L Braile, 1/26/2006

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What is Moment Magnitude?
L Braile, 1/26/2006 (revised, Sept., 2009)
http//web.ics.purdue.edu/braile/new/MomentMagnit
ude.ppt
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Moment Magnitude (Mw also called Magnitude
or M, as in, an M8.0 earthquake)
Focus or hypocenter (point of initiation of
the rupture)
Epicenter (location on Earths surface above
the hypocenter)

Moment Mo µ A D (dyne-cm) µ shear modulus
32 GPa in crust (3.2 x 1011 dynes/cm2),
75 GPa in mantleA LW area (cm2)D average
displacement during rupture (cm) (http//neic.usgs
.gov/neis/general/measure.html http//earthquake.
usgs.gov/image_glossary/seismic_moment.html)
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Then, the Moment Magnitude is calculated by
Mw (or just M) 2/3 log10(Mo) - 10.7
Mo, and therefore Mw, can be determined by (µ is
generally assumed to be 3 x 1011
dynes/cm2) 1. Geological measurements of
fault offset and fault mapping. 2. Estimates
of fault area from the aftershock
distribution and slip (from surface rupture).
3. Modeling of the waveforms of very long period
seismograms to estimate fault slip, fault area
and earthquake source mechanism (strike-slip,
reverse fault, etc., and orientation of the
fault plane). Except for very large earthquakes,
other magnitude calculations (mb, MS, mbLg, ML
Richter magnitude) generally provide a good
estimate of Mw. (see http//web.ics.purdue.edu/b
raile/edumod/MagCalc/AS1Results.htm for
information on AS-1 magnitudes, see
http//web.ics.purdue.edu/braile/edumod/as1mag/as
1mag3.htm)
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Magnitude of earthquake is controlled by fault
length (or area) that ruptures (data for diagram
generated using Seismic/Eruption program)
Magnitude versus fault length (determined from
aftershock zone length) for various earthquakes
(Alaska, 1964 Denali, 2002 Landers, 1992 Loma
Prieta, 1989 Northridge, 1994, etc.). Results
were quickly obtained using Seismic/Eruption
views.
Magnitude versus fault length
Alaska, 1964
Sumatra, 2004
Denali, 2002
Landers, 1992
Loma Prieta, 1989
Northridge, 1994
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Magnitude Comparison Three earthquakes of M7.0,
M8.1 and M9.0 recorded on an AS-1 Seismograph
(WLIN) from about the same distance.
Format of Excel file used for cataloging AS-1
earthquake data (http//web.ics.purdue.edu/braile
/new/EarthquakeList.xls)
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2/2401
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Magnitude Comparison Three earthquakes of M7.0,
M8.1 and M9.0 recorded on an AS-1 Seismograph
(WLIN) from about the same distance, plotted at
the same scale.
2/24/01 M7.0 N. Molucca Sea
12/23/04 M8.1 Macquarie Is. Reg.
Note 9 minutes of strong P wave energy caused by
9 minutes of rupture propagation over the
1200 km long fault plane.
12/26/04 M9.0 Sumatra
Relative Time (minutes)
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Magnitude (M) a consistent measure of size
(energy release) of an earthquake. Should be
able to be measured from many types of
seismographs and for a large distance range from
the earthquake. Calculated from amplitude on a
seismogram with a correction for distance and the
amplification of the seismograph. MS, mb, mbLg
and ML generally approximate M.
Amplitude For mb
To calculate distance, use http//neic.usgs.gov/n
eis/travel_times/
Amplitude For MS (20 s period waves)
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http//web.ics.purdue.edu/braile/edumod/MagCalc/M
agCalc.htm
Online magnitude calculator for mb, MS and mbLg
magnitudes for the AS-1 Seismograph
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Magnitude calculator in AmaSeis for mb, MS and
mbLg magnitudes for the AS-1 Seismograph
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Comparison of AS-1 and USGS Magnitudes
AS-1 magnitudes are accurate!
MS Magnitudes N 116 Standard Deviation
0.25 magnitude units. mb Magnitudes N 229
Standard Deviation 0.27 magnitude units. mbLg
Magnitudes N 27 Standard Deviation 0.34
magnitude units.
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AS-1 mb, MS and mbLg magnitudes correlate well
with M (Mw, moment magnitude) except for the
largest earthquakes
Comparison of AS-1 and USGS Mw Magnitudes