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13.2: Epicenter, fokus in valovi - geoznanost

13.2: Epicenter, fokus in valovi - geoznanost


Pregled

Potres je kot telegram z Zemlje. Tresenje ali tresenje, ki ga doživimo med potresom, je posledica hitrega sproščanja energije v Zemlji, običajno zaradi premikanja vzdolž geoloških napak. Pomislite na napako pri udarnem zdrsu iz poglavja Deformacija skorje. Kamnine na obeh straneh preloma drsijo drug mimo drugega. Ko se premikajo v nasprotnih smereh, se kamnine deformirajo, saj se rahlo upognejo in povečajo pritisk. Sčasoma bodo dosegli prelomno točko. Ko bo moč kamnine presežena, se bodo skale vrnile v normalno obliko in sprostile vso to shranjeno energijo kot potres. Več energije je shranjeno, večji je potres. Spomnite se diagrama napetosti in deformacije iz deformacije skorje. Ko so kamnine pod velikim stresom, so podvržene krhki odpovedi (potres). Trdnost kamnine je na tej točki presežena.

Potresi izvirajo iz točke, ki se imenuje žarišče (množinska žarišča). Od te točke energija potuje navzven v različnih vrstah valov. Kraj na zemeljski površini neposredno nad žariščem se imenuje epicenter (slika 13.2). Žarišča potresa so lahko plitva (manj kot 70 km od zemeljske površine) do globoka (večja od 300 km globoko), čeprav so plitke do vmesne globine veliko pogostejše. Pogostnost in globina potresa sta povezani z mejami plošč. Velika večina (95%) potresov se pojavi vzdolž meje plošče, pri čemer se potresi plitkega žarišča pojavljajo na razhajajočih se in spreminjajočih se mejah plošč, plitvi do vmesni do globoko žarišni potresi pa se pojavljajo na konvergentnih mejah (vzdolž subducirajoče plošče). Potresi, povezani s konvergentnimi mejami, se pojavijo vzdolž območij Wadati-Benioff ali preprosto Benioff cone, območja potresne potresnosti vzdolž subducirajoče plošče (slika 13.3).

Ob potresu nastaneta dve različni vrsti valov: telesni valovi, tako imenovani, ker potujejo po telesu Zemlje, in površinski valovi ki potujejo po zemeljski površini (slika 13.4). Obstajata dve vrsti telesnih valov. P-valoviali primarni valovi so kompresijski valovi, ki se premikajo naprej in nazaj, podobno kot delovanje harmonike. Med prehajanjem vala se atomi v materialu, skozi katerega potuje, stisnejo in raztegnejo. Gibanje je kompresijsko vzporedno s smerjo širjenja valov, zaradi česar so P-valovi najhitrejši od potresnih valov. Ti valovi lahko potujejo skozi trdne snovi, tekočine in pline, ker je mogoče vse materiale do neke mere stisniti. S-valoviali sekundarni valovi so strižni valovi, ki premikajo material v smeri, pravokotni na smer vožnje. S-valovi lahko potujejo le skozi trdne snovi in ​​so počasnejši od valov P. Podobno gibanje kot gibanje S-vala lahko ustvarita dve osebi, ki držita vrv, pri tem pa ena hitro vrvi. Druga možnost je, da si omislite tudi to valovno gibanje, podobno valu, ki ga ustvarijo navijači na stadionu, ki vstanejo in sedejo. Telesni valovi so odgovorni za sunke in tresenje, ki jih čutimo med potresom.

Površinski valovi so počasnejši od telesnih valov in ponavadi povzročajo bolj občutke kotaljenja pri tistih, ki doživljajo potres, podobno kot v čolnu na morju. Ker se površinski valovi nahajajo na površini tal, kjer se nahajajo ljudje (in njihove strukture), in ker se premikajo tako počasi, kar jih združuje in povečuje njihovo amplitudo, so najbolj škodljivi med potresnimi valovi. Ljubezenski valovi so hitrejši površinski valovi, ki premikajo material naprej in nazaj v vodoravni ravnini, ki je pravokotna na smer potovanja valov (glej sliko 13.4). Stavbe ne prenašajo tovrstnega gibanja dobro in Ljubezenski valovi so lahko odgovorni za precejšnje poškodbe struktur. Rayleighovi valovi povzročajo, da se zemeljska površina premika v eliptičnem gibanju, podobno gibanju v morskem valu. Posledica tega je premikanje tal navzgor in navzdol ter od strani do strani.


Žarišče in žarišče potresa

Kje je potres? Žarišče potresa je točka, kjer se skale začnejo lomiti. To je izvor potresa.

Epicenter je točka na kopnem neposredno nad žariščem.

Fokus potresa, USGS

HIPOCENTER potresa

Fokus se imenuje tudi hipocentar potresa. Vibrirajoči valovi potujejo stran od žarišča potresa v vse smeri. Valovi so lahko tako močni, da bodo dosegli vse dele Zemlje in povzročili, da vibrira kot obračalna vilica.

Epicentar potresa

Neposredno nad žariščem na zemeljski površini je potresno žarišče. Potresni valovi se začnejo v žarišču in potujejo navzven v vse smeri. Potresni valovi NE izvirajo iz epicentra.

Novice o potresih

Večina novic o potresih bo naštelo epicentar potresa in nato povedalo, kako globoko je bil potres od epicentra. Veliki potresi, ki se pojavljajo v območjih subdukcije, lahko dajo potresno fokus, vendar se dejansko razbijejo na stotine kilometrov. Čilski potres leta 1960 se je prelomil vzdolž 800 kilometrov od prelomne črte.

Potresi plitkega žarišča

Richterjeva lestvica, ki se uporablja za potrese s plitvim fokusom
Potresi plitkega žarišča se pojavijo med 0 in 40 milj globoko. Potresi s plitvim fokusom so veliko pogostejši od potresov z globoko žariščem. Skorudne plošče, ki se gibljejo ena proti drugi, povzročijo večino potresov plitvih žarišč na Zemlji. Ti potresi so na splošno manjši in znanstveniki pri merjenju teh potresov uporabljajo Richterjevo lestvico.

Energija, ki jo sproščajo potresi s plitkim žariščem
Potresi s plitvim fokusom so veliko bolj nevarni kot potresi z globoko žarišče. Vsako leto sprostijo 75% vse energije, ki jo proizvedejo potresi. Gre za potrese v skorji, ki so manjši od potresov z globoko žarišče.

Potresi z globokim fokusom uporabljajo lestvico trenutne magnitude

Potresi z globokim fokusom se pojavijo 180 milj ali več pod zemeljsko površino. Ti potresi se pojavljajo v otočnih lokih ali globokih oceanskih jarkih, kjer ena plošča zdrsne čez drugo v conah subdukcije. Veliki potresi, pri katerih ena plošča zdrsne čez drugo ploščo v coni subdukcije, sprožijo potrese z globokim fokusom. To so največji potresi, za njihovo merjenje pa znanstveniki uporabljajo lestvico magnitude.


Potresi in potresni valovi

Potres je sestavljen iz vibracij zemeljske površine, ki sledijo sproščanju energije v zemeljski skorji (Stein, str. 215-285). Sproščanje energije je lahko zaradi zdrsa vzdolž geološke napake ali območja preloma ali premikanja magme, povezanega z vulkansko aktivnostjo. Tlak na Zemlji se lahko poveča in povzroči upogibanje geoloških enot, nato nenaden prelom in & quotnapping & quot; na novo orientacijo. V procesu loma nastajajo vibracije, imenovane seizmični valovi. Ti valovi potujejo navzven od vira potresa vzdolž površine in skozi Zemljo z različnimi hitrostmi, odvisno od materialov, skozi katere se premikajo.

Viri potresov in potresnih valov

Potresne vibracije (potresni valovi) lahko sprožijo številna podzemna dejanja, vključno s premikanjem vzdolž prelomov in vulkansko aktivnostjo. Nekatere človeške dejavnosti, vključno z uporabo eksplozivov in mehanskimi metodami (npr. Spuščanje teže in udarjanje kladiva), lahko povzročijo potresne valove. Najmočnejši potresi nastanejo zaradi velikih premikov zemlje po prelomih. Dejavnosti človeka se včasih uporabljajo za ustvarjanje potresnih valov za geofizikalne raziskave podzemlja z uporabo seizmičnih lomov ali odsevnih metod.

Epicentar in žarišče potresa

Zgornja risba prikazuje epicentar in žarišče potresa. Fokus je točka ali središče, kjer se začne sproščanje energije. Epicenter je točka na zemeljski površini neposredno nad žariščem potresa. Ko pride do sproščanja energije, potresni valovi potujejo stran od žarišča v vse smeri.

Fotografija (vir: morgueFile.com) na levi prikazuje tipične potresne poškodbe na ohlapnih kamnitih ali zidanih blokih. Konstrukcije, izdelane iz številnih materialov, so nagnjene k poškodbam zaradi tresenja, ki ga povzročijo potresni valovi, običajno blizu epicentra potresa. Pri potresno odpornih gradbenih tehnikah in materialih je mogoče zmanjšati škodo zaradi močnih potresnih vibracij.

Zgornja razprava ponuja osnovne informacije in ponazoritve za potrese in potresne valove. Za nadaljnje raziskovanje te teme je mogoče raziskati odlične publikacije ali vire interneta. Sledi nekaj hiperpovezav, ki lahko dodatno pomagajo pri oceni in opisu potresov in sezimičnih valov:


Uvodni potresi GeoloGy

13.10 Odgovori sTudenT -a 1. Za Carrier, Oklahoma, kakšen je približen čas prihoda prvega

a. 10 sekund b. 15 sekund c. 21 sekund d. 30 sekund

2. Kakšen je približen čas prihoda prvega S-vala za Marlow, Oklahoma?

a. 19 sekund b. 22 sekund c. 35 sekund d. 42 sekund

3. Kakšna je razlika med časom prihoda valov P in S za Bolivar, Missouri?

a. 10 sekund b. 20 sekund c. 40 sekund d. 55 sekund

4. Kakšna je približna razdalja do epicentra od Carrierja v Oklahomi?

a. 70 km b. 130 km c. 240 km d. 390 km

5. Kakšna je približna razdalja do epicentra od Marlowa v Oklahomi?

a. 70 km b. 130 km c. 240 km d. 390 km

6. Kakšna je približna razdalja do epicentra od Bolivarja v Missouriju?

a. 70 km b. 130 km c. 240 km d. 390 km

7. Poglejte lokacijo, ki ste jo določili kot žarišče potresa. Primerjajte njegovo lokacijo z Oklahoma Cityjem. V katero smer je epicenter od Oklahoma Cityja?

a. jugovzhod b. severozahod c. severovzhod d. jugozahodu

8. Preglejte podobo narodne katedrale pred in po njej. Na podlagi sprememb v strukturi se odločite, kje bi ta potres najverjetneje padel na spremenjeni Mercallijevi lestvici intenzivnosti. Na podlagi te slike bi bila najverjetnejša intenzivnost tega potresa:

a. & ltIV b. V-VI c. VII d. VIII ali več

Uvodni potresi GeoloGy

9. Prebivalci Port-au-Prince-a so se pritoževali nad močnim tresenjem med potresom, medtem ko so prebivalci Santo Dominga, glavnega mesta Dominikanske republike, ki leži 150 milj vzhodno od Port-au-Prince, domnevali, da je tresenje posledica prehajanja velik tovornjak. Na podlagi spremenjene Mercallijeve lestvice intenzivnosti so prebivalci Port-au-Princea večinoma imeli intenzivnost ___, medtem ko so prebivalci Santo Dominga intenzivnost ___.

a. VII, II b. VIII, III c. X, III d. X, IV

10. Močan potres prizadene San Mateo v Kaliforniji, medtem ko ste tam. Med tresenjem vas ujamejo v zaprtih prostorih. Bi bili raje v stavbi ameriške uprave za socialno varnost (na ulici South Claremont Street, San Mateo) ali pri San Mateo Park Rangers (na naslovu J Hart Clinton Drive, San Mateo)?

a. stavba uprave za socialno varnost ZDA b. San Mateo Park Rangers

11. Med obiskom Kalifornije močno zbolite in morate obiskati bolnišnico. Na podlagi vaših strahov pred možnim potresom bi se raje odpravili v bolnišnico Highland v Oaklandu ali bolnišnico Alameda v Alamedi?

a. Highland Hospital, Oakland, CA b. Bolnišnica Alameda, Alameda, CA

12. Po katerem letu se število potresov z magnitudo 3 ali več začne znatno povečevati?

a. 2007 b. 2009 c. 2011 d. 2015

13. Po katerem letu se število frakcijskih vrtin začne znatno povečevati?

a. 2007 b. 2009 c. 2011 d. 2015

14. Ali se zdi, da na podlagi grafa, ki ste ga zgradili, pomembni potresi in število fracking vrtin povezani?

15. Koliko časa je trajala razpoka (dolžina prizadete napake)?

a. 25 milj b. 74 milj c. 198 milj d. 296 milj e. 408 milj

Uvodni potresi GeoloGy

16. Poiščite žarišče potresa leta 1906. Ali se količina vodoravnega zdrsa hitreje zmanjšuje vzdolž severnega ali južnega konca razpoke?

a. severni konec razpoke b. južni konec razpoke

17. Kakšna je bila intenzivnost tresenja v Sacramentu?

a. svetloba b. močan c. huda d. nasilni e. ekstremno

18. Kakšna je bila intenzivnost tresenja v Sebastopolu?

a. svetloba b. močan c. huda d. nasilni e. ekstremno

19. Bi na podlagi zemljevida do leta 2031 bolj verjetno doživeli potres z magnitudo> 6,7 gt, če bi živeli na območju severozahodnega zaliva ali jugovzhodnega zaliva?

20. Ali so na podlagi zemljevida utekočinjanja območja bolj nevarna v notranjosti ali ob obali?

a. celinski b. vzdolž obale

13.1 UVOD To je bil najsmrtonosnejši dan v

zgodovina Mt. Everest. 25. aprila 2015 je Nepal prizadel potres z magnitudo 7,8. To je sprožilo plaz, ki je ubil 19 plezalcev na Mt. Everest. V Nepalu je umrlo več kot 8.800 ljudi, veliko več pa jih je bilo ranjenih in ostali brez strehe nad glavo. Od takrat se je zgodilo na stotine popotresnih sunkov (manjših potresov, ki sledijo večjemu potresu) (slika 13.1).

Potresi v tej regiji niso novost. Podobno število smrtnih žrtev je bilo v potresu leta 1934, v zgodovinskih časih pa se je zgodilo veliko drugih manjših potresov. Potres leta 1833 s podobno magnitudo je povzročil manj kot 500 smrtnih žrtev, čeprav sta bila to najverjetneje posledica dveh zelo velikih prahosov (manjših potresov pred glavnim potresom), ki sta večino prebivalcev prestrašila, kar je bilo zanje bolj varno . Po vsem svetu je bilo v tem stoletju veliko smrtonosnejših in močnejših potresov (Haiti, 2010 - 316.000 mrtvih na Sumatri, 2004 - 227.000 mrtvih, tako s smrtjo zaradi tresenja tal kot z drugimi nevarnostmi, ki jih je povzročil potres). Potresi geologom dajejo dragocene informacije o Zemlji, tako o njeni notranjosti, o čemer smo izvedeli v poglavju Zemeljsko notranjost, kot tudi o razmerah na zemeljski površini (večina potresov se zgodi na mejah plošč, kot smo izvedeli v poglavju Tektonika plošč, slika 4.8).

slika 13.1 | Zemljevid glavnega potresa, ki je prizadel Nepal 25. aprila 2015, skupaj z velikim popotresnim sunkom 12. maja in številnimi (& gt100) drugimi popotresnimi sunki (rdeče - upoštevajte lestvico magnitude v zgornjem desnem kotu). Avtor: USGS Vir: USGS Licenca: Javna domena

13 potresov Randa Harris

Uvodni potresi GeoloGy

Po zaključku tega poglavja bi morali biti sposobni: • primerjati in primerjati različne vrste potresnih valov • razumeti različne lestvice, ki se uporabljajo za merjenje potresov, in

jih uporabite za količino opustošenja • Razumeti, kako se različni geološki materiali obnašajo med

potres in posledični vpliv na konstrukcije • Pojasnite, kako se nahaja potresno središče • Raziščite odnos med industrijo fracking in potresnostjo

13.2 ePICenTer, fokus in valovi Potres je podoben telegramu z Zemlje. Pošilja sporočilo o

razmere pod zemeljsko površino. Tresenje ali tresenje, ki ga doživimo med potresom, je posledica hitrega sproščanja energije v Zemlji, običajno zaradi premikanja vzdolž geoloških napak. Pomislite na napako pri udarnem zdrsu iz poglavja Deformacija skorje. Kamnine na obeh straneh preloma drsijo drug mimo drugega. Ko se gibljejo v nasprotnih smereh, se kamnine deformirajo, saj se rahlo upognejo in povečajo tlak. Sčasoma bodo dosegli prelomno točko. Ko bo moč kamnine presežena, se bodo skale vrnile v normalno obliko in sprostile vso to shranjeno energijo kot potres. Več energije je shranjeno, večja je zemlja-

• Benioff cone • Valovi telesa • Epicenter • Fokus • Inducirana seizmičnost • Intenzivnost • Utekočinjanje • Ljubezenski valovi

• Magnituda • P valovi • Rayleighovi valovi • S valovi • seizmogram • seizmograf • seizmologija • površinski valovi

slika 13.2 | Ilustracija, ki prikazuje žarišče, kjer potres izvira, in epicenter, točko na površini tal neposredno nad žariščem. avtor: Neznan vir: Wikimedia Commons licenca: GNU Free Documentation

Uvodni potresi GeoloGy

potres je. Spomnite se diagrama napetosti in deformacije iz deformacije skorje. Ko so kamnine pod velikim stresom, so podvržene krhki odpovedi (potres). Trdnost kamnine je na tej točki presežena.

Potresi izvirajo iz točke, ki se imenuje žarišče (množinska žarišča). Od te točke energija potuje navzven v različnih vrstah valov. Kraj na zemeljski površini neposredno nad žariščem se imenuje epicenter (slika 13.2). Žarišča potresa so lahko plitva (manj kot 70 km od zemeljske površine) do globoka (večja od 300 km globoko), čeprav so plitke do vmesne globine veliko pogostejše. Pogostnost in globina potresa sta povezani z mejami plošč. Velika večina (95%) potresov se pojavi vzdolž meje plošče, pri čemer se potresi s plitvim žariščem ponavadi pojavljajo pri razhajajočih se in spreminjajočih se mejah plošč, plitvi pa do vmesnih do globoko fokusnih potresov, ki se pojavljajo na konvergentnih mejah (vzdolž subducirajoče plošče). Potresi, povezani s konvergentnimi mejami, se pojavljajo vzdolž območij Wadati-Benioff ali preprosto Benioff cone, območja padajoče potresnosti vzdolž subducirajoče plošče (slika 13.3).

Ob potresu nastanejo dve različni vrsti valov, ki jih imenujemo telesni valovi, ki potujejo po telesu Zemlje, in površinski valovi, ki potujejo vzdolž zemeljske površine (slika 13.4). Obstajata dve vrsti telesnih valov. P-valovi ali primarni valovi so kompresijski valovi, ki se premikajo naprej in nazaj, podobno kot delovanje harmonike. Med prehajanjem vala se atomi v materialu, skozi katerega potuje, stisnejo in raztegnejo. Gibanje je kompresijsko vzporedno s smerjo širjenja valov, zaradi česar so P-valovi najhitrejši od potresnih valov. Ti valovi lahko potujejo skozi trdne snovi, tekočine in pline, ker je mogoče vse materiale do neke mere stisniti. S-valovi ali sekundarni valovi so strižni valovi, ki premikajo material v smeri, pravokotni na smer vožnje. S-valovi lahko potujejo samo skozi trdne snovi in ​​so počasnejši od valov P. Dve osebi lahko ustvarita podobno gibanje kot gibanje S-vala-

slika 13.3 | To je presek potresnosti, posnet vzdolž subducirajoče plošče na konvergentni meji med oceanom in oceanom na Kurilskih otokih, ki leži severovzhodno od Japonske. Žarišča se nahajajo v padajoči plošči. Samo krhke snovi (kot je litosfera) lahko povzročijo potrese, zato mora biti to subducirajoča plošča. Zvezda predstavlja lokacijo potresa z magnitudo 8,3, ki se je zgodil 15.11.2006. avtor: vir USGS: licenca Wikimedia Commons: javna domena

Uvodni potresi GeoloGy

z vrvjo, pri čemer ena hitro zatakne vrv. Druga možnost je, da si omislite tudi to valovno gibanje, podobno valu, ki ga ustvarijo navijači na stadionu, ki vstane in sede. Telesni valovi so odgovorni za sunke in tresenje, ki jih čutimo med potresom.

Površinski valovi so počasnejši od telesnih valov in ponavadi povzročajo bolj občutke kotaljenja pri tistih, ki doživljajo potres, podobno kot v čolnu na morju. Ker se površinski valovi nahajajo na tleh, kjer so ljudje (in njihove strukture), in ker se premikajo tako počasi, kar jih združuje in povečuje njihovo amplitudo, so najbolj škodljivi za potresne valove. Ljubezenski valovi so hitrejši površinski valovi, ki premikajo material naprej in nazaj v vodoravni ravnini, ki je pravokotna na smer potovanja valov (glej sliko 13.4). Stavbe ne obvladujejo tovrstnega gibanja in ljubezenski valovi so lahko odgovorni za precejšnjo škodo na strukturah. Rayleighovi valovi povzročajo, da se zemeljska površina premika v eliptičnem gibanju, podobno gibanju v morskem valu. Posledica tega je premikanje tal navzgor in navzdol ter od strani do strani.

13,3 SEZMOLOŠKI Potresi so ljudje doživeli, dokler so ljudje hodili

Zemlja, čeprav je večina starodavnih kultur razvila mite za njihovo razlago (vključno z zamislijo velikih bitij na Zemlji, ki so se premikala, da bi ustvarila potres). Študija potresov, imenovana seizmologija, se je začela razvijati z razvojem instrumentov, ki lahko zaznajo potrese, ta instrument, imenovan seizmograf, pa lahko izmeri najmanjše vibracije Zemlje (slika 13.5). Tipični seizmograf je sestavljen iz mase, obešene na vrvico iz okvirja, ki se premika s premikanjem Zemljine površine. Na okvir je pritrjen vrtljiv boben, na maso pa pero, tako da se relativno gibanje zabeleži v seizmogramu. Okvir (pritrjen na tla) se premika med potresom-

slika 13.4 | Različne vrste potresnih valov. Telesni valovi v zgornjem delu slike so sestavljeni iz P-valov in S-valov. Gibanje valov P je kompresijsko. Kladivo na levi začne val premikati. Puščica na desni prikazuje splošno smer vala. V S-valu je gibanje valovito. Površinski valovi so prikazani v spodnjem delu karte. Ljubezenski valovi se gibljejo podobno kot S-valovi, kar povzroči vodoravno premikanje zemeljske površine. Rayleighovi valovi so površinski valovi, ki potujejo podobno kot val vzdolž vodne površine. avtor: vir USGS: licenca Wikimedia Commons: javna domena

Uvodni potresi GeoloGy

nagibna masa na splošno ostane mirna zaradi vztrajnosti (težnja telesa, da ostane v mirovanju in se upira gibanju).

13.3.1 Kako se meri potres?

Tragične posledice potresov je mogoče izmeriti na različne načine, na primer število smrtnih žrtev ali silo tresenja tal. Pogosto se uporabljata predvsem dva ukrepa. Eden je kvalitativno merilo škode, ki jo je povzročil potres, in se imenuje intenzivnost. Drugi je količinsko merilo energije, ki jo sprosti potres, imenovana magnituda. Oba ukrepa zagotavljata pomembne podatke.

13.3.2 Intenzivnost potresa

Meritve intenzivnosti upoštevajo tako škodo, nastalo zaradi potresa, kot tudi način odziva ljudi nanj. Spremenjena Mercallijeva lestvica intenzivnosti (slika 13.6) je najpogosteje uporabljena lestvica za merjenje jakosti potresov. Ta lestvica ima vrednosti od rimskih številk I do XII, ki označujejo opaženo škodo in odziv ljudi nanjo. Podatki za to lestvico se pogosto zbirajo takoj po potresu, tako da lokalno prebivalstvo odgovarja na vprašanja o škodi, ki jo vidijo, in o tem, kaj se je zgodilo med potresom. Te podatke je mogoče nato združiti, da se ustvari zemljevid intenzivnosti, ki na podlagi zbranih informacij ustvari obarvana območja (slika 13.7). Te zemljevide zavarovalništvo pogosto uporablja.

slika 13,5 | Seizmograf in seizmogram, ki ga proizvaja. avtor: Uporabnik “Yamaguchi” vir: Wikimedia Commons licenca: CC BY-SA 3.0

Uvodni potresi GeoloGy

Značilnosti intenzivnosti I Tresenje se v normalnih okoliščinah ne čuti. II Tresenje čutijo le tisti, ki počivajo, večinoma ob zgornjih nadstropjih stavb. III Ljudje v zaprtih prostorih opazno čutijo šibko tresenje. Mnogi tega ne prepoznajo kot potres. Vibra-

podobnih kot veliko vozilo, ki gre mimo. IV Svetlo tresenje v zaprtih prostorih mnogi, zunaj redki. Ponoči so se nekateri prebudili. Jedi, vrata in

okna motena stene razpokane. Občutek, kot bi tovornjak udaril v zgradbo. Avtomobili opazno skalijo. V Zmerno tresenje, ki ga čuti večina prebujenih. Nekaj ​​posod in oken je razbitih. Nestabilni predmeti

prevrnil. VI Vsi močno čutijo tresenje, mnogi so prestrašeni. Težko pohištvo se lahko premakne in omet se zlomi. Jez-

starost je majhna. VII Zelo močno tresenje pošlje vse na prostem. Dobro zasnovane zgradbe trpijo rahlo do zmerno

škoda v navadnih stavbah znatna škoda v slabo zgrajenih objektih. VIII Močno tresenje. Dobro zasnovane zgradbe utrpijo majhne poškodbe, pri običajnih zgradbah pa znatno škodo.

povzroča veliko škodo na slabo zgrajenih objektih. IX Silovito tresenje. Dobro zasnovane stavbe trpijo precejšnjo škodo, stavbe so odmaknjene od temeljev,

z delnim propadom. Podzemne cevi so pretrgane. X Ekstremno tresenje. Nekatere dobro zgrajene lesene konstrukcije uničijo večino zidanih in okvirnih konstrukcij

so uničeni. Plazovi precejšnji. XI Nekaj ​​preostalih struktur. Mostovi so porušeni, v tleh pa se odprejo velike razpoke. XII Totalna škoda. Predmeti, vrženi v zrak.

slika 13.6 | (Zgoraj) Skrajšana tabela spremenjene Mercallijeve lestvice intenzivnosti. Intenzivnost pri določenem potresu je določena z največjo nastalo škodo. avtor: Randa Harris vir: Izvirna licenca za delo: CC BY-SA 3.0

slika 13.7 | (Desno) Zemljevid intenzivnosti potresa San Fernando v južni Kaliforniji 2. 9. 76. Upoštevajte, da je bila intenzivnost v bližini epicentra (označena z zvezdico) izjemna. avtor: vir USGS: licenca Wikimedia Commons: javna domena

Uvodni potresi GeoloGy

13.3.3 jakost potresa

Drug način za razvrstitev potresa je energija, ki se sprosti med dogodkom, kar se imenuje magnituda potresa. Medtem ko je bila magnituda izmerjena z uporabo Richterjeve lestvice, se je s povečanjem pogostosti meritev potresov po vsem svetu ugotovilo, da lestvica magnitude Richterja ne velja za vse potrese (ni natančna za potrese velike jakosti). Razvita je bila nova lestvica, imenovana Moment Magnitude Intensity Scale, ki ohranja lestvico podobno Richterjevi. Ta lestvica ocenjuje skupno energijo, ki jo sprosti potres, in jo lahko uporabimo za karakterizacijo potresov vseh velikosti po vsem svetu. Velikost temelji na potresnem momentu (ocenjen na podlagi premikov tal, zabeleženih na seizmogramu), ki je produkt razdalje, ki jo je napaka premaknila, in sile, potrebne za premik. Ta lestvica še posebej dobro deluje pri večjih potresih in jo je sprejela Geološka raziskava Združenih držav Amerike. Magnituda temelji na logaritemski lestvici, kar pomeni, da se za vsako celotno število, ki ga povečate, amplituda gibanja tal, zabeležena s seizmografom, poveča za 10 in sproščena energija se poveča za 101,5, ne pa za eno (tako da nastane potres z magnitudo 3 v desetkratnem tresenju tal pri potresu 2 stopnje, pri potresu z magnitudo 4 je 102 ali 100 -krat večja stopnja tresenja tal kot pri potresu z 2 magnitudo (pri čemer se sprosti 103 ali 1000 -krat več energije). Za grobo primerjavo lestvice magnitude z intenzivnostjo , glejte sliko 13.8 Zakaj je potrebno imeti več vrst lestvic? Lestvica magnitude omogoča svetovno opredelitev vsakega potresnega dogodka, lestvica intenzivnosti pa ne. lokacijo bi lahko uvrstili v kategorijo II ali III na drugo lokacijo, ki temelji na gradnji stavb (npr. slabo zgrajene stavbe bodo utrpele večjo škodo v potresu enake jakosti kot tiste, zgrajene s trdnejšo gradnjo).

13.4 POZITIVNO UČINKOVITO UČINKOVITOST V času potresa se potresni valovi pošiljajo po vsem svetu. Čeprav so

lahko z razdaljo oslabijo, so seizmografi dovolj občutljivi, da še vedno zaznajo te valove. Za določitev lokacije žarišča potresa so uporabljeni seizmografi

Velikost Tipična največja spremenjena Mercallijeva intenzivnost 1,0 - 2,9 I 3,0 - 3,9 II - III 4,0 - 4,9 IV - V 5,0 - 5,9 VI - VII 6,0 - 6,9 VII - IX 7,0 in več VIII ali več

slika 13.8 | Primerjava lestvic magnitude in jakosti za potrese. avtor: Randa Harris vir: Izvirna licenca za delo: CC BY-SA 3.0

Uvodni potresi GeoloGy

za določen dogodek so potrebna vsaj tri različna mesta. Na sliki 13.9 je primer seizmograma postaje, ki vključuje manjši potres.

Ko najdete tri seizmografe, poiščite časovni interval med prihodom vala P in prihodom vala S. Najprej določite prihod vala P in preberite do dna seizmograma, da ugotovite, kdaj (običajno označeno v sekundah) je prišel val P. Nato naredite enako za S-val. Prihod potresnih valov bo prepoznan po povečanju amplitude - poiščite spremembo vzorca, ko bodo črte višje in bližje (npr. Slika 13.10).

Če pogledamo čas med prihodom valov P in S, lahko določimo razdaljo do

potres od te postaje, z daljšimi časovnimi intervali, ki označujejo daljšo razdaljo. Te razdalje se določijo s krivuljo časa potovanja, ki je graf časov prihoda P- in S-valov (glej sliko 13.11).

Čeprav je mogoče z grafom časa potovanja določiti razdaljo do epicentra, smeri ni mogoče določiti. Lahko narišemo krog s polmerom razdalje do potresa. Potres se je zgodil nekje vzdolž tega kroga. Triangulacija je potrebna, da se natančno določi, kje se je to zgodilo. Potrebni so trije seizmografi. Iz vsake od treh različnih lokacij seizmografa je narisan krog, kjer je polmer vsakega kroga enak razdalji od te postaje do epicentra. Mesto, kjer se ti trije krogi sekajo, je epicentar (slika 13.12).

slika 13.9 | Ta seizmogram se bere od leve proti desni in od zgoraj navzdol. Upoštevajte majhen potres, ki je označen, in posledično spremembo amplitude valov na tej točki. avtor: USGS vir: USGS licenca: javna domena

slika 13.10 | Primer seizmograma s prihodom valov P in S. Upoštevajte, kako je prihod valov zaznamovan s povečanjem višine valov (znano kot amplituda) in z bolj tesno zapakiranimi valovi. Ta primer ne vključuje časa vzdolž dna, toda tisti v laboratorijski vaji bodo. avtor: Uporabnik “Pekachu” vir: Wikimedia Commons licenca: javna domena

Uvodni potresi GeoloGy

slika 13.11 | Graf časa potovanja, ki vključuje prihod valov P in S-valov. Upoštevajte, da te krivulje narišejo razdaljo glede na čas in se izračunajo na podlagi dejstva, da je Zemlja krogla. Krivulje se spreminjajo glede na globino potresa, ker se valovi obnašajo drugače (tj. Spreminjajo se njihove hitrosti) z globino in spremembo materiala. Ta krivulja se uporablja za plitke potrese (globine 20 km) s postajami v razdalji 800 km. S-P krivulja se nanaša na časovno razliko med prihodom P-vala in S-vala. Če ste na svojem seizmogramu zabeležili, da je val P prišel ob 10 sekundah, val S pa po 30 sekundah, bi bila razlika med časom prihoda 20 sekund. Prebrali bi 20 sekund od osi y zgoraj do črte S-P, nato pa spustite navzdol, da določite razdaljo do epicentra. V tem primeru bi bilo to približno 200 kilometrov. avtor: Randa Harris vir: Izvirna licenca za delo: CC BY-SA 3.0

slika 13.12 | Za lokacijo tega potresnega žarišča so bili uporabljeni seizmogrami iz Portlanda, Salt Lake Cityja in Los Angelesa. Izračunali so čas med prihodom valov P in S, tabele časov potovanja pa podale razdaljo. Circles with each distance for its radii were drawn from each station. The one resulting overlap, at San Francisco, was the earthquake epicenter. author: Randa Harris source: Original Work license: CC BY-SA 3.0

Introductory GeoloGy earthquakes

13.5 lab exerCIse Part a – locating an epicenter

You will determine the location of an earthquake epicenter using seismograms from Carrier, Oklahoma, Smith Ranch in Marlow, Oklahoma, and Bolivar Missouri available at the end of this chapter. These are actual seismograms that you will be reading, from an actual event. For each, three different readouts are given, as the seismograph measured in three different axes. You may focus on any of the three readouts for each station, as all will have the same arrival times for each wave. First, determine when the P and S waves arrived, and note these times (remember to look for a pattern change as lines get taller and more closely spaced). Mark both the arrival of the P-wave and S-wave, then using the time scale in seconds, note the time difference between the P and S wave arrivals. Add this to the table below for each of the three seismograms.

Station P-wave arrival time (sec) S-wave arrival

time (sec) Difference between P

and S travel times (sec) Distance to Epicenter

from Station (km) Carrier, OK Marlow, OK Bolivar, MO

The difference between the P and S wave arrivals will be used to determine the distances to the epicenter from each station using Figure 13.11. Make sure that you use the curve for S-P Difference – find the seconds on the y-axis, read over to the S-P curve, then draw a line down to the x- axis for distance. Add the values to the table above. Now you need to create the circles from each station using Figure 13.13, a map with the three stations on it. This map includes a legend in kilometers. For each station, note the dis- tance to the epicenter. Using a drafting compass (or alternately, tie a string to a pencil, cut the string the length of the distance to the epicenter, pin it at the station, and draw a circle, with the pencil stretched out the full distance of the string), you will create the circle. First, measure the scale on the map in Figure 13.13 in centimeters, and use that to convert your distances in kilometers to centimeters (ex. the map’s scale of 100 km = 2.1 cm on your ruler, so if you had a measured distance from one station of 400 km, that would equal 8.4 cm on your ruler). For this fictional example, starting at the station, use the drafting compass to make a circle that is 8.4 cm in radius. Create a circle for each of the three stations, using their different distances to the epicenter. They should overlap (or nearly overlap) in one location. The location where they overlap is the approximate epicenter of the earthquake. Once done, answer the questions below.

Introductory GeoloGy earthquakes

1. For Carrier, Oklahoma, what is the approximate time of the arrival of the first P-wave?

a. 10 seconds b. 15 seconds c. 21 seconds d. 30 seconds

2. For Marlow, Oklahoma, what is the approximate time of the arrival of the first S-wave?

a. 19 seconds b. 22 seconds c. 35 seconds d. 42 seconds

3. For Bolivar, Missouri, what is the difference between the P and S wave arrival times?

a. 10 seconds b. 20 seconds c. 40 seconds d. 55 seconds

4. What is the approximate distance to the epicenter from Carrier, Oklahoma?

a. 70 km b. 130 km c. 240 km d. 390 km

5. What is the approximate distance to the epicenter from Marlow, Oklahoma?

a. 70 km b. 130 km c. 240 km d. 390 km

figure 13.13 author: Google Earth source: Google Earth license: Used with attribution per Google’s Permissions Guidelines

Introductory GeoloGy earthquakes

6. What is the approximate distance to the epicenter from Bolivar, Missouri?

a. 70 km b. 130 km c. 240 km d. 390 km

7. Look at the location that you determined was the earthquake epicenter. Compare its location to Oklahoma City. Which direction is the epicenter located from Oklahoma City?

a. southeast b. northwest c. northeast d. southwest

On January 12, 2010, a devastating magnitude 7.0 earthquake hit 16 miles west of Port-au-Prince, the capital of Haiti. At the following website, images are given of areas in Port-au-Prince both before the earthquake and soon after the earthquake, with a slider bar so that you can compare them. Access these images at: http:// www.nytimes.com/interactive/2010/01/14/world/20100114-haiti-im- agery.html (or alternately at http://elearningexamples.com/the-destruc- tion-in-port-au-prince-2/) and note the changes in many areas due to damage from the earthquake.

8. Examine the before and after image of the National Cathedral. Based on the changes seen within the structure, decide where this earthquake would most likely fall on the Modified Mercalli Intensity Scale. Based off this image, the most likely intensity of this earthquake would be:

a. <IV b. V-VI c. VII d. VIII or greater

9. Residents in Port-au-Prince complained of extreme shaking during the earthquake, while residents of Santo Domingo, the capital of the Dominican Republic that sits 150 miles east of Port-au-Prince, assumed the shaking was caused by the passing of a large truck. Based on the Modified Mercalli Intensity Scale, the residents of Port-au-Prince mostly like experienced an intensity of ___, while the residents of Santo Domingo experienced an intensity of ___.

a. VII, II b. VIII, III c. X, III d. X, IV

13.6 hazards from earThQuaKes Earthquakes are among nature’s most destructive phenomena, and there are

numerous hazards associated with them. Ground shaking itself leads to falling structures, making it the most dangerous hazard. The intensity of ground shaking depends on several factors, including the size of the earthquake, the duration of shaking, the distance from the epicenter, and the material the ground is made of. Solid bedrock will not shake much during a quake, rendering it safer than otherhttp://www.nytimes.com/interactive/2010/01/14/world/20100114-haiti-imagery.htmlhttp://www.nytimes.com/interactive/2010/01/14/world/20100114-haiti-imagery.htmlhttp://www.nytimes.com/interactive/2010/01/14/world/20100114-haiti-imagery.htmlhttp://elearningexamples.com/the-destruction-in-port-au-prince-2/http://elearningexamples.com/the-destruction-in-port-au-prince-2/

Introductory GeoloGy earthquakes

ground materials. Artificial fill refers to areas that have been filled in for construc- tion and/or waste disposal (think of a hill that gets cleared for a shopping mall – the soil that was removed is dumped somewhere else as artificial fill). Sediment is not compacted in areas of artificial fill, but compaction will occur during the shak- ing of an earthquake, leading to structure collapse. Artificial fill sediments behave similarly to water-saturated sediments. As they shake, they may experience lique- faction, in which the sediments behave like a fluid. Normally, friction between grains holds them together. Once an earthquake occurs, water surrounds every grain, eliminating the friction between them and causing them to liquefy (Figure 13.14). This can be very dangerous. Seismic waves will amplify as they come in contact with these weaker materials, leading to even more damage.

Other hazards associated with earthquakes include fire (as gas lines rupture), which may be difficult to combat as water lines may also be ruptured. The vast majority of damage during the 1906 San Francisco earthquake was due to fire. Earthquakes can trigger tsunamis, large sea waves created by the dis- placement of a large volume of water during fault move- ment. The Sumatra-Anda- man earthquake in 2004 triggered a tsunami in the

Indian Ocean that resulted in 230,000 deaths. Earthquakes can trigger landslides in mountainous areas, and initiate secondary hazards such as fires, dam breaks, chemical spills, or even nuclear disasters like the one at Fukushima Daiichi Nucle- ar Power Plant in Japan. Earthquake-prone areas can take steps to minimize de- struction, such as implementing strong building codes, responding to the tsunami warning system, addressing poverty and social vulnerability, retrofitting existing buildings, and limiting development in hazardous zones.

13.7 lab exerCIse Part b – liquefaction

Download the kml file from the USGS for Google Earth found here: http:// earthquake.usgs.gov/regional/nca/bayarea/kml/liquefaction.kmz (Al- ternately the file can be downloaded from this site: http://earthquake.usgs. gov/regional/nca/bayarea/liquefaction.php). Note that this file adds a lay- er of liquefaction susceptibility, with areas more likely to experience liquefaction

figure 13.14 | A diagram depicting liquefaction. In the water- saturated sediment on the left, the pore (open) spaces between the grains are filled with water, but friction holds the grains together. In liquefaction, on the right, water surrounds the grains so that they no longer have contact with each other, leading them to behave as a liquid. author: Randa Harris source: Original Work license: CC BY-SA 3.0http://earthquake.usgs.gov/regional/nca/bayarea/kml/liquefaction.kmzhttp://earthquake.usgs.gov/regional/nca/bayarea/kml/liquefaction.kmzhttp://earthquake.usgs.gov/regional/nca/bayarea/liquefaction.phphttp://earthquake.usgs.gov/regional/nca/bayarea/liquefaction.php

Introductory GeoloGy earthquakes

in yellow, orange, or red. Once in Google Earth, type in San Francisco, CA. Zoom in to less than 25 miles to see the layers added and note where liquefaction is most likely, then answer the following questions. When necessary, type locations into the Search box to locate them.

10. A significant earthquake hits San Mateo, California while you are there. During the shaking you are caught indoors. Would you rather be at the US Social Security Administration Building (located at South Claremont Street, San Mateo) or with the San Mateo Park Rangers (located at J Hart Clinton Drive, San Mateo)?

a. the US Social Security Administration Building b. the San Mateo Park Rangers

11. While visiting California, you become violently ill and must visit a hospital. Based off of your fears of a possible earthquake occurring, would you rather go to Highland Hospital in Oakland or Alameda Hospital in Alameda?

a. Highland Hospital, Oakland, CA b. Alameda Hospital, Alameda, CA

13.8 InduCed seIsmICITy The number of significant earthquakes within the central and eastern United

States has climbed sharply in recent years. During the thirty-six year period between 1973 and 2008, only 21 earthquakes with a magnitude of 3.0 or greater occurred. During the 5 year period of 2009-2013, 99 earthquakes of that size occurred within the same area, with 659 earthquakes in 2014 alone and well over 800 earthquakes in 2015 just in Oklahoma (see the blue and red line on the graph in Figure 13.15).

Human intervention is apparently the cause, resulting in induced seismicity (earthquakes caused by human activities). Humans have induced earthquakes in the past (for example, impounding reservoirs has led to earthquakes in Georgia), but this rapid increase in induced seismicity has led to much current research into the problem. Evidence points to several contributing factors, all related to types of fluid injection used by the oil industry. Hydraulic fracturing, also referred to as fracking, has been used for decades by oil and gas companies to improve well pro- duction. Fluid (usually water, though other fluids are often present) is injected at high pressure into low-permeability rocks in an effort to fracture the rock. As more fractures open up within the rock, fluid flow is enhanced and more distant fluids can be accessed, increasing the production of a well. In the past, this practice was utilized in vertical wells. With the recent advent of horizontal drilling technology, the fracking industry has really taken off. Drillers can now access thin horizon- tal oil and gas reservoirs over long distances, highly increasing well production in rocks that formerly were not exploited, creating a boom in US gas and oil pro- duction. While there have been many reports in the public that blame fracking for all of the increased seismicity rates, this is not entirely the case. Fracking mainly

Introductory GeoloGy earthquakes

produces very minor earthquakes (less than magnitude 3), though it has been shown to produce signifi- cant earthquakes on occasion. The majority of induced earthquakes are caused by injection of wastewater deep underground. This wastewater is the byproduct of fracking, so ulti- mately the industry is to blame.

As wells are developed (by fracking or other processes), large amounts of waste fluid, which may contain potentially hazardous chem- icals, are created. When the fluids cannot be recycled or stored in re- tention ponds above ground, they are injected deep underground, the- oretically deep enough to not come into contact with oil reservoirs or water supplies. These wastewater wells are quite common and are con- sidered a safe option for wastewater disposal. By injecting this water in areas that contain faults, the stress conditions on the faults change as friction is reduced, which can result in movement along faults (resulting in earthquakes).

For our lab exercise, we will focus on the state of Oklahoma, and the in- creased seismicity there (Figure 13.16).

The USGS has focused some re- search on the seismicity in Oklaho- ma and determined that the main seismic hazard within the state is the disposal of wastewater from the oil and gas industry by deep injection, though some smaller quakes (mag- nitude 0.6 to 2.9) have been shown to correlate directly to fracking. A 50% increase in earthquake rate has occurred within the state since 2013. One large earthquake of 5.7 magnitude struck in No- vember, 2011, and has been linked to an active wastewater injection site

200 me- ters away. A 4.7 magnitude earthquake struck in November 2015, too.

figure 13.15 | Chart of increased seismicity of magnitude 3 or greater earthquakes within the central and eastern U.S. from 1973-2015. The spatial distribution of the earthquakes is shown on the map, with blue dots representing quakes from 1973-2008, and red dots representing quakes from 2009-2015. author: USGS source: USGS license: Public Domain

figure 13.16 | Earthquakes that occurred within Oklahoma from 1970 – 5/27/15 are depicted above. Please note that the colors indicate year and the size indicates magnitude (see legend on image). The inset image is a close-up view of the outlined box. author: USGS source: USGS license: Public Domain

Introductory GeoloGy earthquakes

13.9 lab exerCIse Part C – Induced seismicity

The table below contains data regarding the number of fracking wells within the state of Oklahoma and the number of significant earthquakes (magnitude 3 or greater) that have occurred since 2000. Before answering the questions for this lab exercise, plot the information in the table below on the graph that is provided note that the graph has two y-axes, one for the number of fracking wells and the other for the number of earthquakes.

Year # of Fracking Wells in Oklahoma # of Earthquakes greater than M 3 2000 0 0 2001 0 0 2002 0 3 2003 0 0 2004 0 2 2005 0 1 2006 0 2 2007 0 1 2008 1 2 2009 4 20 2010 1 43 2011 637 63 2012 1,568 34 2013 470 109 2014 No Data 585 2015 No Data 850

(From: http://www.oudaily.com/news/oklahoma-reports-surge-in-earthquakes-during/arti- cle_79a364da-a1d4-11e5-894a-5ba84c8399c1.html)

Note: Information on number of fracked wells was obtained by SkyTruth through accessing FracFocus. Oklahomans are required to report all fracked wells, but the site was only created in 2011, so some wells may have not been retroactively added pre-2011. Seismic data was obtained through the USGS.

Introductory GeoloGy earthquakes

12. After what year does the number of magnitude 3 or greater earthquakes begin to rise significantly?

a. 2007 b. 2009 c. 2011 d. 2015

13. After what year does the number of fracking wells begin to rise significantly?

a. 2007 b. 2009 c. 2011 d. 2015

14. Based on the graph that you constructed, do significant earthquakes and the number of fracking wells appear to be related?

The exercises that follow use Google Earth. Let’s start by examining the 1906 earthquake that hit Northern California. Access the following website: http:// earthquake.usgs.gov/regional/nca/virtualtour/

There are several links of interest here. Spend some time familiarizing yourself with the site. Scroll down to the section entitled “The Northern California Earth- quake, April 18, 1906” and open the link. The San Andreas Fault is

800 miles long, located in California. In 1906, a major earthquake occurred along a portion of the fault. Scroll down and check out the Rupture Length and Slip.http://earthquake.usgs.gov/regional/nca/virtualtour/http://earthquake.usgs.gov/regional/nca/virtualtour/

Introductory GeoloGy earthquakes

15. How long was the rupture length (the length of the fault that was affected)?

a. 25 miles b. 74 miles c. 198 miles d. 296 miles e. 408 miles

Horizontal slip, or relative movement along the fault, ranged from 2-32 feet. To envision this, imagine that you are facing an object directly across from you. Suddenly, it moves up to 32 feet to your right! Horizontal slip is shown along the rupture as a histogram. Check out all the measurements along the fault by clicking the Rupture Length and Slip on the Google Earth link.

16. Locate the epicenter of the 1906 quake. Does the amount of horizontal slip decrease faster along the northern end or the southern end of the rupture?

a. northern end of the rupture b. southern end of the rupture

Go back to the “The Northern California Earthquake, April 18, 1906” page and scroll down to check out the Shaking Intensity. If your map begins to get difficult to read, remember that by clicking on a checked box in the Places folder, you can remove prior data. Use the search box to display the desired location.

17. What was the shaking intensity like in Sacramento?

a. light b. strong c. severe d. violent e. extreme

18. What was the shaking intensity like in Sebastopol?

a. light b. strong c. severe d. violent e. extreme

Navigate back to the main page and select “Earthquake Hazards of the Bay Ar- ea Today.” Check out the Earthquake Probabilities for the Bay Area.

19. Based on the map, would you be more likely to experience an earthquake of magnitude >6.7 by 2031 if living in the northwest Bay Area or southeast Bay Area?

Go back to the “Earthquake Hazards of the Bay Area Today” page and check out the Liquefaction Susceptibility in San Francisco. Look at the overall trend in the areas affected by liquefaction.

20. Based on the liquefaction map, are areas more dangerous inland or along the coast?

a. inland b. along the coast

Introductory GeoloGy earthquakes

13.10 sTudenT resPonses 1. For Carrier, Oklahoma, what is the approximate time of the arrival of the first

a. 10 seconds b. 15 seconds c. 21 seconds d. 30 seconds

2. For Marlow, Oklahoma, what is the approximate time of the arrival of the first S-wave?

a. 19 seconds b. 22 seconds c. 35 seconds d. 42 seconds

3. For Bolivar, Missouri, what is the difference between the P and S wave arrival times?

a. 10 seconds b. 20 seconds c. 40 seconds d. 55 seconds

4. What is the approximate distance to the epicenter from Carrier, Oklahoma?

a. 70 km b. 130 km c. 240 km d. 390 km

5. What is the approximate distance to the epicenter from Marlow, Oklahoma?

a. 70 km b. 130 km c. 240 km d. 390 km

6. What is the approximate distance to the epicenter from Bolivar, Missouri?

a. 70 km b. 130 km c. 240 km d. 390 km

7. Look at the location that you determined was the earthquake epicenter. Compare its location to Oklahoma City. Which direction is the epicenter located from Oklahoma City?

a. southeast b. northwest c. northeast d. southwest

8. Examine the before and after image of the National Cathedral. Based on the changes seen within the structure, decide where this earthquake would most likely fall on the Modified Mercalli Intensity Scale. Based off this image, the most likely intensity of this earthquake would be:

a. <IV b. V-VI c. VII d. VIII or greater

Introductory GeoloGy earthquakes

9. Residents in Port-au-Prince complained of extreme shaking during the earthquake, while residents of Santo Domingo, the capital of the Dominican Republic that sits 150 miles east of Port-au-Prince, assumed the shaking was caused by the passing of a large truck. Based on the Modified Mercalli Intensity Scale, the residents of Port-au-Prince mostly like experienced an intensity of ___, while the residents of Santo Domingo experienced an intensity of ___.

a. VII, II b. VIII, III c. X, III d. X, IV

10. A significant earthquake hits San Mateo, California while you are there. During the shaking you are caught indoors. Would you rather be at the US Social Security Administration Building (located at South Claremont Street, San Mateo) or with the San Mateo Park Rangers (located at J Hart Clinton Drive, San Mateo)?

a. the US Social Security Administration Building b. the San Mateo Park Rangers

11. While visiting California, you become violently ill and must visit a hospital. Based off of your fears of a possible earthquake occurring, would you rather go to Highland Hospital in Oakland or Alameda Hospital in Alameda?

a. Highland Hospital, Oakland, CA b. Alameda Hospital, Alameda, CA

12. After what year does the number of magnitude 3 or greater earthquakes begin to rise significantly?

a. 2007 b. 2009 c. 2011 d. 2015

13. After what year does the number of fracking wells begin to rise significantly?

a. 2007 b. 2009 c. 2011 d. 2015

14. Based on the graph that you constructed, do significant earthquakes and the number of fracking wells appear to be related?

15. How long was the rupture length (the length of the fault that was affected)?

a. 25 miles b. 74 miles c. 198 miles d. 296 miles e. 408 miles

Introductory GeoloGy earthquakes

16. Locate the epicenter of the 1906 quake. Does the amount of horizontal slip decrease faster along the northern end or the southern end of the rupture?

a. northern end of the rupture b. southern end of the rupture

17. What was the shaking intensity like in Sacramento?

a. light b. strong c. severe d. violent e. extreme

18. What was the shaking intensity like in Sebastopol?

a. light b. strong c. severe d. violent e. extreme

19. Based on the map, would you be more likely to experience an earthquake of magnitude >6.7 by 2031 if living in the northwest Bay Area or southeast Bay Area?

20. Based on the liquefaction map, are areas more dangerous inland or along the coast?

a. inland b. along the coast

Seismogram Begin Time: 2015-06-14 18:17:41 GMT Station: S39B – Bolivar, MO, USA Station Location: Latitude 37.69 N, Longitude 93.32 W

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

Time (s) 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

Seismogram Begin Time: 2015-06-14 18:17:02 GMT Station: CROK – Carrier, Oklahoma Station Location: Latitude 36.50 N, Longitude 97.98 W

0.00 3.00 6.00 9.00 12.00 15.00 18.00 21.00 24.00 27.00 30.00 33.00 36.00

Time (s) 0.00 3.00 6.00 9.00 12.00 15.00 18.00 21.00 24.00 27.00 30.00 33.00 36.00

Seismogram Begin Time: 2015-06-14 18:17:21 GMT Station: X34A – Smith Ranch, Marlow, OK, USA Station Location: Latitude 34.60 N, Longitude 97.83 W


Besednjak

Napaka: A fracture in the rocks that make up the Earth’s crust

Epicenter: The point at the surface of the Earth above the focus

Plošče: Massive rocks that make up the outer layer of the Earth’s surface and whose movement along faults triggers earthquakes

Seismic waves: Waves that transmit the energy released by an earthquake

Focus (Hypocenter): The point within the Earth where an earthquake rupture starts

This post is part of Exploring Earthquakes, a rich collection of resources co-presented by the California Academy of Sciences and KQED. This material is also available as a free iBooks textbook and iTunes U course.


Difference Between Focus and Epicenter

Focus and epicenter are words that are commonly heard in geology when earthquakes and their causes are being taught. With similarities in between, these two terms cause a lot of confusion for the students. These words are frequently used while reporting incidents of earthquakes in media. This article attempts to highlight the differences between focus and epicenter for the readers.

Focus is the point below the surface of the earth where an earthquake originates. This is the point where rocks first rupture or break when an earthquake takes place due to movement of bedrock and release of energy in a violent form. This point is also called hypocenter, and this is from where seismic waves travel to all other directions. The waves are extremely forceful at the start but slowly die down. These waves can make earth vibrate like a tuning fork.

As focus cannot be seen by people, the concept of epicenter was introduced to let people visualize the focus from where the earthquake originated. This epicenter is a point directly above the focus and is situated on the surface of the earth. Thus for practical purposes, epicenter is taken to be the center or the origin of earthquake though the point below the surface of the earth remains the spot where it originated.

What is the difference between Focus and Epicenter?

• Focus is the actual point below the surface of the earth where an earthquake originates whereas epicenter is a point directly above it, and it lies on the surface of the earth.

• It is the focus that is the origin of the earthquake and seismic waves travel in all direction like ripples in a pond when a stone is thrown inside.

• Epicenter is also called hypocenter.

• Area around epicenter is the one that is hit the hardest by an earthquake and can be seen by the people.

• When the focus is shallow, the magnitude of the earthquake registered at the epicenter is higher than when the focus is deep.

• The cause of the earthquake is determined by studying focus whereas epicenter gives information about the extent of damage.


Earthquakes 2 – Determination of Epicenter

Subject: General Questions / General General Questions
Vprašanje
Exploration: Earthquakes 2 – Determination of Epicenter
[NOTE TO TEACHERS AND STUDENTS: This exercise assumes that you have a data table
and graph made while using the Earthquakes 1 – Recording Center Gizmo™. If you do not
have those, or have never used that Gizmo before, do that first.]
Vocabulary: body wave, earthquake, epicenter, fault, focus, P wave, S wave, seismic wave,
seismogram, seismograph Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
Three dogs meet in a park. Each dog is attached by a leash to its owner (triangles).
1. What does each colored circle represent? ________________
__________________________________________________ 2. Where could all the dogs meet in one place? Draw this point
on the diagram. 3. Is there another spot where all three dogs could meet? ______
Explain: ___________________________________________ Gizmo Warm-up
When you used the Earthquakes 1 – Recording Station
Gizmo™, you learned how to find the distance from a
recording station to the epicenter. With the Earthquakes 2 –
Determination of Epicenter Gizmo, you will use data from three
recording stations to find the exact location of the epicenter.
Click Play ( ), and then click Pause ( ) when the
seismograms are complete. Compare the three seismograms.
1. Which recording station is closest to the epicenter? ______
How do you know? _________________________________________________________ 2. Which recording station is farthest from the epicenter? ______
How do you know? _________________________________________________________ Get the Gizmo ready: Activity: Click Reset ( ). Click Play, and then click Pause when the
seismograms are complete. Locating the
epicenter Goal: Based on three seismograms, locate the epicenter of an earthquake.
1. Prepare: To complete this activity, you will need the table and graph you made in the
Earthquakes 1 – Recording Station Student Exploration. Take this out now. 2. Measure: Turn on Show time probe. On each seismogram, locate the first P-wave and the
first S-wave. Measure the time interval (?t) for each seismogram, and then use your graph
to find the distance of each station to the epicenter.
Station Time interval (?t) Distance to epicenter (km) A
B
C 3. Locate: Turn on the Show station A checkbox. Set the Radius to the distance of station A
from the epicenter, based on your table above. Look on the circle on the map.
Where could the epicenter be located? __________________________________________ 4. Locate: Turn on the Show station B checkbox. Set the Radius to the distance of station B
from the epicenter. Look on the two circles on the map.
Which two places could the epicenter be located now? _____________________________
_________________________________________________________________________ 5. Locate: Turn on the Show station C checkbox. Set the Radius to the distance of station C
from the epicenter. If you did everything right, you should see the epicenter symbol ( ). Če
you do not, recheck all of your distances. (You may need to adjust each radius slightly.)
Relative to the three circles, where is the epicenter located? _________________________
_________________________________________________________________________
6. Practice: Click Reset. Try to locate at least five more epicenters. Each time you locate an
epicenter, click the Tools palette and click Screen shot. Right-click the image, choose
“Copy Image,” and paste the image into a blank document to turn in with this sheet.


What is the Difference Between Epicenter and Hypocenter?

Epicenter and hypocenter are two important terms in the field of seismology, especially in describing earthquakes and underground explosions. The key difference between epicenter and hypocenter is that epicenter is the point that exists directly above the hypocenter whereas hypocenter is the point at which an earthquake or an underground explosion originates. Furthermore, during an earthquake, most of the damage occurs at the epicenter while the rupture of the Earth’s surface begins at hypocenter.

Below is a summary of the difference between epicenter and hypocenter in tabular form.


13.2: The Epicenter, Focus, and Waves - Geosciences

How Do I Locate That Earthquake's Epicenter?

To figure out just where that earthquake happened, you need to look at your seismogram and you need to know what at least two other seismographs recorded for the same earthquake. You will also need a map of the world, a ruler, a pencil, and a compass for drawing circles on the map.

Here's an example of a seismogram:

Figure 1 - Our typical seismogram from before, this time marked for this exercise (from Bolt, 1978).

One minute intervals are marked by the small lines printed just above the squiggles made by the seismic waves (the time may be marked differently on some seismographs). The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart. This number will be used to tell you how far your seismograph is from the epicenter of the earthquake.

Finding the Distance to the Epicenter and the Earthquake's Magnitude

Figure 2 - Use the amplitude to derive the magnitude of the earthquake, and the distance from the earthquake to the station. (from Bolt, 1978)

  1. Measure the distance between the first P wave and the first S wave. In this case, the first P and S waves are 24 seconds apart.
  2. Find the point for 24 seconds on the left side of the chart below and mark that point. According to the chart, this earthquake's epicenter was 215 kilometers away.
  3. Measure the amplitude of the strongest wave. The amplituda is the height (on paper) of the strongest wave. On this seismogram, the amplitude is 23 millimeters. Find 23 millimeters on the right side of the chart and mark that point.
  4. Place a ruler (or straight edge) on the chart between the points you marked for the distance to the epicenter and the amplitude. The point where your ruler crosses the middle line on the chart marks the velikosti (strength) of the earthquake. This earthquake had a magnitude of 5.0.

You have just figured out how far your seismograph is from the epicenter and how strong the earthquake was, but you still don't know exactly where the earthquake occurred. This is where the compass, the map, and the other seismograph records come in.

Figure 3 - The point where the three circles intersect is the epicenter of the earthquake. This technique is called 'triangulation.'

  1. Check the scale on your map. It should look something like a piece of a ruler. All maps are different. On your map, one centimeter could be equal to 100 kilometers or something like that.
  2. Figure out how long the distance to the epicenter (in centimeters) is on your map. For example, say your map has a scale where one centimeter is equal to 100 kilometers. If the epicenter of the earthquake is 215 kilometers away, that equals 2.15 centimeters on the map.
  3. Using your compass, draw a circle with a radius equal to the number you came up with in Step #2 (the polmer is the distance from the center of a circle to its edge). The center of the circle will be the location of your seismograph. The epicenter of the earthquake is somewhere on the edge of that circle.

4. Do the same thing for the distance to the epicenter that the other seismograms recorded (with the location of those seismographs at the center of their circles). All of the circles should overlap. The point where all of the circles overlap is the approximate epicenter of the earthquake.


Diagrams and Decisions

Students will need to wait for their water to be still, before dropping a drop into the water. Once they do start they need to write observation notes into their science journals. They can conduct their experiments as many times as needed in order to get all the notes. I do tell the class to pay attention to the initial droplet and the ripples it makes.

Diagrams that depict what they are seeing would be most helpful. I ask students to draw and label the cup but to do so using the words epicenter and surface waves. I also ask them to tell how the ripples that are created in the water are similar to those we have learned about in earthquakes.

As the conclusion to the activity, I discuss with students the energy that is released from the focus and epicenter. We then discuss the result of this energy is what determines the magnitude of the quake.


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