High%20Resolution%20Surface%20Wave%20Tomography%20from%20Ambient%20Seismic%20Noise

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High Resolution Surface Wave Tomography from
Ambient Seismic Noise
50 km
Mike Ritzwoller University of Colorado at
Boulder Pubs ciei.colorado.edu/ritzwoller ritzwol
ler_at_ciei.colorado.edu

• Brief summary of the state of the art
• of tomography with teleseismic
• surface waves its frustrations.
• 2. Describe the use of ambient seismic
• noise for surface wave tomography.

Collaborators Nikolai Shapiro Anatoli
Levshin Greg Bensen M. Campillo L. Stehly
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Frustrations of Surface Wave Tomography

• Poor lateral resolution -- results from large
  epicentral distances wide sensitivity kernels.
• Poor constraints on the crust -- results from
  difficulty in measuring short period (lt15s)
  dispersion caused by attenuation, also due to
  large epicentral distances.

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dispersion maps
high resolution tomography of the Californian
crust from ambient seismic noise
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2. Dispersion measurements from the random
wavefield

• Seismic random wavefields
• coda (regional Campillo Paul, Science,
  2003 teleseismic Shapiro et al., Fall AGU,
  2003)
• ambient noise (Shapiro Campillo, GRL, 2004
  Shapiro, Campillo, Stehly, Ritzwoller,
  Science, 2005) -- presumably from atmospheric
  fluctuations and ocean waves.
• Idea estimate Green functions dispersion
  between stations.
• Application dispersion measurements between 7 -
  18 s period, including tomography in S. CA.
• Proof-of-concept results at longer periods (20 s
  - 150 s) across the entire US.

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The Idea cross-correlating long sequences of
ambient noise observed at pairs of stations
produces the Green function for waves propagating
between the stations.
From Weaver, Science, 2005.
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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 is similar
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
Sensitivity kernel collapses to an ellipse
(approximately) with the recievers at the foci.
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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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From Laurent Stehly
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From Laurent Stehly
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Comparison with Earthquake Records
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correlations computed over four different
three-week periods
PHL - MLAC 290 km
band- passed 15 - 30 s
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correlations computed over four different
three-week periods
PHL - MLAC 290 km
band- passed 15 - 30 s
band- passed 5 - 10 s
repetitive measurements provide uncertainty
estimations
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Example Rayleigh Wave Dispersion Curves
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Raypaths for two one-month data sets
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Repeatability of the Tomography
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Estimated Resolution
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dispersion maps
high resolution tomography of the Californian
crust from ambient seismic noise
Central Valley
Ventura basin
Imperial Valley
LA basin
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dispersion maps
high resolution tomography of the Californian
crust from ambient seismic noise
Sierra Nevada
Sacramento basin
Franciscan formation
Peninsular Ranges
Salinean block
San Joaquin basin
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dispersion maps
high resolution tomography of the Californian
crust from ambient seismic noise
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Comparison Between Traditional Teleseismic
Tomography and Tomography Based on Ambient
Seismic Noise
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Some Intriguing Applications

1. USArray other dense continental deployments
   such as PASSCAL experiments. Much higher
   resolution information about the structure of the
   crust and uppermost mantle in regions far from
   earthquakes.
2. ANSS and other continental-scale networks. (G.
   Bensen)
3. Local deployments much tighter station spacing,
   higher frequencies. (e.g., Yellowstone)
4. OBS deployments. (D. Forsyth)
5. Hazard Assessment. Better models of Vs in
   sedimentary basins.
6. Exploration. Shear static correction from Scholte
   waves in a marine setting.
7. Geotechnical. Shallow shear modulus needed for
   siting studies, slope characterization, etc.

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ANSS broad-band application
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ANSS broad-band application
Nov. 2003
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CMB
Nov. 2003, G. Bensen
33-67 s
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CCM
Nov. 2003, G. Bensen
33-67 s
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WCI
Nov. 2003, G. Bensen
33-67 s
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HRV
Nov. 2003, G. Bensen
33-67 s
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DWPF
Nov. 2003, G. Bensen
33-67 s
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Canada
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Green Functions by Cross-Correlating Ambient
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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Some Key Questions -- practical theoretical

1. Methods to optimize azimuthal homogeneity of the
   ambient noise?
2. Optimal time-series length?
3. Bandwidth?
4. Love waves? Overtones?
5. Source of the seismic signal in the ambient
   wavefield e.g., seasonal variability?
6. Conditions under which meaningful Green functions
   can be recovered i.e., when wont the method
   work?
7. Nature and shape of the sensitivity kernel?