Toward SystemLevel Earthquake Science

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Earthquake magnitude Most related to maximum ampl
itudes in seismograms. Local Magnitude (ML) Ri
chter, 1930ies Noticed similar decay rate of log1
0A (displacement) versus distance
Defined distance-independent magnitude estimate
by subtracting a log10A for reference event
recorded on a Wood-Anderson seismograph at the
same distance MLlog10A(in 10-6m)-log10 A0(in 1
0-6m) log10A(in 10-6m)2.56log10 dist (in k
m) -1.67 for 10
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Geol 600 Notable Historical Earthquakes
Source mechanisms and body wa
ve radiation patterns http//www-rohan.sdsu.e
du/kbolsen/geol600_nhe_magnitude.ppt
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Earthquake magnitude Body wave magnitude (mb)
(used for global seismology) mblog10(A/T)Q(h,?)
T is dominant period of the measured waves (usua
lly P, 1s) Q is an empirical function of distance
? and depth h (details versus amplitude versus r
ange) Surface wave magnitude (Ms) (used for glob
al seismology, typically using Rayleigh waves on
vertical components) Mslog10(A/T)1.66 log10 ?
3.3 log10A201.66 log10 ? 2.0
(shallow events only)
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• Insert table showing saturation

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Earthquake magnitude Saturation problem motivat
ed the moment magnitude Mw Mw2/3 log10M0-10.7 (M
0 moment in dyne-cm, 107dyne cm1Nm)
Mw2/3 log10M0-6.1 (M0 moment in Nm)
Scaling derived so Mw agree with Ms for small
events More physical property, does not saturate
for large events
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Earthquake magnitude Intensity scale Measures
damage to structures Often used is Mercalli I-XII
Can be used to examine historic earthquakes with
out seismic records For example, eastern US.

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Intensity Observed Effects I Not felt at all II
Felt only by a few individuals, indoors and?at
rest, usually on upper floors of tall buildings.
III Felt indoors by many persons, but not
necessarily recognized as an earthquake.
Chandeliers and hanging plants swing.


IV Felt both indoors
and out. Feels like the vibration caused by a
heavy truck or train passing. Windows rattle.
V Strong enough to awaken sleeping persons. Small
objects knocked off shelves. Beverages may splash
out of cups or glasses on tables.
VI Perceptible to everyone. May cause public
fright. Pictures fall off walls. Weak masonry
cracks. Some plaster may fall from ceilings.
VII Difficult to stand upright. Ornamental
masonry falls from buildings. Waves may be seen
in ponds and swimming pools. VIII Mass panic may
occur. Chimneys, smoke stacks and water towers
may lean and fall. Unsecured frame houses slide
off foundations. IX Panic is general. Heavy damag
e to masonry structures and to?underground pipes.
Large cracks open in ground.X Many buildings
collapse. Water splashes over riverbanks.
XI-XII Virtually total destruction.
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Radiated Energy Log10Es (ergs) 5.82.4mb11.8
1.5Ms Es(Ms7)/Es(Ms6)32 Es(Ms7)/Es(Ms5)1,
000
Es0.5??DaveA??M0/2? ?????Es/M0