Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - CSEM (The Collaborative Seismic Earth Model Project)

Teaser

Seismic tomography images of the Earth\'s interior are key to the characterisation of earthquakes, natural resource exploration, seismic risk assessment, tsunami warning, and studies of geodynamic processes. While tomography has drawn a fascinating picture of our planet...

Summary

Seismic tomography images of the Earth\'s interior are key to the characterisation of earthquakes, natural resource exploration, seismic risk assessment, tsunami warning, and studies of geodynamic processes. While tomography has drawn a fascinating picture of our planet, today\'s individual researchers can exploit only a fraction of the rapidly expanding seismic data volume. Applications relying on tomographic images lag behind their potential; fundamental questions remain unanswered: Do mantle plumes exist in the deep Earth? What are the properties of active faults, and how do they affect earthquake ground motion?

To address these questions and to ensure continued progress of seismic tomography in the \'Big Data\' era, we work on new technological developments that enable a paradigm shift in Earth model construction towards a Collaborative Seismic Earth Model (CSEM). Fully accounting for the physics of wave propagation in the complex 3D Earth, the CSEM is envisioned to evolve successively through a systematic group effort of my team, thus going beyond the tomographic models that individual researchers may construct today.

We develop the technological foundation of the CSEM and integrate these developments in studies of large-earthquake rupture processes and the convective pattern of the Earth\'s mantle in relation to surface geology. The CSEM project bridges the gap between regional and global tomography, and deliver the first multiscale model of the Earth where crust and mantle are jointly resolved. The CSEM will lead to a dramatic increase in the exploitable seismic data volume, and set new standards for the construction and reproducibility of tomographic Earth models.

Work performed

So far, our work mainly consists of technological developments, which roughly fall into 3 categories:

[1] We developed the first prototype of the Collaborative Seismic Earth Model (CSEM). This allows researchers for the first time to constrain seismic Earth structure on multiple scales in a collaborative fashion, thus taking advantage of our distributed human and computational power.

[2] We developed a novel evolutionary full-waveform inversion, which can automatically update a 3-D Earth model when new data become available. This is based on a specialised form of stochastic gradient descent where adaptive mini batches are being used. An additional advantage is the ability to use vastly more data than in conventional full-waveform inversion approaches.

[3] We developed a numerical modelling and inversion tool that uses wavefield adapted finite-element meshes. The method exploits the fact that wavefields are comparatively smooth in azimuthal direction, thereby allowing us to stretch elements perpendicular to the propagation direction. Effectively, this allows us to drastically reduce the number of elements, and therefore the required computational resources.

Final results

The major progress beyond the current state of the art in fact consist of the three main technological developments listed above. They enable, for the first time, a collaborative seismic Earth model construction, the exploitation of vastly more data, and the saving of computational resources.

The big goal, at the end of the project, is a fully automated CSEM. It is envisaged to become a full autonomous Earth model that updates itself on all seismically accessible scales.