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
Polarization of teleseismic coda
Polarization of teleseismic coda
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
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:
un
 (x,t)   an un (x)ei
nt
n
- eigenfunctions
n
- eigenfrequencies
an
- modal excitations, uncorrelated random variables:
an am*   n,m F( n )
F()
- spectral energy density
cross-correlation between points x and y :
C(x, y, )   F( n )un (x)un (y)e
 i n 
n
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 from13 earthquakes
Cross-correlations from teleseismic codas: ANMO - CCM
distance 1405 km
vertical component
stack from13 earthquakes
Cross-correlations from teleseismic codas: ANMO - CCM
distance 1405 km
vertical component
stack from13 earthquakes
Cross-correlations from teleseismic codas: ANMO - CCM
distance 1405 km
vertical component
stack from13 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-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
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