Seismo 6: Inner core structure and anisotropy

Transcription

1 Seismo 6: Inner core structure and anisotropy Barbara Romanowicz Institut de Physique du Globe de Paris, Univ. of California, Berkeley Les Houches, 10 Octobre 2014

2 A bit of history 1936 Discovery of the inner core by Inge Lehmann 1940 Birch suggests that the inner core is solid and made of iron Bullen suggests that if the inner core is solid we should see PKJKP waves 1953 Jacobs points out that the increase of the melting point of iron with pressure leads to solidifcation of iron at about the depth of the ICB 1961 Birch confirms that the density of the core is 10% less than that of pure iron Confirmation of the solidity of the inner core using normal mode frequencies (Dziewonski et Gilbert) 1983 Poupinet, Pillet and Souriau observe variation with latitude of propagation times of the phase PKIKP (PKP(DF)) 1986 Discovery of inner core anisotropy

3 Examples of normal modes sensitive to structure in the inner core From Souriau, 2007, ToG

4 Rigidity of the inner core The frequencies of normal modes are a powerful tool to probe earth s deep structure Example: proof of the rigidity of the inner core (Dziewonski and Gilbert, 1971, Nature)

5 PRELIMINARY REFERENCE EARTH MODEL (PREM) ICB Radius [m] Dziewonski and Anderson, 1981

6 Hcp iron density versus pressure comparison with PREM From in situ measurements Poirier, 1994 D après Badding et al., 1991

7 PREM Preliminary Reference Earth Model Dziewonski and Anderson, 1981 AK135 Kennett et al., 1995

8 Inner core: topics of interest in the last 30 years Direct observations of solidity: chasing after PKJKP Density contrast at the ICB Inner core anisotropy Differential rotation of the inner core Heterogeneities near the inner core surface (Degree 1 IC translation)

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10 Courtesy of X. Song

11 PnKP

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13 Radius of the inner core Knowing precisely the radius of the CMB, Engdahl et al. (1974) measured t(pkikp-pcp): R icb = km L ICB is a first order discontinuity PREM: km AK135: km Sharpness of ICB Observation of PKiKP at quasi vertical incidence thickness of ICB <5 km. Absence of observations of PKiKP between 60 and 90 o distance, is in agreement with a thickness of the ICB of less than 3 km.

14 Density jump across the ICB The density jump across the ICB (Δρ ic ) is an important constraint on the dynamics and evolution of the core and the inner core. Gravitational energy liberated during the growth of the inner core -> powers the geodynamo The larger the jump => can maintain dynamo with a slower growth of the inner core > density of the inner core - composition

15 Density jump across ICB Two types of seismological data are sensitive to Δρ ic : Frequencies of normal modes sensitive to inner core/core amplitudes of reflected phases (PKiKP/PcP) at short distances Early estimates: Normal modes Δρ ic =0.6 kg/m 3 (PREM) Body waves (PKiKP/PcP amplitude ratios) Δρ ic >1.8 kg/m 3 (e.g. Bolt and Qamar, 1970) Δρ ic = 1.6 g/cm 3 (Souriau and Souriau, 1989) Δρ ic = 1.0 g/cm 3 (Shearer and Masters, 1990). Recent re-evaluation of mode data: (Masters and Gubbins, 2003) ~ 0.82 g/ cm 3 New measurements of PKiKP/PcP amplitude ratios: - (Cao and Romanowicz, 2004) Δρ ~0.85 g/cm 3 ; Δβ ~2.5±0.2 km/s - Koper and Dombrovskaya (2005) Δρ ~ 0.52 g/cm 3; Δβ ~2.8 km/s Conclusion: Δβ < 3.5 km/s at the top of the inner core: velocity gradient? Δρ~ 0.8 g/cm 3 : a bit larger than PREM (0.6g/cm 3 )

16 Quality A data (nuclear explosion) PredicEons AK135 Cao and Romanowicz, 2004

17 Modèle préféré: Δρ ~0.85 g/cm 3; Δβ~2.5km/s Quality A Shearer and Masters, 2000 Schweitzer, 1992 (stacking) Quality A- (Best data used only n the modeling:red) Cao and Romanowicz, 2004

18 PKJKP Transmission coefficient At ICB Outer core Mantle

19 1 LASA array data (~10Hz) mb = 6.2; Δ= 97 o 2-CEA France short period array data ( Hz) Mw = 7.9; 113<Δ<119 deg Julian et al., 1972 Okal and Cansi, 1998 Find: Vs ~2.95 km/s

20 PKJKP/PKIKP Amplitude ratio as a function of frequency Deuss et al., 2000 Okal and Cansi, 1998 frequency

21 3 rd study: - Global stack (GSN and GEOSCOPE) to 0.1 Hz ppkjkp + SKJKP (Deuss et al., 2000)

22 ppkjkp + SKJKP théorie (Deuss et al., 2000)

23 4 th study: Santa Cruz Islands Feb. 06, 1999 Mw = 7.3, depth = 76 km, Δ = 138 o A very favorable source Phase Pdiff Cao et al., 2005

24 Slowness (s/deg) Phase weighted stack: PWS (Schimmel and Paulssen, 1997) Time (s) 9 s sooner than predicted for PREM Cao et al.., 2005, Science

25 Back-azimuth of the target phase Estimated back-azimuth is ~ 223 Expected back-azimuth is 218

26 Synthetic vespagrams For pseudo-liquid inner core For solid inner core (Qβ = 300) For differential seismograms (amplified 40 times)

27 Modeling of envelope of PKJKP to estimate Qβ in the inner core Q β ~ 300 data Cao and Romanowicz, GRL, 2009

28 More recently Wookey et al. (2008) High frequency observation (0.3Hz) on the japanese network Hi-net Mozambique Mw=7.0 Prof.= 14 km 22 Fev 2006 ~700 stations Δ=114 o

29 PKKP PKiKP Predictions for model AK135 Wookey et al., 2008

30 PKKPab Before station corrections After station corrections Wookey et al., 2008

31 PKJKP Slowness/time Azimuth/time Wookey et al., 2008

32 Shearer et al., 2010

33 Inner core anisotropy Iron crystal structure

34 PKP= Refracted waves in the core

35 PKIKP travel Eme residuals expansion in spherical harmonics up to degree 4 Values in 10- ths of seconds: 2 sec difference between equatorial and polar paths InterpretaEon: lateral heterogeneity near ICB or EllipEcity of the ICB (but 200 km too large..) Poupinet et al., 1983, Nature

36 Inner core anisotropy Morelli and Dziewonski (1986) Travel times of PKIKP = PKP(DF) Woodhouse, Giardini and Li (1986) Splitting of normal modes

37 PKIKP Δ = 170 o 180 o Plo_ed at the source and receiver locaeons Degree 4 expansion Morelli and Dziewonski, 1986

38 Δ= ο Δ= ο Δ= ο Inner core

39 Cylindrical anisotropy with axis parallel to the axis of rotation of the earth "- A difference in velocity of 1% between polar and equatorial paths explains a travel time difference of de ~2.2 s (In case of isotropy: A=C=2L +F) -!Vp 2 (") = Asin 4 " + C cos 4 " + 2(2L + F)sin 2 " cos 2 "

40 mode 0 S 3 7 singlets Normal mode splitting by rotation and 3D heterogeneity

41 Normal mode splitting First order perturbation theory (isolated multiplet) c s t = u k (t) = Re{ exp(i! k t)r " exp(ih k t)" s} s=2l l=+s $ l=#s H mm' =! $ mm' t 0 " ls c s s=0 a " t!m s M s (r)r 2 t dr +!h s 0 d # H s t Recombine the c st s into a function on the sphere: γ (θ, φ) = Σ c st Y st (θ, φ) k=(n,l) splitting matrix Make a map: like a phase velocity map: at each point (q,f): integrated effect of heterogeneity weighted by kernel

42 Mantle mode Core mode

43 Anomalous splitting of Earth s normal modes Ritzwoller et al. (1986) 0.3% density anomaly in the outer core in C20 and 0.5% at the base of the mantle Giardini et al. (1986) Mantle heterogeneity results in a much too large C20 to explain mantle modes Woodhouse et al. (1986) :heterogeneity must be in the inner core But implies too large topography at the ICB Inner core anisotropy: it is plausible that the process of inner core crystallization, in the presence of fluid motions, electric currents and electromagnetic fields all coupled to each other and to the Earth's rotation could result in a systematic orientation of crystals and thus to bulk anisotropy bearing a relation to the Earth's rotation axis

44 Possible causes of anisotropy Convection in the inner core Jeanloz and Wenk, 1988; Wenk et al., 2000 Heat sources? (K?) Anisotropic growth of the inner core (deformation stresses) Yoshida et al., 1996 Alignment of iron crystals during crystallization in the presence of the magnetic field (Maxwell stresses) Karato, 1999; Buffett and Wenk, Texture acquired at the time of crystallization Bergman, 1997.

45 Inner core anisotropy Creager, 1992 Tromp, 1993 Vinnik et al., 1994; PKP(AB)-PKP(DF) Creager, 1992 PKP(BC)-PKP(DF) Strong anisotropy all the way to the center of the inner core

46 Travel Emes BC-DF diff. travel times and amplitude ratios under Africa amplitudes Anisotropy in attenuation Souriau and Romanowicz, 1997

47 Equatorial paths ξ>45 o Bottoming point: km below ICB Polar paths Hemispherical dependence of anisotropy at the top of the inner core Tanaka and Hamaguchi, 1995

48 Polar view Uniform cylindrical Anisotropy - 3% Equatorial view Garcia and Souriau, 2000

49 Degree 1 anisotropy seen by normal modes 16 S 5 17 S 4 J T=146.3 s; Q = T=145.9 s; Q =85.6 CMB CMB ICB ICB Fonctions propres radiales: : U : V Deuss et al., 2010

50 Deuss et al., 2010, Science

51 Innermost Inner Core? Ishii and Dziewonski, 2003

52 Distance: o o Ishii and Dziewonski, 2003

53 PKP(DF) Travel time data (Antipodal) P-velocity IMIC R >300 km Ishii and Dziewonski, 2003

54 Model from free oscillations data Predictions for PKP travel times Béghin and Trampert, 2003

55 1 -Ishii and Dziewonski, Béghin and Trampert, 2003

56 1 -Ishii and Dziewonski, Béghin and Trampert, 2003 Cao and Romanowicz, 2007, GRL

57 Inner core differeneal rotaeon as large as several 10^- 9 rad/s ~ 1deg/year Glatzmaier and Roberts, 1995

58 Δ= o Δ= o Δ= o Δ= o Su and Dziewonski, 1995

59 Differential rotation of the Inner Core

60 Song and Richards, Nature, 1996

61 Song and Richards, Nature, 1996

62 Color contours: best fipng spherical harmonics Up to degree 4 Su, Dziewonski, Jeanloz, 1996, Science

63 Time dependent orientaeon of the axis of symmetry Inferred rate : 3deg/year towards the East Su et al., 1996

64 Inner core rotation

65 Inner core rotation

66 Inner core rotation through time J. Irving CIDER 2014

67 Hemispherical variations Irving and Deuss, 2011

68 PKIKP réfractée: pénètre dans la graine PKiKP réfléchie sur l ICB

69 Mushy zone at the top of the inner core Distance (deg) Porosité décroît avec la Profondeur Plus grande dans l ẖémisphère Ouest (inclusions fluides Connectées): Cao and Romanowicz, 2004

70 PKIKP- PKiKP differeneal travel Emes Waszek and Deuss, NatGeo, 2011

71 Control of convection in the outer core by lateral variations of heat flow at the base of the mantle Δt ~ 10-4 à l ICB Equatorial projection Sumita and Olson, 1999

72 Translation of the inner core! Monnereau et al., 2010 Also: Alboussière et al., 2010

73 Some issues with inner core anisotropy

74 Irving and Deuss, 2011

75 Tkalcic and Romanowicz, 2002 Leykam et al., 2010 resid dt (s) BC-DF Berkeley+Leykam (deg)

76 pred dt (s) obs dt (s) Stations at latitude < -50 deg. BC-DF (deg) (deg) pred dt (s) obs dt (s) Events at latitude < -50 deg. BC-DF (deg) (deg) Romanowicz et al., 2003

77 pred dt (s) obs dt (s) Stations at latitude < -50 deg. BC-DF (deg) (deg) pred dt (s) obs dt (s) Events at latitude < -50 deg. BC-DF (deg) (deg)

78 Observed BC-DF residuals Model 1 Model 2 Romanowicz et al., 2000, 2003

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80 PKP(DF) + PKP(BC) dt sec Romanowicz et al., 2003