Observation of diffuse seismic waves at teleseismic distances

Observation of diffuse seismic waves at teleseismic distances N. Shapiro University of Colorado at Boulder M. Campillo L. Margerin E. Chaljub B. van Tiggelen Université Joseph Fourier Grenoble, France

ballistic waves diffuse field source • source dependent • sample only certain directions • extended sensitivity • source independent • samples all directions • localized sensitivity have been traditionally used in seismology has been used in helioseismology (Duvall et al. , 1993), ultrasonics (Lobkis and Weaver, 2001), marine acoustics (Kuperman and Roux, 2003), and regional seismology (Campillo and Paul, 2003) Main goal of this study is to understand: (1) if the seismic diffuse waves can be observed far from earthquakes (2) if a deterministic information about the Earth’s structure can be extracted from those “teleseismic” diffuse waves

Diffuse fields at teleseismic distances: data and methods Signals • teleseismic coda • ambient seismic noise Methods • polarization analysis (to observe the mode equipartitioning) • field-to-field correlation

Example of teleseismic coda

Example of teleseismic coda vertical component

Example of teleseismic coda vertical component Diffuse and ballistic waves in the teleseismic coda cannot be separated by simple analysis of envelopes

Polarization of teleseismic coda

Stabilization of the vertical-to-horizontal energy ratio 0. 01 - 0. 016 Hz

Stabilization of the vertical-to-horizontal energy ratio 0. 01 - 0. 016 Hz Ez ~1. 5 Eh

Interpretation in terms of modal content ballistic field: no scattering, no energy exchange between modes high-Q modes dominate the late coda High-Q spheroidal modes have large Z/H ratios: domination of the vertical component short-living modes long-living modes Toroidal modes are attenuated faster than spheroidal modes: linear polarization of the horizontal component main physical cause: higher Q for P waves than for S waves

Interpretation in terms of modal content diffuse field scattering and energy redistribution between modes can result in mode equipartitioning randomization of the particle motion in the horizontal plane stabilization of the vertical-tohorizontal energy ratio

Comparison of the observed and the predicted Ez/Eh ratios Ez/Eh ratio in an equipartitioned field can be predicted as an average over all modes observation

Comparison of the observed and the predicted Ez/Eh ratios Ez/Eh ratio in an equipartitioned field can be predicted as an average over all modes or over some subset of modes observation

Comparison of the observed and the predicted Ez/Eh ratios Ez/Eh ratio in an equipartitioned field can be predicted as an average over all modes or over some subset of modes 1. 2. observation Possible explanations: Preferential scattering toward Rayleigh waves in the late coda Unaccounted effect of the anelastic attenuation on the equipartitioning

Extracting Green functions from the diffuse wavefield by field-to-filed correlation: theoretical background modal representation of the diffuse field: - eigenfunctions - eigenfrequencies - modal excitations, uncorrelated random variables: - spectral energy density cross-correlation between points x and y : differs only by an amplitude factor F( ) from an actual Green function between x and y

Cross-correlations from teleseismic codas: data records at five US permanent seismic stations from 17 M≥ 8 earthquakes occurred between 1993 and 2002

Cross-correlations from teleseismic codas: ANMO - CCM distance 1405 km vertical component stack from 13 earthquakes

Cross-correlations from teleseismic codas at US stations vertical component stacks 0. 03 - 0. 1 Hz 3 km/s - Rayleigh wave

Cross-correlations from teleseismic codas: ANMO - CCM vertical component stacks from 13 earthquakes at long periods: 1. 2. 3. scattering is weaker telesesmic coda is not fully diffuse coherent signals disappear in crosscorrelations

Cross-correlations from ambient seismic noise: ANMO - CCM cross-correlations from 30 days of continuous vertical component records (2002/01/10 -2002/02/08) frequency-time analysis of the broadband cross-correlation prediction from global group velocity maps of Ritzwoller et al. (2002)

Cross-correlations from ambient seismic noise at US stations frequency-time analysis of broadband cross-correlations computed from 30 days of continuous vertical component records

Cross-correlation from ambient seismic noise in North-Western Pacific broadband cross-correlation computed from 30 days of continuous vertical component records

Cross-correlations from ambient seismic noise in California cross-correlations of vertical component continuous records (1996/02/11 -1996/03/10) 0. 03 -0. 2 Hz 3 km/s - Rayleigh wave

Conclusions 1. 2. 3. Teleseismic coda at relatively short periods, strong multiple scattering makes the teleseismic coda diffuse at long periods, the scattering is weaker and diffuse waves do not completely dominate in the teleseismic coda Observed Z/H energy ratio may indicate that the coda is dominated by scattering toward fundamental-mode Rayleigh waves 1. 2. Ambient seismic noise is randomized because of the distribution of ambient sources (oceanic microseisms and atmospheric loads) coherent Rayleigh waves can be extracted from the seismic noise in a broad range of periods

Potential for seismic imaging Cross-correlations computed from the ambient seismic noise and the teleseismic coda can provide new surface-wave dispersion measurements that have numerous advantages relative to traditional measurements made from ballistic waves: 1. Measurements possible for every pair of stations 2. No source related errors 3. Localized sensitivity zones 4. Measurements can be extended to shorter periods