Broad-Band Ambient Noise Tomography Across Europe and North America

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Broad-Band Ambient Noise Tomography Across Europe
and North America
Yingjie Yang, Mike Ritzwoller, Gregory Bensen,
Nikolai Shapiro, Anatoli Levshin University of
Colorado at Boulder IPGP, Paris
R. Weaver, Science, 2005
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Outline

• Ambient noise tomography data processing
  procedure.
• Estimated group speed maps across Europe.
• 3. Evidence that the results are an improvement
  over traditional surface-wave earthquake
  tomography.
• Preliminary tomography in the Western US using
  EarthScope Transportable Array data.
• Conclusions.

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Outline

1. Ambient noise tomography data processing
   procedure.
2. Estimated group speed maps across Europe.
3. Evidence that the results are an improvement over
   traditional surface-wave earthquake tomography.
4. Preliminary tomography in the Western US using
   EarthScope Transportable Array data.
5. Conclusions.

4
Outline

1. Ambient noise tomography data processing
   procedure.
2. Estimated group speed maps across Europe.
3. Evidence that the results are an improvement over
   traditional surface-wave earthquake tomography.
4. Preliminary tomography in the Western US using
   EarthScope Transportable Array data.
5. Conclusions.

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Data Processing Procedure
Phase 1. Pre-processing of single-station
data. ? Remove instrument, mean, trend,
band-pass filter, cut to 1-day. ? Time-domain
normalization desensitize to earthquakes
instrumental irregularities.
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Data Processing Procedure
Phase 1. Pre-processing of single-station
data. ? Remove instrument, mean, trend,
band-pass filter, cut to 1-day. ? Time-domain
normalization desensitize to earthquakes
instrumental irregularities.
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Data Processing Procedure
Phase 1. Pre-processing of single-station
data. ? Remove instrument, mean, trend,
band-pass filter, cut to 1-day. ? Time-domain
normalization desensitize to earthquakes
instrumental irregularities. ? Spectral
whitening.
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Data Processing Procedure
Phase 1. Pre-processing of single-station
data. ? Remove instrument, mean, trend,
band-pass filter, cut to 1-day. ? Time-domain
normalization desensitize to earthquakes
instrumental irregularities. ? Spectral
whitening.
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Data Processing Procedure
Phase 2. Processing on station-pairs
cross-correlation and stacking. ? Effect of
temporal normalization.
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Data Processing Procedure
Phase 2. Processing on station-pairs
cross-correlation and stacking. ? Effect of
temporal normalization. ? Effect of
pre-whitening.
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Data Processing Procedure
Phase 2. Processing on station-pairs
cross-correlation and stacking. ? Emergence of
the signal with stacks of increasing length.
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Data Processing Procedure
Phase 2. Processing on station-pairs
cross-correlation and stacking. ? Result of
following the processing procedure and stacking
over long time series.
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Data Processing Procedure
Phase 3. Measure dispersion curves.
Path N. Germany to N. Italy
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Data Processing Procedure
Phase 4. Error analysis and measurement
selection. ? Only high SNR observations are
used. ? Measurements are repeatable -- basis for
error analysis.
Path Holland to Hungary
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Data Processing Procedure
Phase 4. Error analysis and measurement
selection. ? Only high SNR observations are
used. ? Measurements are repeatable -- basis for
error analysis. ? Measurements cohere as a set
-- determined during tomography.
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Outline

1. Ambient noise tomography data processing
   procedure.
2. Estimated group speed maps across Europe.
3. Evidence that the results are an improvement over
   traditional surface-wave earthquake tomography.
4. Preliminary tomography in the Western US using
   EarthScope Transportable Array data.
5. Conclusions.

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Group Speed Tomography Across Europe
I. Data processing was procedure applied to the
12-months of VEBSN data across Europe for 2004.
Phase 1. Pre-processing of single-station
data. Phase 2. Processing on station-pairs
cross-correlation and stacking. Phase 3.
Measure dispersion curves. Phase 4. Error
analysis and data selection. II. Data
processing was followed by tomography to produce
dispersion maps 8-50 sec period.
125 stations
Stations from the Virtual European Broad-Band
Seismic Network (VEBSN).
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Group Speed Maps Across Europe 12 sec
From CUB 3-D Model
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Group Speed Maps Across Europe 12 sec
Ambient Noise Tomography
1664 paths
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Group Speed Maps Across Europe 16 sec
From CUB 3-D Model
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Group Speed Maps Across Europe 16 sec
Ambient Noise Tomography
16 sec
3241 paths
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Group Speed Maps Across Europe 20 sec
From CUB 3-D Model
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Group Speed Maps Across Europe 20 sec
Ambient Noise Tomography
3057 paths
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Group Speed Maps Across Europe 30 sec
From CUB 3-D Model
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Group Speed Maps Across Europe 30 sec
Ambient Noise Tomography
2450 paths
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Group Speed Maps Across Europe 40 sec
From CUB 3-D Model
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Group Speed Maps Across Europe 40 sec
Ambient Noise Tomography
2760 paths
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Outline

1. Ambient noise tomography data processing
   procedure.
2. Estimated group speed maps across Europe.
3. Evidence that the results are an improvement over
   traditional surface-wave earthquake tomography.
4. Preliminary tomography in the Western US using
   EarthScope Transportable Array data.
5. Conclusions.

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How do we Know if These Results are an
Improvement Over Traditional Earthquake
Tomography?
Various lines of evidence

• Agreement with known structures.
• e.g., sedimentary basins, crustal thickness.
• Repeatability of measurements.
• Seasonal variability is the basis for
  uncertainty estimates on the measurements.
• Coherence of measurements.
• Fit to ambient noise measurements during
  tomography, compared with fit to earthquake based
  measurements during tomography.

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Agreement with Location of Sedimentary Basins?
Observed 16 sec
Many of the basins across Europe are reflected in
the short period dispersion maps (e.g., 16 sec
here) N. Sea Basin, Silesian Basin (N.
Germany, Poland), Panonian Basin (Hungary,
Slovakia), Po Basin (N. Italy), Rhone
Basin (S. France), Basins in Adriatic and
Mediterranean Seas.
From Crust1.0, Laske et al.
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Agreement with Expected Crustal Thickness?
Observed 30 sec
Low speed anomalies across Europe are associated
with mountains belts, consistent with thickened
crust e.g., Alps, Balkans, Carpathians.
From Crust2.0, Laske et al.
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Coherence Among Measurements -- 12 sec period?
As measured by the ability to fit data sets when
doing tomography..
Misfit to Earthquake Measurements From
Earthquake Tomography
Misfit to Ambient Noise Measurements From
Ambient Noise Tomography
st dev 28.9 sec
st dev 15.0 sec
misfit (sec)
misfit (sec)
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Coherence Among Measurements -- 16 sec period?
As measured by the ability to fit data sets when
doing tomography..
Misfit to Ambient Noise Measurements From
Ambient Noise Tomography
Misfit to Earthquake Measurements From
Earthquake Tomography
st dev 12.6 sec
st dev 22.7 sec
misfit (sec)
misfit (sec)
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Coherence Among Measurements -- 20 sec period?
As measured by the ability to fit data sets when
doing tomography..
Misfit to Earthquake Measurements From
Earthquake Tomography
Misfit to Ambient Noise Measurements From
Ambient Noise Tomography
st dev 12.0 s
st dev 21.7 s
misfit (sec)
misfit (sec)
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Coherence Among Measurements -- 30 sec period?
As measured by the ability to fit data sets when
doing tomography..
Misfit to Ambient Noise Measurements From
Ambient Noise Tomography
Misfit to Earthquake Measurements From
Earthquake Tomography
st dev 12.2 s
st dev 18.1 s
misfit (sec)
misfit (sec)
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Coherence Among Measurements -- 40 sec period?
As measured by the ability to fit data sets when
doing tomography..
Misfit to Ambient Noise Measurements From
Ambient Noise Tomography
Misfit to Earthquake Measurements From
Earthquake Tomography
st dev 8.2 s
st dev 12.4 s
misfit (sec)
misfit (sec)
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Coherence Among Measurements -- Summary
As measured by the ability to fit data sets when
doing tomography..
Dispersion measurements from ambient noise are
more internally consistent than
measurements following earthquakes earthquake
measurements are difficult to obtain
below 20 sec, source processes,
mislocation, etc. are eliminated. Above 30
sec, earthquake measurements are about as
reliable as ambient noise measurements and the
data sets can be combined without degrading the
ambient noise measurements.
earthquakes
ambient noise
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Outline

1. Ambient noise tomography data processing
   procedure.
2. Estimated group speed maps across Europe.
3. Evidence that the results are an improvement over
   traditional surface-wave earthquake tomography.
4. Preliminary tomography in the Western US using
   EarthScope Transportable Array data.
5. Conclusions.

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Ambient Noise Tomography Across Europe

• Below 40 sec period, ambient noise surface wave
  dispersion measurements across Europe are
  generally more reliable than earthquake
  dispersion measurements.
• This is particularly true below 20 sec period,
  where dispersion provides information about
  crustal structure.

Earth is a noisy place!
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General Conclusions

• GT data bases (e.g., Engdahl Bergmans) will be
  exceptionally valuable to validate an emerging
  European Reference Model.
• Surface waves are very useful in the construction
  of reference models generally. Large-scale 3D
  models (e.g., CUB) of the crust and upper mantle
  constructed from surface wave data provide a good
  start toward the European Reference Model.
• Ambient noise tomography promises to improve
  information from surface waves generally,
  particularly about the crust.
• In the context of the growing VEBBN, ambient
  noise tomography will prove to be a powerful
  method to aid in the development of the European
  Reference Model.

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Principal Conclusions

• Earths Hum (bass section normal mode studies
  250 s) possesses tenor, alto, and soprano
  sections (lt 5 sec to gt 100 sec) useful for
  surface wave tomography.
• 2. Surface wave Green functions are extracted by
  cross-correlating long noise sequences at two
  stations.
• 3. Ambient noise tomography improves the
  resolution of the crust and upper mantle.
  (USArray)
• 4. Source of this noise (our signal) appears to
  be oceanic.
• 5. This noise is not truly ambient, but
  possesses a strong directional component from
  off-shore storms.

R. Weaver, Science, 2005
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Measurement Method Applied to a Pair of North
American Stations
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Surface Wave Tomography from Ambient Seismic Noise

• Summary of the measurement procedure.
• Examples of cross-correlograms.
• Group speed maps from 12 - 40 sec.
• Determination of reliability
• Consistency with known structures,
• (e.g., Sedimentary basins, crustal thickness.
• Coherence among the measurements.
• Repeatability (not shown here).

R. Weaver, Science, 2005
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Station Coverage for Ambient Noise Tomography
Across Europe
Stations from the Virtual European Broad-Band
Seismic Network (VEBSN).
125 stations
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Measurement Procedure
N. Germany To N. Italy

• For each station pair, perform a
• series of narrow band-pass filters on each day
  of data
• 5-15, 10-25, 20-40, 33-66, 50-100, 70-150 sec.
• Perform temporal and spectral
• whitening of each time series.

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Measurement Procedure
N. Germany To N. Italy

• For each station pair, perform a
• series of narrow band-pass filters
• 5-15, 10-25, 20-40, 33-66, 50-100, 70-150 sec.
• Perform temporal and spectral
• whitening of each time series.
• Stack results in daily, monthly,
• tri-monthly, yearly increments.

Symmetric component of 1 year stack.
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Measurement Procedure
N. Germany To N. Italy

• For each station pair, perform a
• series of narrow band-pass filters
• 5-15, 10-25, 20-40, 33-66, 50-100, 70-150 sec.
• Perform temporal and spectral
• whitening of each time series.
• Stack results in daily, monthly,
• tri-monthly, yearly increments.
• Measure surface wave dispersion in each period
  band.

Predicted curve from CUB model
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Example of Broad-Band Cross-Correlograms
Path N. Germany to Romania
1-year stack
time (sec/100)
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Sample Record Section
N. Italy to Stations Across Europe
33-66 sec, 1 year stack, symmetric component