Geophys. J. Int.-2012-Schuberth-1393-412

Geophys. J. Int.-2012-Schuberth-1393-412 - Geophysical...

Info icon This preview shows pages 1–2. Sign up to view the full content.

Geophys. J. Int. (2012) 188, 1393–1412 doi: 10.1111/j.1365-246X.2011.05333.x GJI Seismology Synthetic seismograms for a synthetic Earth: long-period P - and S -wave traveltime variations can be explained by temperature alone Bernhard S. A. Schuberth, Christophe Zaroli and Guust Nolet Universit´e de Nice Sophia-Antipolis, Centre National de la Recherche Scientifique (UMR 6526 ), Observatoire de la Cˆote d’Azur, G´eoazur, Les Lucioles 1, Sophia Antipolis, 250 Rue Albert Einstein, 06560 Valbonne, France. E-mail: [email protected] Accepted 2011 December 10. Received 2011 November 18; in original form 2011 September 6 S U M M A R Y Current interpretations of seismic observations typically argue for significant chemical het- erogeneity being present in the two large low shear velocity provinces under Africa and the Pacific. Recently, however, it has been suggested that large lateral temperature variations in the lowermost mantle resulting from a strong thermal gradient across D may provide an al- ternative explanation. In case of a high heat flux from the core into the mantle, the magnitude of shear wave velocity variations in tomographic models can be reconciled with isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong core heating has been tested in a consistent manner only against tomographic S -wave velocity models, but not against P -wave velocity models. Here, we explore a new approach to assess geodynamic models and test the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P and S waves. Using a spectral element method, we simulate 3-D global wave propagation for periods down to 10 s in synthetic 3-D elastic structures derived from a geodynamic model. Seismic heterogeneity is predicted by converting the temperature field of a high-resolution mantle circulation model (MCM) into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow against seismic data. The statistics of long-period body wave traveltime observations show a markedly different behaviour for P and S waves: the standard deviation of P -wave delay times stays almost constant with turning depth, whereas that of the S -wave delay times increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations computed for the isochemical MCM reproduce these different trends. This is not expected from a ray-theoretical point of view and highlights the importance of finite-frequency effects. Most importantly, the large lateral temperature variations in the lower mantle related to strong core heating are able to explain most of the standard deviation of observed P - and S -wave delay times. This is a strong indication
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern