quake (sismos)

8/9/2019 Quake (Sismos)

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Lecture 16 Earthquakes

• What are earthquakes?

• Elastic rebound theory

• Waves generated by earthquakes:

P waves, waves, ur!ace waves

• Locating earthquakes• Earthquake "agnitude

• Earthquake distributions


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Denali Earthquake 2002/11/03, 5km, M7.9


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• #orthridge earthquake, $an 1%%&' (he )arking lot

at *al tate +niv' #orthridge ca")us colla)sed

during the earthquake'


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What are earthquakes?

Earthquakes are vibrations o! earth, )roduced by the ru)ture and sudden

"ove"ent o! rocks, which are caused bystresses beyond the elastic li"its o! the

rocks'


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• (he )oint inside the Earth where the ru)turegenerates earthquake energy is called focus or

hypocenter' (he )oint at the Earths sur!ace

directly above the !ocus is the epicenter'


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Elastic rebound theory

• -eid in early ./th century suggested that

earthquakes are caused by 0s)ring back0 o!

de!or"ed rocks ter"ed elastic rebound2'

• E3a")le: (he dis)lace"ent in the 1%/6

an 4rancisco earthquake was as "uch as

15 !eet'


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• Elastic rebound' s

rock is de!or"ed, it

bends, storing elasticenergy' 7nce strained

beyond its breaking

)oint, the rock cracks,

releasing the stored8u)

energy, which

generates earthquake

waves'


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• (his !ence was o!!set .'5 " in the 1%/6 an

4rancisco earthquake 9'' 9ilbert2


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• 4ault scar) )roduced by vertical dis)lace"ent in

the 1%6& laska earthquake' +92


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Waves generated by earthquakes

(hree ty)es o! waves are generated by an

earthquake:

P waves )ri"ary waves2,

waves secondary waves2

ur!ace waves -ayleigh waves, and Love

waves2


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• P waves are co")ressional waves, which )ushco")ress2 and )ull e3)and2 rock )articles in the

direction the waves are traveling'


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• S waves are shear waves' (he )articles in waves "ovesat right angles to the direction the waves are traveling'


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Surface waves

• (he "otions o! sur!ace waves are "ore co")le3'

(here are two ty)es o! sur!ace waves that )ro)agate

along Earths sur!ace: Rayleigh waves and Love

waves.

• ur!ace waves are generally strongest waves'

(heir a")litudes decay strongly with de)thi'e' they are generally con!ined near the

sur!ace2'


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• (he )article "otions o! Love waves (top) are )er)endicular tothe )ro)agation direction and )arallel to the sur!ace' (he )article

"otions o! Rayleigh waves (botto) are along the

)ro)agation direction and the vertical )lane'


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!ave speeds

• P waves are !aster than waves'

• waves are !aster than sur!ace waves'

• (hus, in a ty)ical seis"ogra", P wavearrives !irst, then wave, and then sur!ace

wave'


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• (y)ical seis"ogra", showing P wave, wave,

and sur!ace wave'


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• eis"ogra)h' (he inertia o! the sus)ended "ass

tends to kee) it "otionless, while the recording

instru"ent vibrates with the earth'


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Locating earthquakes• Locating an earthquake requires !our basic )ara"eters: origin

ti"e, and latitude, longitude, and de)th o! the hy)ocenter'

• basic "ethod !or locating an earthquake is using travel ties

of P waves and S waves recorded at seis"ic stations'

• 4or e3a")le, e)icenter can be quickly esti"ated !ro" the ti"e

interval between the P wave and the wave recorded at ; or

"ore stations:

12 +sing one station, the distance between the e)icenter and the

station can be esti"ated !ro" the 8P ti"e at the station'

.2 (he e)icenter can then located using distance esti"ates

!ro" ; or "ore stations'


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• (he distance to the e)icenter can be deter"ined using

8P ti"e'


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• Earthquake e)icenter can be located using the

distances obtained !ro" travel ti"es o! three or

"ore seis"ic stations'


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Earthquake "agnitude

• Earthquake "agnitude "easures the a"ount o!

energy released by an earthquake'

• (he best8known "agnitude scale is Richter scale,develo)ed by *harles -icther o! *altech'

• -ichter "agnitude is deter"ined by 12 the largesta")litude o! the seis"ogra" recorded at the

Wood8nderson instru"ent and .2 the distance

!ro" the !ocus'


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• -ichter scale can be deter"ined gra)hically using a

seis"ogra" recorded at a Wood8nderson instru"ent'


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• Richter scale is logarith"ic' Each unit

increase corres)onds to ten!old increase ina")litude, and ;.8!old increase in energy'

• (hus, the largest a")litude o! a "agnitude6 earthquake is 1/ ti"es as great as that o!

"agnitude 5' (he energy released by a

"agnitude 6 earthquake is ;. ti"es that o! a

"agnitude 5 earthquake'


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Earthquake distributions

• great "a>ority o! earthquakes originates

in a !ew narrow one o! the globe along

)late boundaries'


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Earthquake depths

• Earthquakes occur at de)ths !ro" near the sur!ace

to nearly @// k", although a great "a>ority o!

earthquakes are shallow'

hallow earthquakes: /8@/ k"

inter"ediate earthquakes: @/ 8 ;// k"

dee) earthquakes: ;//8@// k"


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• Wadati8Aenio!! one at (onga subduction one' l"ost all dee)

earthquakes are associated with subduction ones' (he !ocal

de)ths increase with the distances !ro" the trench' (hese

subduction seis"ic ones are called Wadati Aenio!! ones

quake (sismos)

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