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Size and Frequency of Occurrence

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Size and Frequency of Occurrence

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Size and frequency of occurrence
It is estimated that around 500,000 earthquakes occur each year, detectable with current
instrumentation. About 100,000 of these can be felt.
[29[!0
"inor earthquakes occur nearly
constantly around the world in #laces like $alifornia and Alaska in the %.&., as well as in 'l
&al(ador, "e)ico, *uatemala, $hile, +eru, Indonesia, Iran, +akistan,
the A,oresin +ortu-al, .urkey, /ew 0ealand, *reece, Italy, India and 1a#an, but
earthquakes can occur almost anywhere, includin- /ew 2ork $ity, 3ondon, and Australia.
[!1
3ar-er earthquakes occur less frequently, the relationshi# bein- e)#onential4 for e)am#le,
rou-hly ten times as many earthquakes lar-er than ma-nitude 5 occur in a #articular time
#eriod than earthquakes lar-er than ma-nitude 5. In the 6low seismicity7 %nited 8in-dom, for
e)am#le, it has been calculated that the a(era-e recurrences are9 an earthquake of !.:;5.<
e(ery year, an earthquake of 5.:;5.5 e(ery 10 years, and an earthquake of 5.< or lar-er
e(ery 100 years.
[!2
.his is an e)am#le of the *utenber-;=ichter law.
.he "essina earthquake and tsunami took as many as 200,000 li(es on >ecember 2?, 190?
in &icily and$alabria.
[!!
.he 1917 El Salvador earthquake
.he number of seismic stations has increased from about !50 in 19!1 to many thousands
today. As a result, many more earthquakes are re#orted than in the #ast, but this is because
of the (ast im#ro(ement in instrumentation, rather than an increase in the number of
earthquakes. .he %nited &tates *eolo-ical &ur(ey estimates that, since 1900, there ha(e
been an a(era-e of 1? [email protected] earthquakes 6ma-nitude :.0;:.97 and one -reat earthquake
6ma-nitude ?.0 or -reater7 #er year, and that this a(era-e has been relati(ely stable.
[!5
In
recent years, the number of [email protected] earthquakes #er year has decreased, thou-h this is
#robably a statistical fluctuation rather than a systematic trend.
[!5
"ore detailed statistics on
the si,e and frequency of earthquakes is a(ailable from the %nited &tates *eolo-ical
&ur(ey 6%&*&7.
[!<
A recent increase in the number of [email protected] earthquakes has been noted,
which could be e)#lained by a cyclical #attern of #eriods of intense tectonic acti(ity,
inters#ersed with lon-er #eriods of lowAintensity. Bowe(er, accurate recordin-s of
earthquakes only be-an in the early 1900s, so it is too early to cate-orically state that this is
the case.
[!:
"ost of the worldCs earthquakes 690D, and ?1D of the lar-est7 take #lace in the 50,000 km
lon-, horseshoeAsha#ed ,one called the circumA+acific seismic belt, known as the +acific
=in- of Eire, which for the most #art bounds the +acific +late.
[!?[!9
"assi(e earthquakes tend
to occur alon- other #late boundaries, too, such as alon- the Bimalayan "ountains.
[50
Fith the ra#id -rowth of me-aAcities such as "e)ico $ity, .okyo and .ehran, in areas of
hi-h seismic risk, some seismolo-ists are warnin- that a sin-le quake may claim the li(es of
u# to ! million #eo#le.
[51
Induced seismicity
Main article: Induced seismicity
Fhile most earthquakes are caused by mo(ement of the 'arthCs tectonic #lates, human
acti(ity can also #roduce earthquakes. Eour main acti(ities contribute to this #henomenon9
storin- lar-e amounts of water behind a dam 6and #ossibly buildin- an e)tremely
hea(y buildin-7, drillin- and [email protected] liquid into wells, and by coal minin- and oil drillin-.
[52
+erha#s the best known e)am#le is the 200? &ichuan earthquake in $hinaCs &ichuan
+ro(ince in "ay4 this tremor resulted in <9,22: fatalities and is the 19th deadliest earthquake
of all time. .he 0i#in-#u >am is belie(ed to ha(e fluctuated the #ressure of the fault 1,<50
feet 650! m7 away4 this #ressure #robably increased the #ower of the earthquake and
accelerated the rate of mo(ement for the fault.
[5!
.he -reatest earthquake in AustraliaCs
history is also claimed to be induced by humanity, throu-h coal minin-. .he city of
/ewcastle was built o(er a lar-e sector of coal minin- areas. .he earthquake has been
re#orted to be s#awned from a fault that reacti(ated due to the millions of tonnes of rock
remo(ed in the minin- #rocess.
[55
Measuring and locating earthquakes
Main article: Seismology
'arthquakes can be recorded by seismometers u# to -reat distances, because seismic
wa(es tra(el throu-h the whole 'arthCs interior. .he absolute ma-nitude of a quake is
con(entionally re#orted by numbers on the moment ma-nitude scale 6formerly =ichter scale,
ma-nitude : causin- serious dama-e o(er lar-e areas7, whereas the felt ma-nitude is
re#orted usin- the modified "ercalli intensity scale 6intensity II;GII7.
'(ery tremor #roduces different ty#es of seismic wa(es, which tra(el throu-h rock with
different (elocities9
• 3on-itudinal +Awa(es 6shockA or #ressure wa(es7
• .rans(erse &Awa(es 6both body wa(es7
• &urface wa(es H 6=aylei-h and 3o(e wa(es7
+ro#a-ation (elocity of the seismic wa(es ran-es from a##ro). ! kmIs u# to 1! kmIs,
de#endin- on the density and elasticity of the medium. In the 'arthCs interior the shockA or +
wa(es tra(el much faster than the & wa(es 6a##ro). relation 1.: 9 17. .he differences in tra(el
time from the e#icentre to the obser(atory are a measure of the distance and can be used to
ima-e both sources of quakes and structures within the 'arth. Also the de#th of
the hy#ocenter can be com#uted rou-hly.
In solid rock +Awa(es tra(el at about < to : km #er second4 the (elocity increases within the
dee# mantle to J1! kmIs. .he (elocity of &Awa(es ran-es from 2;! kmIs in li-ht sediments
and 5;5 kmIs in the 'arthCs crust u# to : kmIs in the dee# mantle. As a consequence, the
first wa(es of a distant earthquake arri(e at an obser(atory (ia the 'arthCs mantle.
Kn a(era-e, the kilometer distance to the earthquake is the number of seconds between the
+ and & wa(e times 8.
[55
&li-ht de(iations are caused by inhomo-eneities of subsurface
structure. Ly such analyses of seismo-rams the 'arthCs core was located in 191! by Leno
*utenber-.
'arthquakes are not only cate-ori,ed by their ma-nitude but also by the #lace where they
occur. .he world is di(ided into :55 Elinn;'n-dahl re-ions 6EA' re-ions7, which are based
on #olitical and -eo-ra#hical boundaries as well as seismic acti(ity. "ore acti(e ,ones are
di(ided into smaller EA' re-ions whereas less acti(e ,ones belon- to lar-er EA' re-ions.

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