199

POTRESI

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Potresi

• potres:

• rasjed:

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Elastični odskok

• naprezanje akumulirano u stijeni sve dok ne nadvlada čvrstoću stijene

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200

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Stijena se deformira elastično, te nastaje elastični odskok u vrijeme razvoja rupture uslijed potresa.

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Varijacije čvrstoće stijena i stresa.

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201

Razvoj potresa(unutar 20 sekundi)

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0. sekunda• ruptura (lomljenje) ekspandira cirkularno na rasjednoj površini šaljući seizmičke valove u svim smjerovima

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5. sekunda• ruptura nastavlja ekspandirati kao pukotina duž rasjedne površine• kada ruptura dosegne površinu, premiještanje se događa duž trase rasjeda i stijene na površini počinju poskakivati

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202

10. sekunda• ruptura napreduje duž rasjedne plohe, reducira stres i omogućava stijenama odskok na obje strane rasjeda• seizmički valovi se kontinuirano odašilju u svim smjerovima na rasjednoj plohi

produžava se rasjedna pukotina

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20. sekunda• ruptura je napredovala duž cijele dužine rasjeda• rasjed je ostvario maksimalno premještanje i potres prestaje

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prije potresa nakon potresa

predudari

hipocentar budućega glavnog potresa

post-udarihipocentar glavnogpotresa

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203

Nazivlje

• hipocentar (fokus):

• epicentar:

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Seizmologija

• znanstvena disciplina koja se bavi napredovanjem mehaničke energije koja je oslobođena potresima i eksplozijama u Zemljinoj unutarnjosti

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Seizmograf

• instrument kojim se mjeri pomicanje na Zemlji

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204

Za registraciju vertikalnih kretanja

Za registraciju horizontalnih kretanja

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Tri različite vrste seizmičkih valova

P valoviS valovipovršinski valovi

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Seizmički valovi

• valovi se počinju širiti uslijed inicijalne tenzije ili kompresije u stijeni

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205

Dvije glavne vrste valova potresa

• P valovi (kompresijski) 6–8 km/s

• S valovi (smični) 4–5 km/s

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Kompresijski valovi (P). Kompresija je praćena širenjem. Svaki mali fragment materije zatrese se prema natrag i prema naprijed kako val napreduje u izmjeni kompresije i širenja.Posljedica pružnog rasjedanja.

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Smični valovi (S). Valna kresta se kreće kroz stijenu. Mali fragment pokazuje deformaciju smicanja koja se pojavljuje kada ga val potresa zahvati.Posljedica normalnog rasjedanja.

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206

Kretanje vala na površini

a) Površina Zemlje vibrira u kotrljajućem, eliptičnom kretanju koje nestaje s dubinom ispod površine.

b) Površina Zemlje se trese u stranu, bez vertikalnog kretanja.

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Lociranje epicentra

• razlika u vremenu dolaska između P i S valova na mjernim postajama je funkcija udaljenosti od epicentra

• zbog toga, potrebno je najmanje tri postaje za odredbu mjesta epicentra

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207

Očitanja na različitim seizmografskim postajama određuju smještaj epicentra

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Znajući udaljenost među postajama, konstruiraju se kružnice i na mjestu njihova sjecišta locira se epicentar.

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Mjerenje intenziteta potresa

1) premještanje površine

– 1964. potres na Aljasci premjestio dijelove morskoga dna 150 m

– 1906. potres u San Francisku pomaknuo tlo za 2,7 m

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208

Mjerenje intenziteta potresa

2) trajanje vibracija– do nekoliko desetaka sekunda

3) ljestvica intenziteta

– temeljena na šteti i čovjekovoj percepciji

4) ljestvica magnituda– temeljena na količini oslobođene

energije

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Richterova ljestvica

• Richterova ljestvica: količina energije primljena na udaljenosti od 100 km od epicentra

• najjači potres koji je ikada izmjeren = 8.9

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Odnosi između Richterovih magnituda (lijevo), oslobađanja energije u ekvivalentnim kg eksploziva (desno), broj potresa u godini dana (sredina).

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209

Prepoznavanje vrsta potresa iz seizmičkih podataka

• kretanje različitih rasjeda:(normalni, reverzni, pružni) će proizvesti seizmičke valove različitih osobina

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paraklaza

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Normalni rasjed

tenzijske sile

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Reverzni rasjed

kompresijske sile

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Pružni rasjed

smične sile

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Raspored potresa

• nije nepravilan

• fokusiran oko rubova ploča (ali se potresi pojavljuju i unutar ploča)

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211

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Šteta uzrokovana potresima

• kretanje tla

• vatra

• tsunami– nije plimni val

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212

• klizanja tla

• poplave

Šteta uzrokovana potresima

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Nastanak tsunamija

reverzni rasjed

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Kompjuterska simulacija tsunamija uzrokovanog potresom magnitude 7,7 na području Aleucijskih otoka.

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213

Potencijalni hazardi potresa na Zemlji

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