Partitioned Waveform Inversion, From Tens to Millions of Seismograms: A Journey of Discovery

Abstract:

In a landmark paper a quarter of a century ago, Nolet (1990) applied his newly developed method, Partitioned Waveform Inversion (PWI), to data from a new broadband array in Europe, the Network of Autonomously Recording Seismographs (NARS). The deployment of NARS was followed by deployments of numerous other arrays, resulting in an explosive growth in the amount of broadband data worldwide. The PWI method, in turn, has proven to be extremely effective in utilizing the growing data volumes. Over the last 20+ years, it has yielded a steady stream of discoveries on the Earth’s structure and dynamics.

PWI extracts information on Earth structure from both surface and body waves within full seismic waveforms, putting this information in the form of linear equations with uncorrelated uncertainties. Early on, PWI has been applied successfully at the continental scale for upper-mantle tomography, at the regional scale for Moho-depth mapping, and even at the local scale for imaging shallow marine sediments. Automated multimode inversion developed on the basis of PWI has taken the method to global applications, and today it has been applied to millions of seismograms (all broadband data available from international data centers).

Waveform tomography stemming from the work of Nolet (1990) now reveals the global structure and anisotropy of the upper mantle in great detail. It provides regional resolution (at the scale of tectonic units) in many regions that are well-sampled by the data. Beneath Tibet, for example, complex subduction of the cold, high-velocity Indian lithosphere is imaged in detail in the upper mantle, while azimuthal anisotropy resolved within the crust shows the flow that accommodates the India-Asia lithospheric convergence. At greater depths, in the mantle transition zone, a belt of high-velocity anomalies reveals subducted lithospheric fragments along the entire Tethys convergence zone. Globally, waveform tomography enables us to examine the heterogeneity of the lithosphere and underlying mantle from different perspectives: multimode surface-wave dispersion, shear-velocity tomography, and tectonic regionalization.

Nolet, G., 1990. Partitioned waveform inversion and two-dimensional structure under the Network of Autonomously Recording Seismographs, J.Geophys. Res. 95, 8499–8512.