n. shapiro observation of diffuse seismic waves at teleseismic distances university of colorado at...

N. Shapiro

Observation of diffuse seismic waves atteleseismic distances

Observation of diffuse seismic waves atteleseismic distances

University of Colorado at Boulder

M. Campillo

L.Margerin

E. Chaljub

B. van Tiggelen

Université Joseph Fourier Grenoble, France

ballistic waves diffuse field

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)

• source dependent• sample only certain directions• extended sensitivity

• source independent• samples all directions• localized sensitivity

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

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

source

Signals

• teleseismic coda

• ambient seismic noise

Methods

• polarization analysis (to observe the mode equipartitioning)

• field-to-field correlation

Diffuse fields at teleseismic distances:data and methods

Diffuse fields at teleseismic distances:data and methods

Example of teleseismic codaExample of teleseismic coda


vertical component


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

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

vertical component

Polarization of teleseismic codaPolarization of teleseismic coda

Stabilization of the vertical-to-horizontal energy ratioStabilization of the vertical-to-horizontal energy ratio

0.01 - 0.016 Hz

0.01 - 0.016 Hz


0.01 - 0.016 Hz

Ez

Eh~1.5


ballistic field: no scattering, no energy exchange between modeshigh-Q modes dominate the late coda

ballistic field: no scattering, no energy exchange between modeshigh-Q modes dominate the late coda

main physical cause: higher Q for P waves than for S wavesmain physical cause: higher Q for P waves than for S waves

Interpretation in terms of modal contentInterpretation in terms of modal content

Toroidal modes are attenuated faster than spheroidal modes:

linear polarization of the horizontal component

long

-liv

ing

mod

essh

ort-

livi

ngm

odes

High-Q spheroidal modes have large Z/H ratios:

domination of the vertical component

diffuse field scattering and energy redistribution

between modes can result inmode equipartitioning

diffuse field scattering and energy redistribution

between modes can result inmode equipartitioning

randomization of the particle motion in the horizontal plane

randomization of the particle motion in the horizontal plane

stabilization of the vertical-to-horizontal energy ratio

stabilization of the vertical-to-horizontal energy ratio

Interpretation in terms of modal contentInterpretation in terms of modal content

Comparison of the observed and the predicted Ez/Eh ratiosComparison 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


can be predicted as an average over all modes

observation



can be predicted as an averageover all modes

orover some subset of modes




observation


observation

Possible explanations:1. Preferential scattering toward Rayleigh waves in the late coda2. Unaccounted effect of the anelastic attenuation on the equipartitioning

Possible explanations:1. Preferential scattering toward Rayleigh waves in the late coda2. Unaccounted effect of the anelastic attenuation on the equipartitioning







Extracting Green functions from the diffuse wavefield by field-to-filed correlation: theoretical background

Extracting Green functions from the diffuse wavefield by field-to-filed correlation: theoretical background

anam* =δn,mF(ωn)

C(x,y,τ) = F(ωn)un(x)un(y)e−iωnτ

n∑

φ(x,t) = anun(x)eiωnt

n∑modal representation of the diffuse field:

ωn - eigenfrequencies

un - eigenfunctions

an - modal excitations, uncorrelated random variables:

F(ω) - 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: dataCross-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 - CCMCross-correlations from teleseismic codas: ANMO - CCM

vertical componentstack from13 earthquakesdistance 1405 km


distance 1405 km vertical componentstack from13 earthquakes

vertical component stacks0.03 - 0.1 Hz

3 km/s - Rayleigh wave

Cross-correlations from teleseismic codas at US stationsCross-correlations from teleseismic codas at US stations

vertical component stacks from 13 earthquakes

at long periods:

1. scattering is weaker2. telesesmic coda is

not fully diffuse3. coherent signals

disappear in cross-correlations

at long periods:

1. scattering is weaker2. telesesmic coda is

not fully diffuse3. coherent signals

disappear in cross-correlations


cross-correlations from 30 days of continuous vertical component records (2002/01/10-2002/02/08)

prediction from global group velocity maps of Ritzwoller et al. (2002)

frequency-time analysis of the broadband cross-correlation

Cross-correlations from ambient seismic noise: ANMO - CCMCross-correlations from ambient seismic noise: ANMO - CCM

Cross-correlations from ambient seismic noise at US stationsCross-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 PacificCross-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 CaliforniaCross-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

ConclusionsConclusions

Teleseismic coda1. at relatively short periods, strong multiple scattering

makes the teleseismic coda diffuse2. at long periods, the scattering is weaker and diffuse

waves do not completely dominate in the teleseismic coda

3. Observed Z/H energy ratio may indicate that the coda is dominated by scattering toward fundamental-mode Rayleigh waves

Ambient seismic noise 1. seismic noise is randomized because of the

distribution of ambient sources (oceanic microseisms and atmospheric loads)

2. coherent Rayleigh waves can be extracted from the seismic noise in a broad range of periods

Potential for seismic imagingPotential for seismic imaging

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

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:

n. shapiro observation of diffuse seismic waves at teleseismic distances university of colorado at...

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