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

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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

T Nov 11, 2008 2008 Sino-US Workshop, Boulder 31

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.