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Some observations on big December 2004 Sumatra earthquake

Some observations on big December 2004 Sumatra earthquake. Dominique Reymond CEA/DASE/LDG Laboratoire de Géophysique French Polynesia. Evolution of the warning : the magnitude is progressively reviewed increasingly. H0 : 00:58 TU decembre

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Some observations on big December 2004 Sumatra earthquake

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  1. Some observations onbig December 2004 Sumatra earthquake Dominique Reymond CEA/DASE/LDG Laboratoire de Géophysique French Polynesia

  2. Evolution of the warning :the magnitude is progressively reviewed increasingly • H0 : 00:58 TU decembre • Detection TREMORS_PPT (Phase Pdiff) 26 dec 01:13 H0 + 00:15’ • Bulletin of PTWC – Mw = 8.0 26 dec 01:14 H0 + 00:16’ • Warning TREMORS_PPT (Phase SP) 26 dec 01:27 H0 + 00:29’ • Bulletin of PTWC – Mw = 8.5 26 dec 02:08 H0 + 01:10 • Analysis of Rayleigh_PPT - Mw = 8.2 26 dec 02:38 H0 + 01:38 • Message from PPT - Mw = 8.2 26 dec 02:44 H0 + 01:44 • CMT Harvard - Mw = 8.9 26 dec 05:30 H0 + 04:30 • revised CMT Harvard revised - Mw = 9.0 26 dec 20:00 H0 + 19h • Information NEIC – Mw = 9.0 26 dec ~23h H0 + 22h • Bulletin from PTWC – Mw = 9.0 27 dec 05:36 H0 + 25h • MT PDFM_PPT – Mw = 8.9 27 dec 01:50 H0 + 27h • Study of eigen modes of Earth – Mw = 9.3 (02 fev 2005, Emile Okal & Seth Stein)

  3. Mw is obtained from Mm (Mantle magnitude) • Mm = log X(w) + Cd + Cs – 3.9(100% theoretical) • X(w) spectral amplitude at pulsation w • Cd : distance correction, • Cs source correction • Talandier, Okal, Reymond, 1987 • Simple • Calculated routinely in TREMORS • Works at variable periode in [50s – 400s] Log Mo = Mm + 13.0 N.m Mw = 2/3 ( Log Mo – 9.0)

  4. Mm vs azimuth82 IRIS stations 410s Average Mm 410s = 9. 33 ( Mw = 8.9) 341 s Azimuth of PPT Average Mm 341s: 9.0 (Mw = 8.7)

  5. FLN EVOP FLN EVOP PPT PPT Why is PPT magnitude so weak ? 1)It was in a node of radiation 2)it was computed at only 256s PPT Azimut ~107 ° Distance ~114°

  6. Sumatra 26 dec 2005: Mm vs period Mm at each period is computed as the average Value of 82 stations.

  7. The eigen modes of the Earth RER (La Réunion) ~ 36 hours of signal

  8. Sumatra dec, 2004: why all classical methods have underestimated the seismic moment ? Mo N.m Average 82 stations Stein & Okal, 2005

  9. The seismic moment increases toward VLF Mm-410s From Stein & Okal

  10. Sumatra 26 décembre 2004: source, ray, and considered stations Diego Garcia, I52 (IMS) DGAR (IRIS) D =25.6 ° Az = 245°

  11. Signal in displacement broad band [1 - 500s] Amp= 83800 µm = 8.38 cm ! DGAR, Az 245° Mm DGAR = 9.57 soit Mo =3.7 E 22 N.m 82 stations IRIS Mm vs azimuth

  12. Seismic and infrasonic signal at Diego Garcia Seismic Signal Amplitude  12 mm/s Infrasonic signals Amplitudes : [4 – 4.5] Pa

  13. Excitation of the atmospheric layer by Rayleigh waves transfert function: P(w) = r c V(w) P in Pa, r in kg/m3, c et V in m/s Range of cair : cair (331 + 0.6 q) avec q = T - 273. 15 °K, sous 1013 hPa (approximated formula from -20 à 40 °C, error < 0.2 %) Range of r * r 25 °C : 1.171 (90 % Hr ) < r < 1.181 (20% Hr) * r 10 °C : 1.241 (90 % Hr ) < r < 1.245 (20% Hr) * r -10 °C : 1.340 (90 % Hr ) < r < 1.341 (20% Hr) Choosen value : c = 346 m/s ( 25°C), r = 1.24 kg/m3 Thus V(w) = P(w) / 408 = P(w) x 0.002450 m/s at 25 °C V(w) = 2 450 P(w) . en µm/s

  14. The instrumental responses Station DGAR : sismo. STS 1 Cut_off frequency : Fc = 360 s = 0.00277 Hz Slope order 2 Station I52H* : microbaro mb2000 Cut_off frequency: Fc # 120 s = 0.0083Hz Slope order2

  15. Elaboration of the same instrumental responses Process of DGAR : *high-pass 120 s order 2 *integrator with cutt-off period 200 s Process of I52 : * high-pass 200 s ordre 2 * compensator 120 - 200s order 2 * integrator with cutt-off period 200 s Identical response seismic – IF [10 -120 s]

  16. Chagos: comparison between signal in displacement [1 -120 s] Ampl. 61800 µm = 6.18 cm ! Ampl. 6.42 cm Ampl. 5.75 cm Ampl. 5.83 cm Ampl. 5.72 cm Average amplitude : 5.93 cm i.e. difference of 4 % sismic and IF

  17. Conclusions • Sumatra 26 dec 2004 was: • Not only big, but slow. • We observe a clear increase of energy toward VLP • Infrasound signal allows to recover amplitude of signal in displacement with a good agreement. • It can be useful in for saturated stations in near field for example. • But the Signal/Noise ratio toward VLP can shutdown dramatically in case of a windy conditions.

  18. END Thank you for your attention

  19. Signaux de Mongolie: forts mais bruités ULN : STS1 (vitesse) Ampl. 18.45 mm/s IS34 (assez bruité) Ampl: [7 – 8] Pa Avec Passe-Haut 200 s

  20. Mongolie : signaux en déplacement: intégration jusqu’à 50 s Ampl:  7 cm Ampl: 5.8 cm Ampl: 5.5 cm Ampl: 6.8 cm Ampl: 6.9 cm

  21. Mongolie: possibilité d’obtenir une magnitude Ms mais pas de calcul Mm Valeurs moyennes retenues : c = 325 m/s (à -10°C), r = 1.34 kg/m3 d ’où : • V(w) = P(w) / 435.5 = P(w) x 0.002296 m/s (à -10 °C) soit V(w) = 2 296 P(w) . en µm/s • Donc l’amplitude moyenne en vitesse obtenue avec les IS34 est: Avit = 2.296 * 8 = 18368 µm/s (à comparer à 18450 µm/s sur ULN) • De là on obtient la magnitude à 20 s de période: Ms =log10(A/T) + 1.66 log10(D) +3.0 d’où avec une amplitude moyenne de 6.25 cm: Ms = Log10( 62500 / 20 ) + 1.66 log10(45) + 3.0 = 9.5

  22. Conclusions • Les variations de pressions atmosphériques induites par le passage des ondes de Rayleigh lors des très forts séismes sont bien observées sur les réseaux infrasons et permettent de remonter au déplacement vertical avec une bonne précision. • Ces types de mesures sont surtout utiles dans le cas des très forts séismes, pour lesquels les enregistrements sismiques des ondes de Rayleigh sont saturés, car dépassant la dynamique de la chaîne sismique (ex. Chili 1960, Alaska 1964). • La principale limitation des mesures utilisant les signaux IF est le bruit basse fréquence (et même sur toute la largeur du spectre) générés par le vent, ce qui ne permet pas toujours d’obtenir les mesures aux TBF.

  23. Séismes géants et grands tsunamis trans-océaniques • 1) Chili 22 Mai 1960, (Mw = 9.5, Mo >= 2.0 E23 N.m) • 2) Sumatra 26 Dec. 2004 (Mw = 9.3, Mo = 1.3 E23 N.m) **lent • 3) Alaska 28 Mar 1964, (Mw = 9.2, Mo = 8.0 E22 N.m) • 4) Aléoutiennes 4 Fev. 1965, (Mw = 8.7, Mo = 1.4 E22 N.m) • 5) Sumatra 28 mar 2005, (Mw = 8.7, Mo = 1.0 E22 N.m) • 6) Aléoutiennes 1er Av 1946, (Mw = 8.5, Mo = 8.0 E21 N.m) **lent

  24. Sumatra: pourquoi toutes les méthodes classiquesont sous-estimé le moment sismique ? Moment sismique Mo en N.m

  25. Signal in velocity at Chagos Signal sismique brut DGAR Amplitude  12 mm/s <sismic> signal IF I52 Amplitudes :  [10.5 – 11.6] mm/s

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