Solar powered seismology: high resolution imaging from ambient seismic noise

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Solar powered seismology high resolution imaging
from ambient seismic noise
Nikolai Shapiro University of Colorado at Boulder
Collaborators Michel Campillo, Laurent
Stehly, Mike Ritzwoller, Greg Bensen
Aknowledgements Misha Barmin, Craig Jones,
Ludovic Margerin, Eric Larosse, Richard Weaver,
Arnaud Derode, Anne Paul, Bart van Tiggelen
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Natural sources of seismic signals
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Resolution of seismic tomographic models of the
crust and the uppermost mantle

• Seismic tomography is traditionally based on
  direct waves from earthquakes
• Distribution of earthquakes and seismic stations
  is inhomogeneous
• In many regions, surface-wave tomography is based
  on long pathes

• Problems with long paths
• Extended sensitivity kernels
• Difficult to make short-period measurements
• Consequence limited resolution

Solution measurements independent of
earthquake locations
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Seismic coda and ambient seismic noise - random
seismic wavefields
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A consequence of the randomness correlations and
Greens function. Green function A-gtB time
correlation of fields in A and B Origins of
the idea The fluctuation-dissipation theorem
links random fluctuations of a system with its
response to an external source (e.g. Kubo, 1966).
The origin of the idea can be tracked in works
on Brownian motion by Einstein (in 1905!).
Applications with mechanical waves (under
different names) Helioseismology Duvall et
al. (1993). Laboratory Acoustics Weaver and
Lobkis (2001) Coda waves Campillo and Paul
(2003) Marine acoustics Roux et al.,
(2003) Ambient seismic noise Shapiro and
Campillo (2004)
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Correlations of random wavefields
Random wavefield - sum of waves emitted by
randomly distributed sources
Cross-correlation of waves emitted by a single
source between two receivers
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Correlations of random wavefields
Sources are in constructive interference when
respective travel time difference are close to
each other
Effective density of sources is high in the
vicinity of the line connecting two receivers
Cross-correlation extracts waves propagating
along the line connecting two receivers
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Cross-correlations of regional coda
From Campillo and Paul (2003)
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Cross-correlations from teleseismic codas data
records at five US permanent seismic stations
from 17 M8 earthquakes occurred between 1993 and
2002
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Cross-correlations from teleseismic codas at US
stations
vertical component stacks 0.03 - 0.1 Hz
3 km/s - Rayleigh wave
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Cross-correlations from teleseismic codas ANMO -
CCM
vertical component stacks from 13 earthquakes

• at long periods
• scattering is weaker
• telesesmic coda is not fully random
• coherent signals disappear in cross-correlations

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Cross-correlations from ambient seismic noise
ANMO - CCM
cross-correlations from 30 days of continuous
vertical component records (2002/01/10-2002/02/08)
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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
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Cross-correlation from ambient seismic noise in
North-Western Pacific
broadband cross-correlation computed from 30 days
of continuous vertical component records
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Cross-correlation from ambient seismic noise in
North-Western Pacific
broadband cross-correlation computed from 30 days
of continuous vertical component records
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Cross-correlation of seismic noise in California
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Cross-correlation of 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
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Comparison with signals from earthquakes
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Examples of Rayleigh-wave dispersion curves
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Measurements from two different months
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Repetitive tomography
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Resolution
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dispersion maps
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dispersion maps
Sierra Nevada
Sacramento basin
Franciscan formation
Peninsular Ranges
Salinean block
San Joaquin basin
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dispersion maps
Central Valley
Vantura basin
Imperial Valley
LA basin
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Comparison between noise-based and
earthquake-based tomographies
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Extraction of surface waves from seismic noise
Measurements without earthquakesImproved
resolutionPossible applications -
imaging of the crust and the uppermost mantle
- structure of sedimentary basins for seismic
hazard - seismic calibration for nuclear
monitoring Remaining questions - optimal
duration of noise sequences - spectral
range - optimal inter-station distances
- Other than Rayleigh waves (Love, body waves)
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Perspectives of noise-basedcontinental-scale
imaging in USA
courtesy of Greg Bensen (CU Boulder)
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CMB
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CCM
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WCI
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HRV
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DWPF
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Cross-correlation of seismic noise in Canada
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Cross-correlation of seismic noise in Antarctica
Record section Cross-correlate 1 month of
ambient noise, Z
20 sec period Rayleigh wave
Bandpass centered on 20 sec
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Tracing the originof the seismic noise
courtesy of Laurent Stehly (LGIT, Grenoble)
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Tracing the originof the seismic noise
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Tracing the originof the seismic noise
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Tracing the originof the seismic noise
10 - 20 s
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the end
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Extracting Green functions from the random
wavefield by field-to-filed correlation
theoretical background
seismic noise is excited by randomly distributed
ambient sources (oceanic microseisms and
atmospheric loads)
cross-correlation between points x and y
differs only by an amplitude factor F(?) from an
actual Green function between x and y
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estimation of errors
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estimation of errors
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Cross-correlations from teleseismic codas ANMO -
CCM
vertical component stack from13 earthquakes
distance 1405 km
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Cross-correlations from teleseismic codas ANMO -
CCM
vertical component stack from13 earthquakes
distance 1405 km
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Cross-correlations from teleseismic codas ANMO -
CC
vertical component stack from13 earthquakes
distance 1405 km
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Cross-correlations from teleseismic codas ANMO -
CCM
vertical component stack from13 earthquakes
distance 1405 km
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Measurement Procedure

• Select a long time series at each station (1
  month - 1 year).
• Filter data in a narrow frequency band (e.g., 5
  s - 10 s period).
• Create 1-bit signal (improves homogeneity of the
  signal with azimuth).
• Remove sequences following large earthquakes.
• Cross-correlate to produce the Green function.
• Measure the group speed at the center of the
  band.
• Repeat for different frequency bands.

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Diffuse field vs. ballistic waves
traditional approach using teleseismic surface
waves

• extended lateral sensitivity
• sample only certain directions
• source dependent
• difficult to make short-period
• measurements

Consequence limited resolution