Earthquake Size

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Lecture-14 1

Lecture #14-

Earthquake Size





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







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,


http://slidepdf.com/reader/full/earthquake-size 7/28Lecture-14 <

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,



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+,

• •



: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,





: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



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



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@



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 /,/



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,@





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,&



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,





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,





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)



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



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.



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



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

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Earthquake Size