T Nov 11, Sino-US Workshop, Boulder1 Mike Ritzwoller 1 Ying Yang 1 Morgan Moschetti 1 Fan-Chi Lin 1 Greg Bensen 1 Xiaodong Song 2 Sihua Zheng University of Colorado at Boulder 2 - University of Illinois, Urbana-Champaign 3 - Chinese Earthquake Administration R. Weaver,Science, 2005 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, Sino-US Workshop, Boulder2 Why Ambient Noise Tomography?

T Nov 11, Sino-US Workshop, Boulder3 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, Sino-US Workshop, Boulder4 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, Sino-US Workshop, Boulder5 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

T Nov 11, Sino-US Workshop, Boulder6 time (s) 16.3 Month Stack Station Y12C Station 109C 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 for tomography Ambient noise data processing

T Nov 11, Sino-US Workshop, Boulder7 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 for uncertainty analysis. 5. Agreement with earthquake measurements. 6.Station-triad analysis. 7.Fit to data by tomographic maps.

T Nov 11, 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

T Nov 11, Why Believe Ambient Noise Empirical Green’s Functions?: Omni-directionality of Ambient Noise Simulated Noise DistributionsExample Cross-Correlations Expected error < 0.5 sec (From Yang & Ritzwoller, G-cubed, 2008)

T Nov 11, 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)

T Nov 11, Why Believe Ambient Noise Empirical Green’s Functions?: Spatial Repeatability (From Bensen et al., GJI, 2007)

T Nov 11, Earthquake near PFO Red - earthquake Blue - EGF Why Believe Ambient Noise Empirical Green’s Functions?: Comparison with Earthquake Records (From Bensen et al., GJI, 2007)

T Nov 11, Sino-US Workshop, Boulder13 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.

T Nov 11, Sino-US Workshop, Boulder14 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, Sino-US Workshop, Boulder15 Current Status: Transportable Array Component 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) T Nov 11, Sino-US Workshop, Boulder16

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. Regional Array (Yang et al., JGR, 2009) Incoming wave is distorted by velocity heterogeneities T Nov 11,

Phase velocity maps at 33 sec Multiple-plane-wave Ambient noise T Nov 11, (Citation: Yang et al., JGR, 2009)

Phase velocity maps from MPWT 50 sec 83 sec T Nov 11, (Citation: Yang et al., JGR, 2009)

Phase velocity maps 8 sec 100 sec Monte-Carlo inversion for isotropic Vs structure Rayleigh wave phase speed crust mantle Shear velocity ANTMPWT Rayleigh wave phase speed

Crustal shear velocity Low 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 km10-20 km OP CCR T Nov 11, (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 Mountains BR: the Basin and Range SRP: the Snake River Plain Upper mantle shear velocity B B’ depth (km) BR (Citation: Yang et al., JGR, 2009)

GV: the Great Valley TR: the Transverse Range ST: 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) T Nov 11, Sino-US Workshop, Boulder27

T Nov 11, Love phase Rayleigh phase Rayleigh group crust mantle... Inverting Rayleigh & Love wave data: Isotropic model Misfit with an Isotropic Model (Moschetti et al., in preparation, 2008)

T Nov 11, Love phase Rayleigh phase Rayleigh group crust mantle VshVsv Inverting Rayleigh & Love wave data: Radial anisotropy in crust & mantle crustmantle Misfit 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, Sino-US Workshop, Boulder30

T Nov 11, Sino-US Workshop, Boulder31 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” sec Rayleigh wave Repeat for many (>400) effective Sources. (Lin et al., GJI, in press, 2008)

Local constraints on phase velocity 22s Rayleigh wave N. Nevada N. Arizona S.Cal.W. Oregon N. Oregon W. Utah A B 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 tomography Traditional inversion method Barmin et al. (2001) 25s Rayleigh wave T Nov 11, (Lin et al., GJI, in press, 2008)

Azimuthal anisotropy of Rayleigh waves at 12 and 22 sec period 12 sec22 sec (Lin et al., GJI, in press, 2008)

T Nov 11, SKS: Data from Matt Fouch Inversion for azimuthally anisotropic model: Two layers crustal anisotropyupper 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 Fouch 38 Figure removed. (Lin et al., in preparation, 2008)

Conclusions 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. T Nov 11, Sino-US Workshop, Boulder39

T Nov 11, Sino-US Workshop, Boulder40 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, , doi: /j X x, 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, Lin, F-C. and M.H. Ritzwoller, Azimuthal anisotropy in the western US in the crust and uppermost mantle, in preparation, 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: /2007GC001655, Moschetti, M.P., M.H. Ritzwoller, and F. Lin, Seismic evidence for widespread crustal flow caused by extension in the western USA, in preparation, 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: /2007GC001814, 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.