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.
"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 $ity, 3ondon, and Australia.
3ar-er earthquakes occur less frequently, the relationshi# bein- e)#onential4 for e)am#le,
rou-hly ten times as many earthquakes lar-er than ma-nitude 5 occur in a #articular time
#eriod than earthquakes lar-er than ma-nitude 5. In the 6low seismicity7 %nited 8in-dom, for
e)am#le, it has been calculated that the a(era-e recurrences are9 an earthquake of !.:;5.<
e(ery year, an earthquake of 5.:;5.5 e(ery 10 years, and an earthquake of 5.< or lar-er
e(ery 100 years.
.his is an e)am#le of the *utenber-;=ichter law.
.he "essina earthquake and tsunami took as many as 200,000 li(es on >ecember 2?, 190?
in &icily and$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.
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.
"ore detailed statistics on
the si,e and frequency of earthquakes is a(ailable from the %nited &tates *eolo-ical
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
"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.
"assi(e earthquakes tend
to occur alon- other #late boundaries, too, such as alon- the Bimalayan "ountains.
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.
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-.
+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.
.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.
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
• 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.
&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
'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.