Depth sensitivity of seismic coda waves to velocity perturbations in an elastic heterogeneous medium

Depth sensitivity of seismic coda waves to velocity perturbations in an elastic heterogeneous medium Anne Obermann* Thomas Planès* Eric Larose Christoph Sens-Schönfelder Michel Campillo Montpellier August 212 *PhD Students

Motivation Central question in monitoring of ambient seismic noise: Depth of the temporal changes! Monitoring of ambient seismic noise during slow slip events in the Guerrero region Mexico (Rivet et al. (211)) Apparent temporal change of velocity (tt) Coda: Rayleigh waves Locating the depth of the change (lower crust)

Depth Motivation Rivet et al. assumption Coda: mainly Rayleigh waves Heterogeneous medium Coda: Rayleigh wave Bulk wave conversions Influence on the depth sensitivity of coda waves

2 D Simulations v p v ' p v p d h z =16km x Model parameters -2D velocity model with random velocity fluctuations -free surface 3 absorbing boundaries = 3 m σ = 2% v p = 65 m/s h = 2 m = 2/3 dv/v=1.5% f=2hz x=16km SPECFEM 2D Komatitsch et al.

normalized displacement Stretching Technique 1 5.5 5.6 5.7 t in s -1 ' 3 6 9 t in s Hadziiannou et al. (JASA 29)

Apparent velocity variations versus depth Result from the simulation 6 x 1-3 std dev data 4 2 1 2 3 depth (km)

Apparent velocity variations versus depth Result from the simulation 6 x 1-3 std dev data 4 1 D surface wave regime 2 2 D diffusion regime of body waves 1 2 3 depth (km)

Apparent velocity variations versus depth Result from the simulation 6 x 1-3 std dev data 4 1 D surface wave regime 2 2 D diffusion regime of body waves 1 2 3 depth (km) Model Surf ( 1 ) α=fitting parameter; partition coefficient Bulk

S R Bulk Regime: Sensitivity Kernel ) ( ) ( ) ( ) ( t r s p du u t r x p u x s p t x r s K t Pacheco and Snieder (25) Intensity Propagator ) ( t r s p Diffusion radiative transfer... ) ( V bulk t x r s K t v dv v dv local relative velocity change in the volume element ΔV around x

epsilon Surface Wave Sensitivity Regime 3.5 4 x 1-3 3 std dev Data Radiative Transfer 2.5 2 1.5 1.5 1 2 3 4 depth (km) = apparent velocity change

epsilon epsilon Surface Wave Sensitivity Regime 3.5 3 x 1-3 std dev Data Radiative Transfer.15.1 Surface wave sensitivity 2.5 2 1.5 1 2 3 depth (m).5 1 2 3 depth (m) = apparent velocity change

epsilon epsilon Surface Wave Sensitivity Regime 3.5 3 x 1-3 std dev Data Radiative Transfer Model.15.1 Surface wave sensitivity 2.5 2 1.5 1 2 3 depth (m).5 1 2 3 depth (m) = apparent velocity change

epsilon normalized displacement Model 4 x 1-3 3 std dev Data Model 1 2-1 ' 3 6 9 t in s 1 1 2 3 4 depth (km) = apparent velocity change Model Surf ( 1 1-α =.75 α =.25 ) α=fitting parameter; partition coefficient Bulk

partition coefficient Evolution of the partition coefficient 1 =5% =1% =15% =2% =3%.5 1- (bulk) (surf) 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 time in coda (s) Model Surf ( 1) Bulk

partition coefficient partition coefficient Evolution of the partition coefficient 1 =5% =1% =15% =2% =3%.5 1- (bulk) (surf) 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 time in coda (s) 1.8.6.4.2 =5% =1% =15% =2% =3% Mean free time t*=l*/c 1 2 3 4 5 6 t/t*

Shallow or deep perturbation? Perturbation at shallow depth (2 m) Perturbation at great depth (15 m)

Shallow or deep perturbation? Application to lunar data Passive Seismic Experiment of the Apollo 17 Sens-Schönfelder & Larose (28) Velocity variations as a consequence of temperature variations induced by solar irradiation influence limited to shallow subsurface only

Conclusion We can relate the depth sensitivity of coda waves to a combination of bulk-wave and surface-wave sensitivity Changes can be detected at large depth Time dependence of the relative velocity change in the coda - possibility to discriminate a shallow and a deep perturbation Apply the new sensitivity kernel to seismic monitoring in regions with strong scattering (volcanoes fault zones). Obermann et al. (212). Depth sensitivity of seismic coda waves to velocity perturbations in an elastic heterogeneous medium. Submitted to GJI Piton de la Fournaise La Reunion Island

absorbing Bulk Regime: Diffusion vs Radiative transfer absorbing absorbing absorbing

Bulk Regime: Diffusion vs Radiative transfer Simulation of the bulk regime without free surface source and receiver in the middle obs 1.4 x 1-3 1.2 1.8.6 std dev Data ( bulk ) radiative transfer approx. diffusion approx. = apparent velocity change.4.2 4 6 8 1 12 14 depth (km) No free parameters = direct fit!