body waves in media with depth dependent properties a)body waves: p and s waves b) surface waves: :...
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Body waves in media with depth dependent properties
a) Body waves: P and S waves
b) Surface waves: : Love e Rayleigh
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Strain associated with seismic waves
Compressional waves imply changes in volume
Shear waves imply changes in shape
P Rayleigh
S Love
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Registrazione telesismica
Componente verticale
Componente radiale
Componente trasversale
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Riflessione e rifrazione delle onde sismiche
A beam of light is refracted or reflected when it crosses the boundary between air and water. Seismic waves behave similarly at boundaries within the Earth.
P and S Waves radiate from an earthquake focus in many directions
from Press and Siever (1994)
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Propagazione delle onde sismiche all’interno della Terra e zone d’ombra
onde P onde S
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Fasi di un sismogramma
• L’interazione dell’onda di volume con una discontinuità genera onde riflesse e rifratte, identificabili sul sismogramma (FASI)
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Fasi di un sismogramma
• Le fasi si distinguono in base alla distanza angolare epicentro-osservatore
epicentro
osservatore
D
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fasi di un sismogramma Primi arrivi D = 0° - 10°D = 0° - 10°: propagazione crostale
• D < 2° fasi Pg, Sg
• D > 2°- fasi Pn, Sn diffrazione lungo la Moho
Eventi crostali e regionali
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fasi di un sismogramma Primi arrivi D = 0° - 10°
Crosta
Mantello
distanza critica
• D < 2° fasi Pg, Sg
• D > 2°- fasi Pn, Sn (diffrazione lungo la Moho)
approssimazione raggi rettilinei
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fasi di un sismogramma Primi arrivi D < 2°
tempo (sec)
• D < 2° fasi Pg, Sg
Esempio 1: D ~ 0.1°
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fasi di un sismogramma Primi arrivi D < 2°• D < 2° fasi Pg, Sg
Esempio 2: D ~ 0.2°
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fasi di un sismogramma Primi arrivi D < 2°• D < 2° fasi Pg, Sg
Esempio 2: D ~ 0.2°
ROM9
Sg
NS
Z
Pg
NS
Z
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BOB
fasi di un sismogramma Primi arrivi D > 2°
• D > 2°- fasi Pn, Sn
(diffrazione lungo la Moho)
Esempio 2: D ~ 4°
BOB NS
Z
Pn Pg
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BOB
NS
Z
ROM9
NS
Z
BOB
ROM9
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fasi di un sismogramma D = 10° - 103°/105°
D = 10° - 103°/105°: propagazione nel mantello
• Il mantello è in prima approssimazione un mezzo lateralmente omogeneo, ovvero un corpo a simmetria sferica con velocità di propagazione che cresce con la profonditàapprossimazione raggi curvilinei• arrivi regionali-telesismici
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fasi di un sismogramma D = 10° - 103°/105°D = 10° - 103°/105°: propagazione nel mantello
• D < 40° fasi P, S dirette (telesismiche)
• D > 40° fasi riflesse alla sup. libera (PP, PS, SS, PPP,…), predominanti per D>100°
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fasi di un sismogramma D = 10° - 103°/105°D = 10° - 103°/105°: propagazione nel mantello
*
P
pPp• Fasi profonde: se evento
profondo, le fasi dirette p,s vengono riflesse indietro nel mantello (pP,pS, sP, sS)
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ESEMPIO: Terremoto di SUMATRA (26/12/2004) registrato dalla stazione di BOB (80° distanza angolare, backazimuth=90°)
NS
EW
Z
NS (tangenziale)
EW (radiale)
Z
P
SLove
Rayleigh
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fasi di un sismogramma arrivi telesismici
• 105°-140°: zona d’ombra (inversione di velocità)
- Arrivano solo fasi riflesse dal CMB (Pc, Sc) e diffratte dal nucleo esterno (Pdif)
D = 103°/105° - 180°: propagazione nel nucleo
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fasi di un sismogramma arrivi telesismici
• 130° - 180°: propagazione nel
- nucleo esterno K (solo onde P: PKP,
PKS,SKS, SKP)
- nucleo interno I (PKIKS, SKIKS,
SKIKP)
D = 103°/105° - 180°: propagazione nel nucleo
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Propagazione delle onde
http://bingweb.binghamton.edu/~ajones/