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7/23/2019 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
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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 late !"th #entury
most scientists accepted this association as fact, even if
the mechanisms behind it were still a mystery.
‡ EarthquakeMagnitude
In connection withFaulting
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‡ EarthquakeMagnitude
• $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.
In connection withFaulting
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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.
• (his work led to the formulation of the
elastic rebound theory of fault rupture by
*rinceton geologist +arry $. eid.
‡ EarthquakeMagnitude
In connection withFaulting
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+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
‡ EarthquakeMagnitude
In connection withFaulting
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•(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
‡ EarthquakeMagnitude
In connection withFaulting
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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
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• 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
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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
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Mo = u D A
where:
u = shear modulus (3 - 6 x !" d#n$cm%
& = a'erage dislacement
) = area o* ruture
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• 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"
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@!g E 84B M 884: ),uten2erg*Rihter magni
energ" re%ati!n0
• Then7
@!g M! @!g )= 6 8/0 84B MC 884:
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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