Instantaneous generation of broadband global-scale waveforms

Friday, 19 December 2014: 10:35 AM
Tarje Nissen-Meyer, University of Oxford, Department of Earth Sciences, Oxford, United Kingdom, Martin van Driel, ETH Zurich, Department of Earth Sciences, Zurich, Switzerland, Simon C. Stähler, Ludwig Maximilian University of Munich, Munich, Germany, Alexander R Hutko, IRIS Data Management System, Seattle, WA, United States and Ludwig Auer, ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland
Spherical models for planetary spheres represent a common characterization of bulk global material properties, often satisfying up to 90% of recorded data. Our new methodology combines accurate seismic wave propagation with symmetry properties of radiation patterns, reciprocity, and high-order interpolation to deliver a comprehensive waveform database from which arbitrary source-receiver configurations and high-frequency record sections for a given model can be extracted within seconds. The database thus acts as a once-and-for-all solution to wave propagation in spherically symmetric models. This not only releases users from re-running wave propagation codes, but opens doors to new applications in which vast numbers of parameter alterations are desired such as modifications in source properties (moment tensor, source-time function, location), filtering, or background models, e.g. in a framework for probabilistic uncertainty assessment.

Using reciprocity, only 2 simulations with global wave-propagation solver AxiSEM (Nissen-Meyer et al. 2014, www.axisem.info) suffice to generate a complete database of Green's functions: one as a "source" for the vertical, and one for both horizontal components. Storage of the propagating spatio-temporal displacement field at all distances (0-180 degrees) and depths (0-700km for earthquakes) on the actual basis of the spectral-element mesh ensures the same accuracy as for the propagation system upon posteriori interpolation. The ease of computation (10.000 CPU hours) and tolerable storage requirements (~1TB for 1Hz waveforms) implies that multiple such databases may be computed for many models at high resolution (1Hz for global-Earth synthetics), e.g. continental versus oceanic crust, anisotropic versus isotropic, or a various lower-mantle models. One may also use our methodology in parameter-space studies for other planetary objects such as Mars, Mercury, Sun. Further applications include the efficient generation of reference synthetics for global tomography, wavefields for hybrid 1D-3D methods, and responses to finite-fault sources. A first example of such a database is being developed and stored at the IRIS DMC (Seattle), to deliver on-demand customizable synthetics.