T Nov 11, 2008 2008 Sino-US Workshop, Boulder 1
Mike Ritzwoller1
Ying Yang1
Morgan Moschetti1
Fan-Chi Lin1
Greg Bensen1
Xiaodong Song2
Sihua Zheng3
1 - University of Colorado at Boulder2 - University of Illinois, Urbana-Champaign3 - Chinese Earthquake Administration
R. Weaver,Science, 2005
Seismic Tomography without Earthquakes: Progress in
Ambient Noise Tomography
Seismic Tomography without Earthquakes: Progress in
Ambient Noise Tomography
• Ambient noise is enriched at short periods: 5 – 25 sec.
Better constraints on crustal and uppermost mantle structure than information from earthquakes.
• Particularly useful in aseismic areas; e.g., continental interiors.
• For temporary deployments -- do not have to wait for earthquakes to occur.
• Measurements are repeatable: rigorous uncertainty estimates.
T Nov 11, 2008 2008 Sino-US Workshop, Boulder 2
Why Ambient Noise Tomography?Why Ambient Noise Tomography?
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Outline
1. Why do we believe results from ANT?
2. Application to the EarthScope Transportable Array (TA) across the western US:
Isotropic and radially anisotropic 3D model in W. US.
3. New method of tomography:
Eikonal tomography & azimuthal anisotropy in W. US.
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Outline
1. Why do we believe results from ANT?
2. Application to the EarthScope Transportable Array (TA) across the western US:
Isotropic and radially anisotropic 3D model in W. US.
3. New method of tomography:
Eikonal tomography & azimuthal anisotropy in W. US.
T Nov 11, 2008 2008 Sino-US Workshop, Boulder 5
Processing Steps:
Remove instrument response, de-mean, de-trend, bandpass filter, time-domain normalization, spectral whitening
Cross-correlation: 1 day at a time.
Stack over many days.
Waveform selection (SNR) for tomography
time (s)
16.3 Month Stack
Station Y12C
Station 109C
Ambient noise data processing
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time (s)
16.3 Month Stack
Station Y12C
Station 109CProcessing Steps:
Remove instrument response, de-mean, de-trend, bandpass filter, time-domain normalization, spectral whitening
Cross-correlation: 1 day at a time.
Stack over many days.
Waveform selection for tomography
Ambient noise data processing
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Why Believe Ambient Noise Empirical Green’s Functions?: Several Primary Lines of Evidence
1. Results make sense “geologically”.
2. Ambient noise arrives within continental interiors from“all azimuths” (although SNR varies with
azimuth).
3. Spatial repeatability of measurements.
4. Temporal repeatability of measurements – basis foruncertainty analysis.
5. Agreement with earthquake measurements.
6. Station-triad analysis.
7. Fit to data by tomographic maps.
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Why Believe Ambient Noise Empirical Green’s Functions?: Omni-directionality of Ambient Noise
Results from Europe:
SNR vs azimuth
(From Yang & Ritzwoller, G-cubed, 2008)
Secondary microseism
Primary microseism
Non - microseism
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Why Believe Ambient Noise Empirical Green’s Functions?: Omni-directionality of Ambient Noise
Simulated Noise Distributions Example Cross-Correlations
Expected error < 0.5 sec
(From Yang & Ritzwoller, G-cubed, 2008)
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Narrow band-pass (15 sec) 3-month cross - corrrelations.
Note stability of phases.
Envelopes are less stable.
Phase time uncertainties: ~ 1sec
Why Believe Ambient Noise Empirical Green’s Functions?: Temporal Repeatability
BAR & NEE
(Work of Fan-Chi Lin)
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Why Believe Ambient Noise Empirical Green’s
Functions?: Spatial Repeatability
(From Bensen et al., GJI, 2007)
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Earthquake near PFO
Red - earthquakeBlue - EGF
Why Believe Ambient Noise Empirical Green’s Functions?: Comparison with Earthquake Records
(From Bensen et al., GJI, 2007)
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Outline
1. Why Ambient Noise Tomography (ANT)?
2. Idea behind ANT. Simulation.
3. Why do we believe results from ANT?4. Basic Science result:
Isotropic and radially anisotropic 3D model in W. US.
5. New method of tomography:
Eikonal tomography & azimuthal anisotropy in W. US.
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1. Why do we believe results from ANT?
2. Application to the EarthScope Transportable Array (TA) across the western US:
Isotropic and radially anisotropic 3D model in W. US.
3. New method of tomography:
Eikonal tomography & azimuthal anisotropy in W. US.
Outline
T Nov 11, 2008 2008 Sino-US Workshop, Boulder 15
Current Status:Transportable ArrayComponent of USARRAY/EarthScope
Sep 23, 2008.
To show:(1) 3D isotropic Vs structure of the crust & uppermost mantle. Rayleigh waves alone. ANT + earthquake tomography: 6 – 100 sec.
(Yingjie Yang)
(2) 3D radial Vsh:Vsv anisotropy. ANT alone: 6 – 40 sec.
Rayleigh and Love waves. (Morgan Moschetti)
(3) New method (Eikonal tomography):
3D Vs azimuthal anisotropy. (Fan-Chi Lin)
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Traditional ambient noise tomography
Rayleigh wave 8 sec
Phase velocity anomaly (%)
(Citation: Moschetti et al., G-cubed, 2007)
Traditional ambient noise tomography
Rayleigh wave 8 sec
Love and Rayleigh waves
Both phase and group velocities
Periods: 8 to 40 sec
Phase velocity anomaly (%)
(Citation: Moschetti et al., G-cubed, 2007)
Multiple-Plane-Wave Tomography
Two or more plane waves represent the incoming wavefront
Finite-frequency kernels are included.
RegionalArray
(Yang et al., JGR, 2009)
Incoming wave is distorted by velocity heterogeneities
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Phase velocity maps at 33 sec
Multiple-plane-waveAmbient noise
T Nov 11, 2008 20(Citation: Yang et al., JGR, 2009)
Phase velocity maps from MPWT
50 sec 83 sec
T Nov 11, 2008 21(Citation: Yang et al., JGR, 2009)
Phase velocity maps
8 sec
100 sec
Monte-Carlo inversion for isotropic Vs structure
Rayleigh wave phase speed
crustmantle
Shear velocity
ANT MPWT
Rayleigh wavephase speed
Crustal shear velocityLow velocities in the shallow crust:
Great Valley (GV) Salton Trough (ST )Los Angeles Basin (LAB)
Yakima Fold Belt (YFB)Olympic Peninsula (OP)California Coastal Ranges (CCR)
High velocities throughout the crust
Sierra Nevada (SN)
Peninsular Ranges (PR)
N. Columbia Plateau (NCP)
W. Snake River Plain (SRP)
GV
ST
LAB
YFB
SN
PR
NCP
SRP
0-10 km 10-20 km
OP
CC
R
T Nov 11, 2008 23(Citation: Yang et al., JGR, 2009)
Upper mantle shear velocity
CR: the Cascade Range RM: the Rocky Mountains
24(Citation: Yang et al., JGR, 2009)
CR: the Cascade Range RM: the Rocky MountainsBR: the Basin and Range SRP: the Snake River Plain
Upper mantle shear velocity
BB’
dept
h (k
m)
BR
(Citation: Yang et al., JGR, 2009)
GV: the Great Valley TR: the Transverse RangeST: the Salt Trough
ST
GV Sierra Nevada
(Zandt et al. Nature, 2004)
Upper mantle shear velocity:High velocity mantle “drip”
(Citation: Yang et al., JGR, 2009)
To show:(1) 3D isotropic Vs structure of the crust & uppermost mantle. Rayleigh waves alone. ANT + earthquake tomography: 6 – 100 sec.
(Yingjie Yang)
(2) 3D radial Vsh:Vsv anisotropy. ANT alone: 6 – 40 sec.
Rayleigh and Love waves. (Morgan Moschetti)
(3) New method (Eikonal tomography):
3D Vs azimuthal anisotropy. (Fan-Chi Lin)
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Love phase
Rayleigh phase
Rayleigh group
crustmantle
.
.
.
Inverting Rayleigh & Love wave data: Isotropic model
Misfit with an Isotropic Model
(Moschetti et al., in preparation, 2008)
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Love phase
Rayleigh phase
Rayleigh group
crustmantle
VshVsv
Inverting Rayleigh & Love wave data:Radial anisotropy in crust & mantle
crust mantleMisfit with an Anisotropic Model
(Moschetti et al., in preparation, 2008)
To show:(1) 3D isotropic Vs structure of the crust & uppermost mantle. Rayleigh waves alone. ANT + earthquake tomography: 6 – 100 sec.
(Yingjie Yang)
(2) 3D radial Vsh:Vsv anisotropy. ANT alone: 6 – 40 sec.
Rayleigh and Love waves. (Morgan Moschetti)
(3) New method (Eikonal tomography):
3D Vs azimuthal anisotropy. (Fan-Chi Lin)
T Nov 11, 2008 2008 Sino-US Workshop, Boulder 30
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1. Why do we believe results from ANT?
2. Application to the EarthScope Transportable Array (TA) across the western US:
Isotropic and radially anisotropic 3D model in W. US.
3. New method of tomography:
Eikonal tomography & azimuthal anisotropy in W. US.
Outline
Evidence for wavefield complexity: ray tracing
The travel time at each location is simulated based on our 8s Rayleigh wave phase velocity map.The center station is LRL and 50s contours are shown.
Eikonal Tomography: construct the travel time surface and the local phase velocity
Use the TA as an array.
Construct a travel time surfaces.
Center station R06C taken as an“effective source”
€
∇t phase (r) =1
c(r)
33
22 sec Rayleigh wave22 sec Rayleigh wave
Repeat for many(>400) effectiveSources.
(Lin et al., GJI, in press, 2008)
Local constraints on phase velocity22s Rayleigh wave
N. Nevada
N. Arizona
S.Cal. W. Oregon
N. Oregon
W. Utah
AB
C
D
E
F
Note:(1)Azimuth dependent phase speed measurements.(2)Uncertainties in the measurements.
(Lin et al., GJI, in press, 2008)
Comparison between Eikonal and traditional (straight ray) tomography
Eikonal tomographyTraditional inversion method Barmin et al. (2001)25s Rayleigh wave
T Nov 11, 2008 35(Lin et al., GJI, in press, 2008)
Azimuthal anisotropy of Rayleigh waves at 12 and 22 sec period
12 sec 22 sec
(Lin et al., GJI, in press, 2008)
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SKS: Data from Matt Fouch
Inversion for azimuthally anisotropic model: Two layers
crustal anisotropy upper mantle anisotropy
Figure removed.
(Lin et al., in preparation, 2008)
SKS splitting directions versus crustal and uppermost mantle azimuthal anisotropy
SKS data & mantle anisotropy
SKS - Crust
SKS - Mantle
SKS data from Matt Fouch38
Figure removed.
(Lin et al., in preparation, 2008)
ConclusionsConclusions• There are numerous lines of evidence that now establish the
veracity of ambient noise tomography.
• Ambient noise provides unique information about short period (5 – 20 sec period) surface wave propagation.
• In combination with earthquake-derived information at longer periods, high resolution 3D models of crust and upper mantle are now emerging:
o 3D isotropic structure in the western US.o Radial anisotropy in the crust and uppermost mantle.
• A new method of tomography based on tracking surface wavefronts (Eikonal tomography) provides direct constraints on azimuthal anisotropy and yields meaningful uncertainty estimates:
o 3D model of azimuthal anisotropy in the crust and uppermost mantle.
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References
• Bensen, G.D., M.H. Ritzwoller, M.P. Barmin, A.L. Levshin, F. Lin, M.P. Moschetti, N.M. Shapiro, and Y. Yang, Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements, Geophys. J. Int., 169, 1239-1260, doi: 10.1111/j.1365-246X.2007.03374.x, 2007.
• Lin, F.-C., M.H. Ritzwoller, and R. Snieder, Eikonal Tomography: Surface wave tomography by phase-front tracking across a regional broad-band seismic array, Geophys. J. Int., in press, 2009.
• Lin, F-C. and M.H. Ritzwoller, Azimuthal anisotropy in the western US in the crust and uppermost mantle, in preparation, 2008.
• Moschetti, M.P., M.H. Ritzwoller, and N.M. Shapiro, Surface wave tomography of the western United States from ambient seismic noise: Rayleigh wave group velocity maps, Geochem., Geophys., Geosys., 8, Q08010, doi:10.1029/2007GC001655, 2007.
• Moschetti, M.P., M.H. Ritzwoller, and F. Lin, Seismic evidence for widespread crustal flow caused by extension in the western USA, in preparation, 2008.
• Yang, Y. and M.H. Ritzwoller, The characteristics of ambient seismic noise as a source for surface wave tomography, Geochem., Geophys., Geosys., 9(2), Q02008, 18 pages, doi:10.1029/2007GC001814, 2008.
•Yang, Y., M.H. Ritzwoller, F.-C. Lin, M.P. Moschetti, and N.M. Shapiro, The structure of the crust and uppermost mantle beneath the western US revealed by ambient noise and earthquake tomography, J. Geophys. Res.,in press, 2009.