earthquake size
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Lecture-14 1
Lecture #14-
Earthquake Size
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Lecture-14 &
Ma'imum (ntensit
Ma'imum (ntensit isused to estimate the size
o) historica
earthquakes* "ut su))ers
)rom de!endence onde!th* !o!uation*
construction !ractices*
site e))ects* re+iona
+eoo+* etc,
MaximumIntensity = VII
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Lecture-14 /
Earthquake Ma+nitudes
(n the 1&.7s* 8adati in 9a!an and :ichter
in ;ai)ornia noticed that athou+h the peak
amplitudes on seismo+rams )rom di))erent
e5ents di))ered* the peak amplitudes
decreased with distance in a simiar manner
)or di))erent quakes,
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Seismo+ram =eak >m!itude
he peak amplitude is the size o) the ar+est
de)ection )rom the zero ine,
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:ichter7s O"ser5ations
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:ichter7s Loca Ma+nitude
:ichter used these o"ser5ations to construct
the )irst ma+nitude scae* ML :ichter7s
Loca Ma+nitude )or Southern ;ai)ornia@,
He "ased his )ormua )or cacuatin+ the
ma+nitude on the astronomica "ri+htness
scae - which was logarithmic,
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Lo+arithmic Scaes
(n a o+arithmic scae such as ma+nitude
3 > chan+e in one ma+nitude unit means a
chan+e o) a )actor o) 1. in the am!itude o)
+round shakin+,
• •
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:ichter7s %ormua
He de)ined a re)erence 5aue >o
ML A o+> - o+ >o
3 > is the am!itude on the seismo+ram* >o is the
am!itude o"ser5ed )or a re)erence e5ent,
%or Southern ;ai)ornia
ML A o+> - 2,40 B 2,</ ' o+distance@ 3 he distance is in kiometers and this )ormua
works )or southern ;ai)ornia,
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:ichter7s Ma+nitude Scae
On 5aid )or Southern ;ai)ornia
earthquakes
On 5aid )or one s!eci)ic t!e o)seismometer
Has not "een used " !ro)essiona
seismoo+ists in decades (s much a"used " the !ress toda
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Modern Seismic Ma+nitudes
oda seismoo+ists use di))erent seismic wa5esto com!ute ma+nitudes
hese wa5es ha5e ower )requencies than thoseused " :ichter
hese wa5es are +enera recorded at distances o)1...s o) kiometers instead o) the 1..s o)kiometers )or the :ichter scae
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eeseismic MS and m "
he two most common modern ma+nitudescaes areC
MS* Sur)ace-wa5e ma+nitude :aei+h 8a5e@
m "* od-wa5e ma+nitude =-wa5e@
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Ma+nitude Discre!ancies
(dea* ou want the same 5aue o)ma+nitude )or an one earthquake )rom each
scae ou de5eo!* i,e, MS A m " A ML
ut this does not awas ha!!enCurke 01<C MS A <,0* m " A /,&
aiwan 2.C MS A <,<* m " A /,/
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8h Don7t Ma+nitude Scaes
>+ree?
Sim!est >nswerC
3 Earthquakes are com!icated !hsica
!henomena that are not we descri"ed " a
single number ,
3 ;an a thunderstorm "e we descri"ed " one
num"er ? o, (t takes wind s!eed* rain)a*i+htnin+ strikes* s!atia area* etc,@
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Lecture-14
8h Don7t Ma+nitude Scaes
>+ree?
Most com!icated reasonC
3 Ma+nitude scaes saturate
3 his means there is an u!!er imit to ma+nitude
no matter how Far+eG the earthquake is
3 %or instance Ms sur)ace wa5e ma+nitude@
ne5er +ets a"o5e 0,2-0,&
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Lecture-14
E'am!eC m " FSaturationG
m " sedom +i5es
5aues a"o5e /,< - it
FsaturatesG,
m " must "e
measured in the )irst
seconds - that7s
the rue,
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Lecture-14
8hat ;auses Saturation?
he ru!ture !rocess, 3 Sma earthquakes are ru!ture sma areas are reati5e
enriched in short !eriod si+nas,
3 Lar+e earthquakes ru!ture ar+e areas and are reati5e
de!eted in hi+h )requencies,
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Lecture-14
(s there anthin+ "etter?
es the seismic moment ! Mo
3 (n5ented in the 1/.s to circum5ent ma+nitude
imitations
3 Has !hsica units o) ener+ m* ca* 9@
3 (s the !roduct o) three )actors that indicate the
size o) the earthquakeC
Mo = (shear modulus) x (rupture area) x (slip offset)
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Lecture-14
Moment Ma+nitude* M8
o aow com!arisons* we use momentma+nitude* which is cacuated )rom the
seismic moment,
Estimatin+ M8 takes more work than the
others* "ut it7s o)ten the "est measure o)
size,
M W = 2 / 3 * log(Seismic Moment) - 10.73
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Lecture-14
Ma+nitude Summar
Magnitude Symbol Wave Period Local (Richter) ML S or Surface Wave* 0.8 s
Body-Wave mb P 1 s
Surface-Wave Ms Rayleigh 20 s
Moment Mw Rupture Area, Slip 100’s-1000’s
• Magnitude is a measure of ground shaking amplitude.• More than one magnitude scales are used to study
earthquakes.
• All magnitude scales have the same logarithmic form.
• Since different scales use different waves and different
period vibrations, they do not always give the same value.
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Lecture-14
he Lar+est Earthquakes
M W is the appropriate choice for comparingthe largest eents! it "oes not sat#rate.
1$%0 &hile $.'
1$% lasa $.2
1$'2 +amchata $.1
1$%' le#tians $.0
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Lecture 14
he ar+est earthquake e5er
recorded " seismometers the
1/. ;hiean e5ent with , Mw @
had a seismic moment equi5aentto the ener+ o) triion kiotons
o) I