Imagerie de la Terre profonde avec le bruit sismique. Michel Campillo (ISTERRE, Grenoble)

Transcription

1 Imagerie de la Terre profonde avec le bruit sismique Michel Campillo (ISTERRE, Grenoble)

2 Body waves in the ambient noise: microseisms (Gutenberg, Vinnik..) The origin of the noise in the period band 5-10s: VARIABLE SOURCE LOCATIONS Landès et al., 2010

3 Body wave in high frequency (anthropic) noise Regional scale (N Hz) Local scale (N.100 Hz) Hillers, G., Campillo, M., Lin, Y.-Y., Ma, K.-F., Roux, P., Anatomy of the high-frequency ambient seismic wave field at the TCDP borehole. J. Geophy. Res. 117 Olivier, G., F. Brenguier, M. Campillo, R. Lynch, and P. Roux, 2015, Body-wave reconstruction from ambient seismic noise correlations in an underground mine: Geophysics, 80, KS11 KS25.

4 Erebus volcano: icequakes Chaput et al., JGR 2015

5 Coda Correlations 44 large events All 3318 events ZZ correlations: reciprocity holds

6 Surface wave tomography body waves (deep reflections) POLENET/LAPNET array in Finland (RESIF-SISMOB) Comparison of high frequency (1Hz) 1-year noise correlation with earthquake data Poli et al. 2012a Z-Z noise correlations Z comp. actual earthquake Finland: homogeneous Archean crust=> simple seismograms

7 Comparison with synthetic Green functions C ZZ (data) GF ZZ (theory) C RR (data) GF RR (theory) Polarisation: noise correlation vs synthetics Reconstruction of P and S multiple reflections Good reconstruction of phase and relative amplitudes of the components of the reflected waves. (amplitude discussed by Prieto) A favorable context: distance vs. mean free path, amplitude in actual earthquake records Deep phases Poli et al., 2012a

8 BODY WAVES: deep reflections/echography Short periods 5-10 s strong scattering P and PcP Japan to Finland Finland to Japan Standard (surface-wave) pre-processing (Shapiro and Campillo, 2004; Sabra et al. 2005;.) eliminates the contamination by EQ ballistic waves.

9 Earth s mantle transition zone discontinuities from ambient seismic noise ( phase transition (P,T)) A PP PKP P Station 1 Station 2 Upper Mantle Transition zone Lower Mantle 72 B 68 C 0.1 Data 0 slow[s/km ] Model time[s] 0.1 From Poli, Campillo, Pedersen and LAPNET WG, Science

10 Earth s mantle transition zone discontinuities from ambient seismic noise ( phase transition (P,T)) A PP PKP P Station 1 Station 2 Upper Mantle Transition zone Lower Mantle 72 B 68 C 0.1 Data slow[s/km ] Model time[s] 0.1 From Poli, Campillo, Pedersen and LAPNET WG, Science 2012 In agreement with receiver functions (Alinaghi et al. 2003) 64

11 Core phases PcP and PdP D : - different hypotheses for the nature of the layer - PdP difficult to observe - lack of earthquake data From Poli, Thomas, Campillo and Pedersen 2014

12 Advantage of noise vs earthquake records: -surface to surface -impulsive wavelet -double beam forming Stacked vespagrams for: Earthquakes Noise From Poli, Thomas, Campillo and Pedersen 2014 A 5% increase of velocity at 2530 km depth.

13 Consider now the problem of long period body (P and S) waves at the global scale with noise sources at the surface: -Very weak scattering -A problem of a different nature, although indeed the uneven distribution surface noise sources is still there. This representation is not formally valid on the free surface: the integral vanishes. GF reconstruction would require a more complex procedure (Ruigrok et al., 2008) (Here also, the correlation of multiply scattered waves should lead to the Green function.)

14 GLOBAL TELESEISMIC CORRELATIONS (periods s vertical components) Boué, Poli et al., GJI 2013

15 Numerous phases can be identified Vertically incident S waves on the vertical component??

16 Long periods (25-100s) Processing: separating EQ and coda from ambient noise Low daily coherence High daily coherence (EQs) AXISEM synthetics High amplitude spurious High coherence=days following large earthquakes

17 Long periods (25-100s) Spurious arrivals and numerical simulation CC (ambient noise) CC(EQ days) PREM synth Synth EQ CC Very clear pulses in the correlations, likely holding information about the deep Earth, but should not be interpreted directly as components of the Green function Attempt to use a representation with a free surface condition boundary From Boué et al., 2014

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20 Time after EQ in mn

21 Spectrum ScS Identification peak: 102.4s: 17S14,16S12, 14S18 (ScS) Vapp =+/-30km/s Spectrum P P in blue Corresponding to the ray parameter of a ScS at 12 (3.62s/ ) error=13s! Peak at 78.7s error 7s + question of bias due to anisotropy. Reduces the error!

22 Focusing pattern at USArray C(0,r) Jo(kr) k Vapp=29+/-2km/s l=13-15 To be redone in narrow bands