Earthquake Descriptors

EARTHQUAKE DESCRIPTORS Earthquake Earthquake Magnitude; Seismi M!ment;  Seismi Energ"; The Centr!id M!ment Tens!r Pr!#et;  Magnitude and $au%ting; $!resh!ks& A'tersh!ks& and S(arms; Seismi Intensit" )R!ssi*$!rre% Sa%e& M!di+ed Mera%%i Sa%e& USSR ,EO$A- Sa%e& MSK*./ Sa%e0

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 Earthquake Magnitude In connection with Faulting

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 An earthquake is also defined as the sudden slip of one part of the Earth's crust, relative to another, along a fault surface.  A gradual build-up of mechanical stress in the crust, primarily the result of tectonic forces, provides the source of energy for earthquakes; sudden motion along a fault releases it in the form of seismic waves. t's unclear when the connection between faults and earthquakes was first made, but by the l ate !"th #entury most scientists accepted this association as fact, even if the mechanisms behind it were still a mystery.

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 Earthquake Magnitude In connection with Faulting

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$ault research received a tremendous boost in the aftermath of the great %an $rancisco earthquake of !"&. (his was one of the first earthquakes for which both seismographic and fault-rupture studies were conducted. (he fault rupture occurred in through a very well-surveyed, developed area.

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 Earthquake Magnitude In connection with Faulting

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)ecause of this, researchers could not only map the offset across the fault trace, but also the amount of displacement between points far removed from the fault.

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(his work led to the formulation of the elastic rebound theory of fault rupture by *rinceton geologist +arry $. eid.

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 Earthquake Magnitude In connection with Faulting

+ow big is an earthquake epends on how big a patch of the fault breaks.

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f the patch that breaks is a few square miles, you may have a magnitude five earthquake. f it's up to a couple hundred square miles, you

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have a magnitude seven. f it's a couple of thousand square miles, you get a / 0.1, !"& %an $rancisco quake.2

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 Earthquake Magnitude In connection with Faulting

(he seismic moment is the product of the area of fault surface that ruptures, the average displacement along that surface, and a constant -- a measure of the elastic property of rock 3i.e. how easily it can be stretched4 called the modulus of rigidity. /oment magnitude 3/ 54 is based upon •

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Radiated Seismic Energy 

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Seismi energ" is a 1h"sia% !ne1t re%ated t! 2r!ad2and in'!rmati!n !n the s!ure radiati!n; this is di3erent 'r!m seismi m!ment4

In the!r"& its !m1utati!n sim1%" requires an integrati!n !' radiated energ" 5u6 in velocity-squared seismograms.

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In 1ratie& energ" has hist!ria%%" a%m!st a%(a"s 2een estimated (ith em1iria% '!rmu%as4 The em1iria% a11r!ah d!minated '!r t(! ma#!r reas!ns7

84 Unti% the 89:e and 2e%!( the !rner 'requen" !' an earthquake& a2!ut (hih energ" densit" is m!st str!ng%" 1eaked4

How is radiated seismic energy measured? ` •

 T! determine the t!ta% seismi energ" radiated 'r!m an earthquake !ne (!u%d ha>e t! integrate the energ" radiated at a%% 'requenies !>er the entire '!a% s1here4  The s1etrum !' the a>erage radiati!n !>er the '!a% s1here an 2e a11r!6imated 2" a !nstant %e>e% at %!( 'requenies )(hih is 1r!1!rti!na% t! the m!ment& M !0 and a uni'!rm derease (ith inreasing 'requen" a2!>e s!me !rner 'requen" )$ 0& s! the seismi energ" is a 'unti!n !' 2!th M! and $4 $!r a gi>en m!ment& the radiated energ" (i%% inrease as $  inreases4

Mo = u D A where: u = shear modulus (3 - 6 x !" d#n$cm% & = a'erage dislacement ) = area o* ruture

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I* one e'ent were a +slow+ earthquake with +more or less creelike de*ormation+ while the other had a more t#ical ruture  'elocit# near the shear wa'e 'elocit#, much more energ# would e radiated *rom the latter earthquake due to its rich high *requenc# radiation corresonding larger Fc than *rom the +slow+ e'ent/

E=

erg

( erg =  d#n cm"

@!g E  84B M  884: ),uten2erg*Rihter magnitude* energ" re%ati!n0 •

 Then7 @!g M!  @!g )= 6 8/0  84B MC  884:

Centr!id M!ment Tens!r )CMT0  0entroid Moment 1ensor (0M1" is a reliale method *or calculating moment tensors/ •

Seismic Moment is a measure !' the siFe !' an earthquake 2ased !n the area !' 'au%t ru1ture& the a>erage am!unt !' s%i1& and the '!re that (as required t! !>er!me the 'riti!n stiking the r!ks t!gether that (ere !3set 2" 'au%ting4 Seismi m!ment an a%s! 2e a%u%ated 'r!m the am1%itude s1etra !' seismi (a>es4

Moment Tensor  is a mathematia% re1resentati!n !' the m!>ement !n a 'au%t during an earthquake& !m1rising !' nine genera%iFed !u1%es& !r nine sets !' t(! >et!rs4 The tens!r de1ends !' the s!ure strength and 'au%t !rientati!n4

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