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Seismic Analysis and Tomography on Volcanoes

Seismic Analysis and Tomography on Volcanoes. Jonathan M. Lees University of North Carolina, Chapel Hill Presented at Georgia State University, July, 2011. Lees, J. M. (2007), Tomography of Crustal Magma Bodies: Implications For Magmatic Systems, J. Volc. Geoth. Res. , 167 , 37-56

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Seismic Analysis and Tomography on Volcanoes

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  1. Seismic Analysisand Tomography on Volcanoes Jonathan M. Lees University of North Carolina, Chapel Hill Presented at Georgia State University, July, 2011 Lees, J. M. (2007), Tomography of Crustal Magma Bodies: Implications For Magmatic Systems, J. Volc. Geoth. Res., 167, 37-56 Lees, J.M., (2002). Three-Dimensional Anatomy of a Geothermal Field. In: A. Glazner, J.D. Walker and J.M. Bartley (Editors), Geologic Evolution of the Central Mojave Desert and Southern Basin and Range. Geological Society of America, Boulder, CO., pp. 259-276.

  2. Lees Study Areas Galapagos

  3. Tomographic inversion along the San Andreas Fault • Parkfield • Loma Prieta • Landers • North Palm Springs • Coso

  4. Tomographic Inversion of Volcanic and Geothermal regions • Delineate Geometry and Structure • Estimate temperature and/or % melt • Crack Density/Orientation • Fluid Flow • Commonly uses P and/or S-wave Data • Occasionally have attenuation (Q) information • More rarely Anisotropy

  5. From Lab Experiments: • P-wave velocity decreases with temperature, especially near melting temperatures • Near melting temperatures the reduction can be as much as 50-60% If magma accumulations contain significant amounts of melt we should see these anomalies in seismic velocity From: Murase & Mcbirney, GSA Bull, 1973, 84, p 3576

  6. Effect of Q (attenuation)

  7. Summary of 3D analysis at Coso • Vp , Vs Velocity Perturbations • Vp/Vs Poisson’s Ratio • Vp*Vs Porosity Variations • Qp(Pulse Broadening)  Attenuation • Anisotropy (P-wave): • Crack Density • Stress Distribution • Permeability Variation • Anisotropy (Shear Wave Splitting) • Stress Distribution • Multiplets: Crack Density

  8. Vp Tomographic analysis of P-wave anomalies in the Coso Geothermal field. Light areas are positive velocity perturbations representing higher velocity zones. Dark areas are relatively lower velocity. Blocks that are not sampled are blanked out. Map features and stations are plotted for geographic reference. Map orientation of cross-sections 1, 2, 3 is presented in lower right figure for reference in subsequent cross section figures.

  9. Vs Tomographic analysis of S-wave anomalies in the Coso Geothermal field.

  10. Vp/Vs Ratio at Coso • r = Vp/Vs is related to Poisson’s ratio • r decreases as temperature increases • r increases as pressure increases • Fluid saturated have high r • At Coso average r≈1.62 (<1.732) • So rocks are hot • (Combs and Rostein 1976: r≈1.57 • Low r between stations S2 and S6 • High ridge of r separates these • Low r near S3-S1-S4 triangle

  11. Vp/Vs Tomographic analysis of r=Vp/Vs ratio anomalies in the Coso Geothermal field. Dark regions represent regions of high r and light are low r values. Perturbations of Poisson’s ratio are approximately proportional to perturbations in r.

  12. Vp×Vs Product at Coso • P=Vp×Vs Porosity Proxy • P porosity in sedimentary rocks is related inversely to VpVs product • See (Picket, 1963; Tatham, 1982; Iverson, 1989) • At Coso, average is negative -> Coso is more porous than country rocks • Greatest P 1.5-3.0 km depth • High P between S2-S6 (low porosity) • Low P surounding S2 (high porosity) • High P S3-S1-S4 (low porosity) • High attenuation, low r

  13. Vp*Vs Tomographic analysis of Vp*Vs product, a proxy for porosity, anomalies in the Coso Geothermal field. Dark areas are high-P; light are low-P. High P is interpreted to indicate low porosity and low-P is high porosity.

  14. Q-attenuation Tomographic analysis of attenuation anomalies in the Coso Geothermal field. Attenuation is the reciprocal of the quality factor Q. Light regions have high Q (small 1/Q, lower attenuation) and dark zones have low Q (large 1/Q, higher attenuation).

  15. P-wave Anisotropy at Coso • Determination of Anisotropy factor, af • Provides a means to estimate: • Deviatoric Stress • Crack Density • Permeability • Orientation of Fractures

  16. 1 Normally we assign a single parameter (or two) to each block in 3D model of Perturbations in the earth. These are Vp, Vs or attenuation Qp and Qs. If there is a significant anisotropy then the inversion for structure will be distorted. We may attempt to take this into account by including anisotropic effects. In that case we must introduce several parameters in each block to account for the complex waveform propagation through each model element. 6 By making several assumptions we can reduce the general anisotropic case to six independent parameters describing the distortion of waves through the model element. We separate the isotropic component from the deviatoric part and extract the anisotropic components.

  17. We developed a new way to model P-wave anisotropy in complex media. The method involves inverting for a matrix of physical parameters that include isotropic and anisotropic wave propagation. The anisotropic parts can be summarized by an anisotropy parameter. Wu, H. and Lees, J.M., (1999). Cartesian Parameterization of Anisotropic traveltime tomography. Geophys. J. Int., 137(1): 64-80.

  18. Anisotropy factor at Coso Tomographic analysis of P-wave anisotropy anomalies in the Coso Geothermal field. Anisotropy factor is defined as the ratio of the eigenvalues of the matrix describing the anisotropy in each block of the model. Dark represents large relative anisotropy contrasted with light regions of lower anisotropy factor.

  19. Vertical Cross sections of P-wave Anisotropy at Coso • Lees, J. M., and H. Wu, (1999), P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California, J. Geophys. Res. 104(8), 17,955-17,973.

  20. P-wave Fast Directions

  21. What is a magma chamber?Tomography of crustal magma bodies Jonathan M. Lees University of North Carolina Chapel Hill Lees, J. M. (2007), Tomography of Crustal Magma Bodies: Implications For Magmatic Systems, J. Volc. Geoth. Res., 167, 37-56

  22. Will we ever be able to see details like this? Can we image individual magma chambers as discrete blobs or sheeted? Conclusions: Not Yet… However, it is not inconceivable that we may in the future. Need more stations and more earthquakes…more money!

  23. -7% +7% Mount St. Helens Data: 1980-1990

  24. Green lines are from Palister et al. 1992, Bull. Volcanol., v. 54, p. 126-146 Resolution Kernel Lees, J. M. (1992): The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography, J. Volc. Geoth. Res., 53(1-4), 103-116.

  25. Mt. St. Helens Lees, J. M. (1992): The magma system of Mount St. Helens: Non-linear high resolution P-wave tomography, J. Volc. Geoth. Res., 53(1-4), 103-116.

  26. Mt. Rainier Mount Rainier Lees, J. M. and R. S. Crosson (1990): Tomographic imaging of local earthquake delay times for 3-D velocity variation in western Washington, J. Geophys. Res., 95(B4), 4763-4776.

  27. Mt Rainier Tomographic inversion Low Velocity anomaly Moran, S. C., J. M. Lees and S. D. Malone (1999), P-wave velocity structure in the greater Mount Rainier area from local earthquake tomography, J. Geophys. Res. 104 (10), 10,775-10,786.

  28. Technical Issues in Tomography • Error in data • Need Damping → Lowers values of anomalies • Resolution: • Depends on: • Wavelength of signals • Ray coverage • Stations are only on one side of target • Need more stations, more sources

  29. Kliuchevskoi Tomography

  30. Kliuchevskoi Tomography

  31. Kliuchevskoi X-secs Kliuchevskoi Tomography

  32. Kronotsky Kliuchevskoi Tomography Kliuchevskoi Volcano High Velocity Sheveluch Low Velocity North

  33. North Pacific Ocean Mt. Fuji Inversion

  34. Mt. Fuji

  35. Joint Inversion and Constraints • In the past we have performed simultaneous inversion by constraining model parameters with additional constraints • Gravity Lees, J. M. and J. C. Vandecar (1991): Seismic tomography constrained by Bouguer gravity anomalies: Applications in Western Washington, Pure Appl. Geophys., 135(1), 31-52. • An obvious next step is to constrain with Geodetic models

  36. Constraints Using Geodetics • Geodetic models have finite extent • Use Geodetic models to constrain gradients of tomographic model • Use tomographic models to constrain geodetic inversions

  37. Joint Geodetic/Seismic Inversion

  38. Algebraic Reconstruction Techniques (ART) • Row action methods • Update the model after each row is used • Add rows as new data is accumulated •  Real time tomography • (akin to Kalman Filtering Techniques)? • Bayesian ART • Lees, J. M. and R. S. Crosson (1991): Bayesian ART versus conjugate gradient methods in tomographic seismic imaging: An application at Mount St. Helens, Washington, in Spatial Statistics and Imaging, edited by A. Possolo, Inst. of Math. Statistics, 186-208.

  39. Summary • Tomography in Hot Zones can be used as a lab for wave propagation • Velocity, Attenuation Inversions • Anisotropy – splitting and 3D inversion • Scattering – delineating structure • (noise studies?) • Need more data - dense networks, borehole seismometers • Need joint inversion with external (independent) constraints

  40. Tomography at LandersLees, 1993

  41. Coso Seismic Analysis Papers Published • Three-Dimensional Imaging • Wu, H. and J. M. Lees (1996): Attenuation Structure of Coso Geothermal Area, California, from P Wave Pulse Widths, Bull. Seismol. Soc. Am, 86, 1574-1590. • Lees, J. M., and H. Wu, (1999), P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California, J. Geophys. Res. 104(8), 17,955-17,973. • Wu, H., and J. M. Lees (1999), Three-dimensional P- and S-wave velocity structures of the Coso Geothermal Area, California, from microseismic traveltime data, J. Geophys. Res. 104, 13,217-13,233. • Lees, J. M., and H. Wu, (1999), P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California, J. Geophys. Res. 104(8), 17,955-17,973. • Hough, H. E., J. M. Lees and F. Monastero (1999)Attenuation and source properties at the Coso Geothermal Area, California, in press. Bull. Seismol. Soc. Am • Multiplets • Lees, J. M. (1998): Multiplet analysis at Coso Geothermal, Bull. Seismol. Soc. Am., 88(5), 1127-1143. • Scattering • Lees, J.M., 2004. Scattering from a fault interface in the Coso geothermal field. Journal of Volcanology and Geothermal Research, 130(1-2): 61-75. • Stress Distribution • Feng, Q., and J. M. Lees (1998): Microseismicity, stress, and fracture within the Coso geothermal Field, California, Tectonophysics, 289, 221-238. • Stress Loading from Large Events • Bhattacharyya, J., S. Grosse , J. M. Lees, and M. Hasting (1999): Recent earthquake sequences at Coso: evidence for conjugate faulting and stress loading near a geothermal field, Bull. Seismol. Soc. Am. 89(3), 785-795. • Bhattacharyya, J., and J.M. Lees, Seismicity, stress and triggering in the Coso/Indian Wells Valley region, in Geologic Evolution of the Central Mojave Desert and Southern Basin and Range, edited by A.G. J., D. Walker, and J.M. Bartley, pp. 243-258, Geological Society of America, 2002. • Review • Lees, J.M., Three-Dimensional Anatomy of a Geothermal Field, in Geologic Evolution of the Central Mojave Desert and Southern Basin and Range, edited by A. Glazner, J.D. Walker, and J.M. Bartley, pp. 259-276, Geological Society of America, Boulder, CO., 2002.

  42. Why Geothermal? • High Seismicity • Complex Structure • Borehole access • Well logs; abandoned wells • Fluids – geochemistry/geology • Social Significance: Clean Energy • Incentive for funding and support •  Mini-Laboratory • Seismic Wave Propagation: scattering; anisotropy; attenuation Japan Geothermal

  43. P-wave fast directions

  44. Coso Seismic Structure • Anisotropy • Largest (>8%) East of station S1-S4 and Large near S6 down to 3 km depth • Fast directions • North-South in Shallow zones • Easterly in deep section • Stress • Lab Experiments at 30 MPa  13%Anisotropy • Coso: 3-4%  6Mpa stress perturbation in anomalous region • (average at Coso ≈ 3MPa) • Elevated deviatoric stress correlates with seismicity and variations of stress observed in focal mechanism studies • Lees, J. M., and H. Wu, (1999), P-wave anisotropy, stress, and crack distribution at Coso Geothermal Field, California, J. Geophys. Res. 104(8), 17,955-17,973.

  45. Seismic Analysis at Coso Geothermal Field • Three-dimensional Imaging • Velocity, Attenuation, Anisotropy, Porosity, Permeability, Crack Distribution • Multiplets • Scattering • Stress Distribution • Stress Loading from Large events • Injections

  46. Geothermal Anisotropy • Krafla (splitting: orientation and time delay) • Hengill (splitting: orientation) • Coso (splitting and P-wave)

  47. Outline • Coso • P-wave/S-wave Velocity • Attenuation (pulse width broadening) • Anisotropy • P-wave: Cartesian Parameterization

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