THORSTEN W. BECKER
Department of Earth Sciences
University of Southern California
Los Angeles

Seismology research

 T. Rockwell's trench 1, Hog Lake, 2002
[news] [research] [publications] [group] [CV] [teaching] [downloads] [contact] [misc]
[geodynamics] [seismology] [SEATREE] [downloads] [lab]
[crustal stress] [faults] [anisotropy] [tomography]

  • Earthquakes, lithospheric deformation, and crustal stress

    • Stress and strain patterns associated with southern California earthquakes and fault loading

      We analyze seismically and geodetically imaged strain release with focus on the western US plate boundary and fault loading and evolution in southern California. A partially SCEC funded project.

      • Bailey, I. W., Ben-Zion, Y., Becker, T. W., and Holschneider, M.: Quantifying focal mechanism heterogeneity for fault zones in central and southern California. Submitted to Geophys. J. Int.2009. (PDF)
      • Bailey, I. W., Becker, T. W., and Ben-Zion, Y.: Patterns of co-seismic strain computed from southern California focal mechanisms. Geophys. J. Int., 177, 1015-1036, 2009. (PDF).
      • Fay, N. P., Becker, T. W., and Humphreys, E. D.: Southern California Modeling of Geodynamics in 3D (SMOG3D): Toward quantifying the state of tectonic stress in the southern California crust, 2008 SCEC Annual Meeting, 1-122, 2008.
      • Becker, T W, Bailey, I W, & Y Ben-Zion: Stress and strain in southern California, Southern California Earthquake Center Meeting, Palm Springs CA, September 2006. (invited)

    • Fault slip rates and crustal state of stress

      We construct a simple model of interseismic strain and stress accumulation on southern California faults. The joint inversion of GPS velocities and focal mechanisms for fault slip rates of the southern San Andreas contributes to an improved understanding of plate boundary processes.

      • Becker, T. W., Hardebeck, J. L., and Anderson, G.: Constraints on fault slip rates of the southern California plate boundary from GPS velocity and stress inversions. Geophysical Journal International, 160, 634-650, 2005. (PDF).

    • Transform fault mechanics

      We explore how crustal thinning and thickening may result along a continental transform fault within a thin viscous sheet and argue that such antisymmetric patterns are observed along the San Andreas fault.

      • Platt, J. P., Kaus, B. J. P. and Becker, T. W.: The San Andreas Transform system and the tectonics of California: An alternative approach. Earth Planet. Sci. Lett., 274, 380-391, 2008. (PDF)

    • Boundary element package interact

      interact is a boundary element program which implements Okada's (1992) solutions for stress in an elastic half-space (Greens' functions for constant slip on rectangular dislocation elements). The interact package is modularly programmed in C and FORTRAN, GNU-licensed, and used to study geometrically complex fault systems and earthquake cycles.

      • Becker, T. W. and Schott, B.: On boundary-element models of elastic fault interaction (abstract). Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract NG62A-0925, 2002. (PDF)

    • Single fault and shear zone earthquake recurrence time variations

      We study earthquake interactions using 2-D elastic models, and apply analytical and finite element methods. We show that the orientation of faults in the background stress-field can lead to variations in the seismic cycle even without fault interactions.

      • Becker, T. W. and Schmeling, H.: Earthquake recurrence time variations with and without fault zone interactions. Geophys. J. Int., 135, 165-176, 1998. (PDF)

    • Effective shear modulus of crack-filled media

      We study micro-crack interaction and the mechanical properties of a crack-filled medium using finite element and boundary element techniques. We find that interactions should be taken into account; a modified self-consistent approach is best suited to the problems under consideration.

      • Dahm, T. and Becker, T.: On the elastic and viscous properties of media containing strongly interacting in-plane cracks. Pure Applied Geophys., 151, 1, 1998. (PDF)

    • Chaos in friction

      Two state-variable rate and state friction on faults can result in deterministic chaos in the seismicity of a simple spring-slider model. I substantiate that the system exhibits universal period doubling cascades and show that sliding events can be predicted with some accuracy. Coupled sliders show more regular seismicity, implying a regularizing effect of interactions.

      • Becker, T. W.: Deterministic Chaos in two State-variable Friction Sliders and the Effect of Elastic Interactions, in GeoComplexity and the physics of earthquakes, edited by J. B. Rundle, D. L. Turcotte, and W. Klein, p. 5-26, AGU, Washington D. C., 2000. (PDF)

  • Mantle tomography

    • Plume detection

      We analyze geodynamic and seismological models of the mantle and demonstrate that tomography images deep mantle plumes that connect to surface hotspots if conduit distortion in the mantle wind is accounted for.

      • Boschi, L., Becker, T. W., and Steinberger, B.: On the statistical significance of correlations between synthetic mantle plumes and tomographic models. Physics Earth Planet. Int., 260, 230-238, 2008. (PDF)
      • Boschi, L., T. W. Becker, and B. Steinberger, Mantle plumes: Dynamic models and seismic images, Geochem. Geophys. Geosyst., 8, Q10006, doi:10.1029/2007GC001733, 2007. (PDF)

    • Tomography validation and filtering

      We study long-period, surface wave seismograms from actual earthquakes and synthetics using the global spectral element method in order to validate and analyze different global mantle tomography models.

      • Qin, Y., Capdeville, Y., Montagner, J.-P., Boschi, L., and Becker, T. W.: Reliability of mantle tomography models assessed by spectral-element simulation. Geophys. J. Int., 177, 125-144, 2009. (PDF)

      The effects of tomographic resolution and filtering are explored in the context of global mantle circulation models; velocities between original input geodyamic model and tomo-filtered output match well.

      • Bullen, A. L., McNamara, A., Becker, T. W., and Ritsema, J.: Global scale models of the mantle flow field predicted by synthetic tomography models. Submitted to Phys. Earth Planet. Int., 2009. (PDF)

    • A comparison of seismologic and geodynamic mantle models

      We present a comprehensive and quantitative comparison between recent seismological and geodynamic models of the Earth's mantle to help in the move from mapping to hypotheses testing. Our results are compatible with whole mantle convection with reorganization of flow at 660-km due to the viscosity jump.

      • Becker, T. W. and Boschi, L.: A comparison of tomographic and geodynamic mantle models, Geochemistry, Geophysics, Geosystems, 3, 2001GC000168, 2002. (PDF)

      You can find the additional online material and tomographic model expansions on the Becker & Boschi: Correlations between models page. All tomographic models from that repository can be used directly as input for hc, for example in the Solid Earth Research and Teaching Environment (SEATREE) GUI.

    • Teaching interface to seismic tomography codes

      Larry Boschi's seismic tomography codes are available for teaching and research purposes in the Solid Earth Research and Teaching Environment (SEATREE)

      • Milner, K., Becker, T. W., Boschi, L., Sain, J., Schorlemmer, D. and H. Waterhouse: The Solid Earth Research and Teaching Environment: a new software framework to share research tools in the classroom and across disciplines. Eos Trans. AGU, 90, 12, 2009. (PDF).
      • Waterhouse, H. D., K. Milner, T. W. Becker, J. Sain, and D. Schorlemmer: A Solid Earth Research and Teaching Environment, Opportunities and Challenges in Computational Geophysics workshop, Caltech, 2009. (PDF).

    • Finite frequency tomography

      We strive to analyze and evaluate enhancements in global imaging of mantle structure by means of finite frequency tomography within the constraints given by global data coverage.

      • Boschi, L., Becker, T. W., Soldati, G., and Dziewonski, A. M.: On the relevance of Born theory in global seismic tomography. Geophys. Res. Lett., 33, L06302, doi:10.1029/2005GL025063, 2006. (PDF).

  • Seismic anisotropy and the asthenosphere

    • Radial anisotropy as a constraint for rheology

      We present the first global forward model of radial anisotropy and are able to match both anisotropy averages and some of the anomaly patterns. The mismatch between seismology and geodynamic reference, residual anisotropy, yields information on the frozen-in structure of the tectosphere and the volatile content and dynamics of the asthenosphere. An NSF-Geophysics funded project.
      • Becker, T. W., Kustowski, B. and Ekström, G.: Radial seismic anisotropy as a constraint for upper mantle rheology. Earth Planet. Sci. Lett., 267, 213-237, 2008. (PDF)

    • Azimuthal anisotropy from surface waves and flow models

      We show that anisotropy constrains net rotations of the lithosphere to be smaller than in some hotspot reference frame models, and analyze different seismological models using generalized spherical harmonics. Geodynamics can provide an both heterogeneity power spectra and patterns. An NSF-Geophysics funded project.
      • Becker, T. W.: Azimuthal seismic anisotropy constrains net rotation of the lithosphere. Geophys. Res. Lett., 35, L05303, doi:10.1029/2007GL032928, 2008. (Correction: 2008GL033946, PDF)
      • Becker, T. W., Ekström, G., Boschi, L., and Woodhouse, J.: Length scales, patterns, and origin of azimuthal seismic anisotropy in the upper mantle as mapped by Rayleigh waves. Geophysical J. Int., 171 451-462, 2007. (PDF)

    • Length scales and origin of continental anisotropy

      We analyze the lateral variations in anisotropic length scales as inferred from SKS splitting. Older continental regions appear more coherent than younger, geologically active units. We interpret this finding using surface waves and geodynamic models; anisotropy in old cratons may reflect frozen-in structure in the thick tectosphere rather than asthenospheric flow as underneath oceanic plates.

      • Becker, T. W., Browaeys, J. T., and Jordan, T. H.: Stochastic Analysis of Shear Wave Splitting Length Scales. In press at Earth and Planetary Science Letters, 259, 526-540, 2007. (PDF)

    • LPO texturing, mantle flow, and xenolith fabrics

      We study upper mantle fabrics from different mineral physics texturing models and mantle convection simulations with lateral viscosity variations. Texturing methods are found to differ strongly in terms of the treatment of recrystallization and predictions of mechanical anisotropy. Flow modeling results indicate that scaling relationships exist between hexagonal anisotropy parameters, and that natural samples follow the same trends as synthetics. Previously NSF-CSEDI funded.

      • Castelnau, O., Blackman, D. K. and Becker, T. W.: Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow. Geophys. Res. Lett, 36, L12304, doi:10.1029/2009GL038027, 2009. (PDF)
      • Becker, T. W., Chevrot, S., Schulte-Pelkum, V., and Blackman, D. K.: Statistical properties of seismic anisotropy predicted by upper mantle geodynamic models. J. Geophys. Res., 111, B08309, doi:10.1029/2005JB004095, 2006. (PDF).

    • Regional flow underneath the western United States

      We study regional shear wave splitting observations for the western United States and interpret them in a comprehensive modeling framework. We find evidence for a deep counterflow underneath North America driving by the Farallon slab. An NSF-CSEDI funded project.

      • Becker, T. W., Schulte-Pelkum, V., Blackman, D. K., Kellogg, J. B., and O'Connell, R. J.: Mantle flow under the western United States from shear wave splitting, Earth and Planetary Science Letters, 247, 235-251, 2006. (PDF)

    • Global upper mantle deformation and azimuthal anisotropy

      We compare finite strain from flow-models with Rayleigh-wave observations of azimuthal seismic anisotropy. Finite strain is a better explanation for anisotropy orientations than alignment with absolute plate motions, and we find evidence for buyoancy driven upwellings.

      • Becker, T. W., Kellogg, J. B., Ekström, G., and O'Connell, R. J.: Comparison of azimuthal seismic anisotropy from surface waves and finite-strain from global mantle-circulation models, Geophysical Journal International, 155, 696-714, 2003. (PDF)

[news] [research] [publications] [group] [CV] [teaching] [downloads] [contact] [misc]
[geodynamics] [seismology] [SEATREE] [downloads] [lab]
[crustal stress] [faults] [anisotropy] [tomography]
Updated: November 17, 2009. (thorstinski at gmail dot com)