.  It is one of the things IASPEI hopes to address.
There are other magnitude scales in addition to the 4 above, but they are
certainly the most common.  I might add MLg - the Lg magnitude important
in test ban monitoring, and Mw - the energy magnitude important for
for very large earthquakes.  MLg is a little like ML, but the computation
steps seem more like mb.  Mw is a lot like MS, but measured at very long
periods.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov