Caveats on tomographic images

TY - ABST

T1 - Caveats on tomographic images

AU - Foulger, Gillian R.

AU - Panza, Giuliano F.

AU - Artemieva, Irina

AU - Bastow, Ian D.

AU - Cammarano, Fabio

AU - Evans, John

AU - Hamilton, Warren D.

AU - Julian, Bruce R.

AU - Lustrino, Michele

AU - Thybo, Hans

AU - Yanovskaya, Tatiana B.

PY - 2014/4

Y1 - 2014/4

N2 - Gillian Foulger (1), Giuliano Panza (2,3), Irina Artemieva (4), Ian Bastow (5), Fabio Cammarano (4), John Evans(6), Warren Hamilton (7), Bruce Julian (1), Michele Lustrino (8,9), Hans Thybo (4), and Tatiana Yanovskaya (10)(1) Durham University, Earth Sciences, Durham, United Kingdom ([email protected], +44 191 334 2301), (2) Dept.Mathematics and Geosciences, University of Trieste, Italy and the Abdus Salam ICTP - SAND Group, Trieste, Italy, (3) Inst.Geophysics, China Earthquake Administration, Beijing, China, (4) Dept. Geography and Geology, University of Copenhagen,Denmark, (5) Dept. Earth Science and Engineering, Imperial College, London, SW7 2AZ, UK, (6) U.S. Geological Survey,Menlo Park, CA 94025, U.S.A., (7) Dept. of Geophysics, Colorado School of Mines, Golden CO 80401, U.S.A., (8)Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185, Rome, Italy, (9)CNR – Istituto di Geologia Ambientale e Geoingegneria (IGAG) c/o Dipartimento di Scienze della Terra, Università degliStudi di Roma La Sapienza, Rome, Italy, (10) Dept. Physics of the Earth, Sankt-Petersburg State University, Sankt-Petersburg,RussiaGeological models of the mantle and its geodynamic evolution rely essentially on joint interpretations of publishedseismic tomography images and petrological/geochemical data. This approach tends to neglect the fundamentallimitations of, and uncertainties in, seismic tomography and geochemistry. Counter-intuitively, teleseismic tomographycannot image the three-dimensional structure of the mantle. Tomography cannot determine with certainty thestrengths of calculated anomalies, since the results depend on subjective choices of inversion parameters. However,anomaly strengths are still commonly translated directly into critical geological properties such as temperature,density, and convective motion with little justification. Tomography does not return thermal or geological information,but seismological parameters, and assumptions are required to translate these seismic results into otherphysical properties. Resolution- and error-assessment methods cannot encapsulate the true errors, and are insensitiveto critical experimental limitations that invalidate parts of most derived structures. Much of Earth’s mantle isunsampled by crossing seismic waves. One must know what regions are well sampled in order to understand whichparts of published images are reliable and which are not. Other tomographic limitations and uncertainties involvetheory, correcting for the crust, and choice of what background model to subtract to reveal anomalies. Methodsdo exist for retrieving absolute wave speeds, and not just differences from a starting model, and the former shouldbe preferred. Finally, the selection of cross-sections for publication is subjective, and can give a misleading impressionof the three-dimensional structure retrieved. It is common to overestimate the power of geochemistry toidentify magma sources. Geochemistry is limited by our poor knowledge of the thermal structure of the mantleand the size and the distribution of geochemical and mineralogical heterogeneities, and a range of conclusions isusually permitted by the data. We will present a relatively non-technical summary of the most important of theseproblems in a form accessible to non-seismologists. Appreciation of these issues is essential if final geodynamicalmodels are to be robust, and required by the scientific observations in contrast to merely being selected candidateinterpretations.

AB - Gillian Foulger (1), Giuliano Panza (2,3), Irina Artemieva (4), Ian Bastow (5), Fabio Cammarano (4), John Evans(6), Warren Hamilton (7), Bruce Julian (1), Michele Lustrino (8,9), Hans Thybo (4), and Tatiana Yanovskaya (10)(1) Durham University, Earth Sciences, Durham, United Kingdom ([email protected], +44 191 334 2301), (2) Dept.Mathematics and Geosciences, University of Trieste, Italy and the Abdus Salam ICTP - SAND Group, Trieste, Italy, (3) Inst.Geophysics, China Earthquake Administration, Beijing, China, (4) Dept. Geography and Geology, University of Copenhagen,Denmark, (5) Dept. Earth Science and Engineering, Imperial College, London, SW7 2AZ, UK, (6) U.S. Geological Survey,Menlo Park, CA 94025, U.S.A., (7) Dept. of Geophysics, Colorado School of Mines, Golden CO 80401, U.S.A., (8)Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185, Rome, Italy, (9)CNR – Istituto di Geologia Ambientale e Geoingegneria (IGAG) c/o Dipartimento di Scienze della Terra, Università degliStudi di Roma La Sapienza, Rome, Italy, (10) Dept. Physics of the Earth, Sankt-Petersburg State University, Sankt-Petersburg,RussiaGeological models of the mantle and its geodynamic evolution rely essentially on joint interpretations of publishedseismic tomography images and petrological/geochemical data. This approach tends to neglect the fundamentallimitations of, and uncertainties in, seismic tomography and geochemistry. Counter-intuitively, teleseismic tomographycannot image the three-dimensional structure of the mantle. Tomography cannot determine with certainty thestrengths of calculated anomalies, since the results depend on subjective choices of inversion parameters. However,anomaly strengths are still commonly translated directly into critical geological properties such as temperature,density, and convective motion with little justification. Tomography does not return thermal or geological information,but seismological parameters, and assumptions are required to translate these seismic results into otherphysical properties. Resolution- and error-assessment methods cannot encapsulate the true errors, and are insensitiveto critical experimental limitations that invalidate parts of most derived structures. Much of Earth’s mantle isunsampled by crossing seismic waves. One must know what regions are well sampled in order to understand whichparts of published images are reliable and which are not. Other tomographic limitations and uncertainties involvetheory, correcting for the crust, and choice of what background model to subtract to reveal anomalies. Methodsdo exist for retrieving absolute wave speeds, and not just differences from a starting model, and the former shouldbe preferred. Finally, the selection of cross-sections for publication is subjective, and can give a misleading impressionof the three-dimensional structure retrieved. It is common to overestimate the power of geochemistry toidentify magma sources. Geochemistry is limited by our poor knowledge of the thermal structure of the mantleand the size and the distribution of geochemical and mineralogical heterogeneities, and a range of conclusions isusually permitted by the data. We will present a relatively non-technical summary of the most important of theseproblems in a form accessible to non-seismologists. Appreciation of these issues is essential if final geodynamicalmodels are to be robust, and required by the scientific observations in contrast to merely being selected candidateinterpretations.

M3 - Conference abstract in journal

SN - 1607-7962

VL - 16

JO - Geophysical Research Abstracts

JF - Geophysical Research Abstracts

M1 - 9197

T2 - EGU General Assembly 2014

Y2 - 26 April 2014 through 2 May 2014

ER -