Instantaneous generation of broadband global-scale waveforms

Abstract:

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.