This research involves the development of open-source software for simulations of seismic wave propagation inside the Earth. Such simulations may be used for quantitative seismic hazard assessment, visualizations for outreach and educational activities, as well as a basis for seismic tomography. Like medical tomography, the goal of seismic tomography is to image the Earth's interior on all scales, from hydrocarbon fields in exploration seismology to Earth's mantle in global seismology, to help us understand our planet's inner workings and evolution. As a broader impact this project will publish earthquake wave animations online in near real-time for events with magnitudes of 5.5 and greater at Princeton University's Near Real Time Seismicity Portal (GlobalShakeMovie) .
The main goal of this research is to further develop global seismic tomography using complete seismic waveforms and adjoint-state methods by enabling massive data assimilation and harnessing the largest and fastest available computers. Using a pilot dataset of 253 earthquakes, a first-generation global tomographic model based on "adjoint tomography", an iterative full-waveform inversion technique was built. Based on this experience, a next-generation global model will be constructed by assimilating full waveform data from thousands of earthquakes that were well recorded by broadband global and regional seismographic networks. Prior NSF support demonstrated the feasibility of regional adjoint tomography for isotropic heterogeneity, anelasticity, and azimuthal anisotropy. Similar inversions on a global scale will be done through secured access to the necessary computational hardware through the Department of Energy INCITE program. This research supports a portion of the required human resources, where the rate-limiting steps in this ambitious endeavor are data quality control, I/O, and workflow management. Four main tasks will be tackled bearing these obstacles in mind: 1) prepare and further develop the open source spectral-element seismic wave propagation solvers SPECFEM3D and SPECFEM3D_GLOBE for exascale simulations; 2) further develop the Adaptable Seismic Data Format (ASDF) for fast I/O; 3) further develop the inversion & migration toolkit SeisFlows by taking advantage of the open source Structured Query Language database engine SQLight and existing workflow management tools, such as Pegasus; and 4) perform global adjoint seismic tomography with a database of thousands of earthquakes at a shortest period of 9 s. Task 4 encompasses the main goal of the proposal, but it cannot be completed without simultaneously addressing the other tasks.