"Application of ambient noise analysis in seismology at regional and global scales"

Transcription

1 Adv. Grant Whisper "Application of ambient noise analysis in seismology at regional and global scales" Michel Campillo ISTerre Université Joseph Fourier and CNRS,38041 Grenoble, France

2 Imagerie sismique industrielle: Valhall 2400 capteurs sources ac9ves: canons à air

3 avec la permission de J. Virieux (projet Seiscope)

4 Large networks con9nuous recordings Huge data sets consis9ng for a large part of ambient noise Availability: ORFEUS, IRIS RESIF, Computers for massive processing

5 A long history of noise studies. including azempts of using noise for retrieving Earth proper9es (Aki 1957+, Toksoz, Claerbout,..) Global noise sources in the microseism band (extended 2-50s) seismological observa9ons oceanographic modeling Strong contribution from oceanic waves Example of a global comparison (secondary microseism- Miche/Longuet-Higgins mechanism) Longer periods: infragravity waves, e.g. Fukao et al Hillers et al., high frequency: industrial noise, local characteris9cs + EARTHQUAKES: direct paths and coda (scazered waves)

6 Long range correla9ons ()*&#"'!"#"$%"&'!!!!!!"! #! Source in A the signal recorded in B characterizes the propagation between A and B. Green function between A and B: G AB!"#"$%"&'!"#"$%"&'!!!!!!"! #! G AB can be reconstructed by the correlation (C AB ) of noise from randomly distributed sources or «diffuse» (equipartitioned) fields recorded at A and B A way to provide new data with control on source location and origin time Experimentally verified with seismological data: Coda waves: Campillo and Paul, 2003,.. Ambient noise: Shapiro and Campillo, 2004,

7 Mathema9cal basis Arbitrary medium: an integral representa9on wrizen in the frequency domain FT of G(-t) Volume term Surface term Absorp9on coefficient FT of G(t) 1 2

8 Surface term: κ =0 (no attenuation) and we obtain a widely used integral relation: è Derode et al., 2003: Analogy with Time reversal mirrors è Wapenaar 2004 For surface waves: distant sources of noise at the surface of the sphere (2D problem)

9 Surfave wave reconstruction within a large array (Boué et al. 2014) Noise correla9on=gf(t)- GF(- t) (Rayleigh waves)

10 Several hundreds of applica9ons in the last 10 years! An issue for surface wave tomography: In practice, the noise sources are not evenly distributed and the field is not made fully isotropic by scattering. We can study the effect of non isotropy of the intensity of the field incident on the receivers. It results in a bias on the measurements of direct path travel times.

11 Correla9on of direct waves: Bias in the travel time Increasing anisotropy of the source intensity B Azimuthal distribu9on of source intensity B(θ) = 1+ B 2 cos(2θ) Travel 9me error wrt the observed Green func9on δt = 1 2tω 0 2 B(0) d 2 B(θ) dθ 2 θ =0 valid with t (travel 9me) > T (period) From Froment, Campillo, Roux, Gouédard, Verdel and Weaver 2011.

12 Multiple scattering and equipartition Equipartion principle for a completely randomized (diffuse) wave-field: in average, all the modes of propagation are excited to equal energy. Implication for diffuse elastic waves (Weaver, 1982, Ryzhik et al., 1996): P to S energy ratio stabilizes at a value independant of the details of scattering. Observations (Hennino et al., 2001) Numerical simulation (Margerin et al.2000) RTE Monte Carlo Diffusion equation

13 An argument independant of the representation theorems Multiple scattering and equipartition: the simplest case (finite body) equipartion correlation Compare with: 1 derivative 2 causality è Long range correla9on in seismic coda= Green func9on (Campillo and Paul, Science 2003)

14 In presence of scazering: Correla9on of coda waves - isotropy provided by mul9ple scazering Increasing anisotropy of the source intensity B (a) B(θ) = 1+ B 2 cos(2θ) (b) No bias in the correlation of coda waves! Noise records contain direct and scazered waves: è the biases of direct wave travel 9mes are generally small enough for imaging purpose è Importance of processing strategies From Froment, Campillo, Roux, Gouédard, Verdel and Weaver 2011.

15 Fault zone structures - fault segments, complexity (rupture speed) - bi- material interfaces: preferen9al direc9on of rupture propaga9on - amplifica9on effects - lack of resolu9on for shear wave in the first kilometers for tradi9onal tomography From Zigone, Ben- Zion, Campillo and Roux, 2014

16 Surface wave tomography with noise correla9on 9- component correla9ons Rayleigh wave 8 measurements From Zigone, Ben- Zion, Campillo and Roux, 2014

17 From Zigone, Ben- Zion, Campillo and Roux, 2014 Group velocity maps at different periods

18 3D shear velocity - Damaged fault zone - Flower- like pazerns - Diffuse seismicity associated with low- velocity (damaged) area between SAF and SJFZ From Zigone, Ben- Zion, Campillo and Roux, 2014

19 Surface wave tomography è body waves (deep reflec9ons) Comparison of high frequency (1Hz) 1- year noise correla9on with earthquake data POLENET/LAPNET array in Finland Z- Z noise correla9ons Z comp. actual earthquake From Poli, Pedersen, Campillo and LAPNET WG, 2012

20 Comparison with synthe9c Green func9ons C ZZ (data) GF ZZ (theory) C RR (data) GF RR (theory) From Poli, Pedersen, Campillo and LAPNET WG, 2012

21 GLOBAL TELESEISMIC CORRELATIONS (periods s vertical components) Noise correla9ons PREM synthe9cs From Boué, Poli, Campillo, Pedersen, Briand, and Roux, 2013

22 Numerous phases can be iden9fied Ver9cally incident S waves on the ver9cal component?? From Boué, Poli, Campillo, Pedersen, Briand, and Roux, 2013

23 Long periods (25-100s): dominance of coherent earthquake signals Processing: separa9ng EQ and their long las9ng reverbera9ons from ambient noise Low daily coherence High daily coherence (EQs) AXISEM synthe9cs High amplitude spurious From Boué, Poli, Campillo, Pedersen, Briand, and Roux, 2013

24 Long periods (25-100s) Spurious arrivals and simula9on CC (ambient noise) CC(EQ days) PREM synth (no scazering!) Synth EQ CC From Boué, Poli, Campillo, Pedersen, Briand, and Roux, 2013

25 Short periods 5-10 s è stronger scazering P and PcP Japan to Finland (P- PcP) 10.5 time (min) Finland to Japan (P- PcP) θ ( ) Standard pre- processing (Shapiro and Campillo, 2004; Sabra et al. 2005) eliminates the contamina9on by EQ ballis9c waves.

26 è Earth s mantle transi/on zone discon/nui/es from ambient seismic noise ( phase transi/on è (P,T)) In agreement with receiver func9ons (Alinaghi et al. 2003) From Poli, Campillo, Pedersen and LAPNET WG, 2012

27 Going deeper: inves9ga9ng the core with records of the ambient noise

28 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

29 Advantage of noise vs earthquake records: - surface to surface - impulsive wavelet - double beam forming Stacked vespagrams for: Earthquakes Noise 0.5 B) 0.5 A) Slowness to P wave [s/deg] P PdP PcP Slowness to P wave [s/deg] P PdP PcP Time to P wave [s] From Poli, Thomas, Campillo and Pedersen Time to P [s] A 5% increase of velocity at 2530 km depth.

30 Tradi9onal Seismic velocity tomography Local seismic velocity (V) = D/(travel 9me) Seismic velocity is a proxy for s/ffness (high veloci9es) and compliance (low veloci9es) of rocks

31 Tradi9onal Seismic velocity tomography New Seismic suscep9bility tomography Dynamic stress (Δσ) ΔV Local seismic velocity (V) = D/(travel 9me) Seismic velocity is a proxy for s/ffness (high veloci9es) and compliance (low veloci9es) of rocks velocity ΔV 9me Nonlinear elas9city, slow dynamics Seismic suscep/bility (ΔV/Δσ) is sensi9ve to fractured, damaged or pressurized rocks

32 Noise based seismic velocity temporal changes Because seismic noise is con9nuous in 9me, it is possible to reconstruct repea/ng virtual seismic sources and perform con/nuous monitoring of seismic veloci/es. ΔV

33 Monitoring seismic veloci9es before and azer the M9 Tohoku- oki earthquake 600 seismic sta9ons (Hi- net) For each seismic sta9on, we obtain a con/nuous seismic velocity change /me series ΔV Tohoku- oki earthquake From Brenguier, Campillo, Takeda, Aoki, Shapiro, Briand, Emoto and Miyake 2014

34 Seismic suscep/bility tomography of Japan We use seismic waves caused by the 2011 Tohoku- oki earthquake as dynamic stress perturba/ons Ozawa et al Furumura et al. 2011

35 Tomography of seismic suscep9bility (velocity change/dynamic stress) Delineates volcanic regions characterized by high volcanic fluid pressure (low effec9ve pressure) and sedimentary basins Maximizes below Mt Fuji volcano where a M6 earthquake occurred 4 days azer the Tohoku- oki earthquake Minimizes in s/ff old plutonic regions From Brenguier, Campillo, Takeda, Aoki, Shapiro, Briand, Emoto and Miyake 2014 Feasibility of imaging new parameters relevant for the dynamics of erup/ons and earthquakes

36 Conclusions Ambient noise provides reliable travel 9me measurements for surface and deep waves, including the core phases A careful considera9on of reconstruc9on condi9ons is required for body waves Coverage for all sta9on- to- sta9on paths (different from EQ- to sta9on!) è new data Dense arrays allow for beam- forming Wide possibili9es of improvement of the processing Time dependent elas9c proper9es: monitoring of the deforma9on and mechanical condi9ons at depth?