BBC Focus - 04.2020_

Q&A

THE EXPLAINER

WHY CAN’T WE PREDICT

EARTHQUAKES?

WHYAREEARTHQUAKES SO DIFFICULT

TO PREDICT?

Theproblemwithearthquakesis that– unlikevolcanicblasts– they

typicallyhappenwithoutconsistentwarningsigns.Inthe1980s,

seismologistsconstructedwhatwastoutedastheworld’smostdetailed

andsophisticatednetworkofmonitoringsensors,aroundtheParkfield

segmentofCalifornia’sSanAndreasFault.Anearthquakewasexpected

tohappen,theyhadcalculated,sometimebetween 1988 and1993,and

thesensorsweredesignedtopickupprecursoryphysicalchangesinthe

crustthatcouldformthebasisofanearthquakepredictionmodel.Inthe

end,themagnitude6.0quakearrivedlate, in 2004, with no forewarning

or physical precursors whatsoever.

HOWCLOSE CAN

WE GET?

WHATCAUSESAN
EARTHQUAKE?

AstheEarth’stectonicplates
pushandpulleachother, they
generatestrainthat
accumulatesonlinesof
weaknessinthecrustknownas
faults.Whenthisstrainreaches
a criticallevel,a faultwillbreak
andtherockmassesoneither
sidescrapepastoneanother,
releasinga hugeamountof
energythatcausestheground
toshake.Everyyear,thereare
morethanthreemillion
earthquakes;Californiaalone
experiencesmorethan10,000.
Mostaretoosmalltodetect,
exceptbyseismometers,but
around 15 a yeararemajor
shocksofmagnitudesevenand
above,capableofcausing
major damage and loss of life. GET T YY IMAGES X3, ALAMY, SUPERSTOCK, NASA/JPL

Toqualifyasa success,an

earthquakepredictionwould

havetoprovidedataonthe

timing,sizeandlocationofan

event,sufficientlyinadvanceso

thatactioncouldbetakento

limitinjuryandlossoflife.This

hasneverbeenaccomplished.

However,earthquakesonmost

faultshavecharacteristicreturn

periods,sothatononefaultthey

mayhappeneverydecade;on

another,everycentury;andso

on.Thefrequencydependsupon

howmuchstraina faultcan

accommodatebeforeit ruptures.

Thisrelationshipmeansthat

earthquakescanbe‘forecast’.

Forecastsarelessprecisethan

predictionsbutareusefulin

evaluatingthelevelofrisk.So,if

therehasbeennoquakefor 120

yearsona faultthatbreaks,on

average,every 100 years,then

thereis a highprobability that a

quake is imminent.

Earthquakesare

causedbythemassive

amountsofenergy

releasedastectonic

platesscrapepast

each other

Despitebeingableto

accuratelydetectand

recordearthquakesas

theyarehappening,

theyarenotoriously

difficult to predict