Data analysis and tomography

Multi-frequency waveform tomography

(T. Nissen-Meyer, K. Hosseini, K. Sigloch, L. Auer, L. Boschi)

We map the deep mantle at high resolution via core-diffracted phases in the framework of waveform-based, multi-frequency tomography. For this purpose, we are computing an entire database of waveforms that covers all possible source-receiver configurations and parameterization choices for a given background model such as PREM using AXISEM and the approach outlined in Section 5. This database shall eventually be made publicly available for the community to extract any desired sensitivity kernels and seismograms instantaneously. We use a large broadband data set of original waveforms based on a semi-automated data collection and processing strategy, upon which we consider multiple time windows, frequency bands, and various misfits for global inversions of lowermost mantle structure. Notably, utilizing diffracted phases in multiple frequency bands drastically improves resolution of the region. Preliminary analyses suggest that we may use different lowermost mantle reference models depending on which region is considered. This can be assessed based on a small number of forward simulations and data fitting. We furthermore plan to address bulk/shear velocity ratios and a possible anti-correlation in the lowermost mantle using our full-wave sensitivity kernels. With the prospect of such high-quality data and imaging techniques, we may explore the long-term potential of core tomography.

GPS waveforms

(K. Kelevitz, N. Houlie, T. Nissen-Meyer)

GPS satellites measure displacements along the Earth's surface with staggering and increasing accuracy. In recent years, the numbers of continuous recordings outgrew that of conventional seismometers, now reaching up to 10,000 worldwide. We embark on an ambitious project to compile a global database of displacement time series for all sufficiently large earthquakes recorded at any GPS station with decent signal-to-noise ratio. Such longer-period data falls into a regime where both conventional seismometers and tomographic models fail. Thus, we attempt to fill this gap by including GPS waveforms into tomography.

Anisotropic tomography

(L. Auer, L. Boschi, T. Becker, M. van Driel, T. Nissen-Meyer)

Seismic anisotropy is a well-described and long known property of elastic media. There is general agreement that the Earth's interior is subject to significant anisotropy. However, it has proved difficult to capture these effects with agreeable tomographic means. This may be due to poor data sampling, insufficient techniques, or non-unique parameterization in terms of the complexity of anisotropy. Even more importantly, anisotropy may be directly linked to geodynamics and bulk properties of the interior by, for instance, shape-induced orientationor lattice-preferred orientation. These imprints on seismic waveforms are thus a direct hint of flow directions. We compiled a diverse dataset of body and surface waves and derived a new anisotropic model of the Earth named SAVANI. Further studies include local parameterization, finite-frequency studies of European anisotropy, tomographic filtering of geodynamic models and the general link to mineral physics and geodynamics.

Transition-zone tomography

(S. Stähler, K. Sigloch, T. Nissen-Meyer)

The transition zone at the bottom of the upper mantle is intriguing on multiple levels. In order to understand mantle dynamics, we need to constrain its detailed structure especially in terms of the fate of subducting slabs, some of which seem to buckle and stagnate above the 660km discontinuity. It therefore serves as a prime region to decipher in terms of heat transfer and the global energy budget of the Earth. Topographic undulations of its discontinuities are another imprint of this dynamic behavior. Triplicated body waves sample this region exceptionally well, but have previously been neglected in tomographic studies due to their complex wave character. With the advent of massive array deployments in conjuction with full-wave modeling techniques, we show that that such waves can be utulized and represent a valuable complementary asset to imaging this crucial region of the Earth's deeper interior.