Earthquake Descriptors

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EARTHQUAKE DESCRIPTORS

Earthquake Magnitude; Seismi M!ment;

Seismi Energ"; The Centr!id M!ment Tens!r Pr!#et; Magnitude and $au%ting; $!resh!ks& A'tersh!ks& and

S(arms;

Seismi Intensit" )R!ssi*$!rre% Sa%e& M!di+ed Mera%%i Sa%e&USSR ,EO$A- Sa%e& MSK*./ Sa%e0



• 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 late !"th #entury

most scientists accepted this association as fact, even if

the mechanisms behind it were still a mystery.

‡ EarthquakeMagnitude

In connection withFaulting




• $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.




• )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.

• (his work led to the formulation of the

elastic rebound theory of fault rupture by

*rinceton geologist +arry $. eid.





+ow big is an earthquake

•epends on how big a patch of the fault breaks.

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

have a magnitude seven. f it's a couple of

thousand square miles, you get a / 0.1, !"&

%an $rancisco quake.2





•(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

the seismic moment, and represents a kin





Radiated Seismic Energy

• Seismi energ" is a 1h"sia% !ne1t re%ated t! 2r!ad2and in'!

!n the s!ure radiati!n; this is di3erent 'r!m seismi m!ment4

• In the!r"& its !m1utati!n sim1%" requires an integr

radiated energ" 5u6 in velocity-squared seismog



• In 1ratie& energ" has hist!ria%%" a%m!st a%(a"sestimated (ith em1iria% '!rmu%as4 The em

a11r!ah d!minated '!r t(! ma#!r reas!ns7

• 84 Unti% the 89:<s m!st seismi data (ere ana%!g& a (hih (as n!t amena2%e t! s1etra% 1r!essing !n a r!utine

•=4 An aurate estimate !' radiated energ" requireana%"sis !' s1etra% in'!rmati!n 2!th a2!>e and 2e%!!rner 'requen" !' an earthquake& a2!ut (hih edensit" is m!st str!ng%" 1eaked4



How is radiated seismic energy measured?`

• T! determine the t!ta% seismi energ" radiated 'earthquake !ne (!u%d ha>e t! integrate the radiated at a%% 'requenies !>er the entire '!a% s

The s1etrum !' the a>erage radiati!n !>er ths1here an 2e a11r!6imated 2" a !nstant %e>e%'requenies )(hih is 1r!1!rti!na% t! the m!me

and a uni'!rm derease (ith inreasing 'requen"s!me !rner 'requen" )$0& s! the seismi ener'unti!n !' 2!th M! and $4 $!r a gi>en m!me

radiated energ" (i%% inrease as $ inreases4



Mo = u D A

where:

u = shear modulus (3 - 6 x !" d#n$cm%

& = a'erage dislacement

) = area o* ruture



• I* one e'ent were a +slow+ earthquake with +more or less clike de*ormation+ while the other had a more t#ical rutu

'elocit# near the shear wa'e 'elocit#, much more energ# we radiated *rom the latter earthquake due to its rich high*requenc# radiation corresonding larger Fc than *rom the

+slow+ e'ent/

• E = erg ( erg = d#n cm"



@!g E 84B M 884: ),uten2erg*Rihter magni

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 reliale method *or calculating mo

• 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

'!re that (as required t! !>er!me

the 'riti!n stiking 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 s1etra !' seismi (a>es4

•Moment Tensor mathematia% re1resenthe m!>ement !n a 'au%tearthquake& !m1risinggenera%iFed !u1%es& !r

!' t(! >et!rs4 The tens!!' the s!ure strength !rientati!n4

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