One-dimensional Reference Models of the Upper Mantle Constrained by Surface Waves

One-dimensional (1D) reference models of the Earth are used by seismologists in order to quantify anomalies in seismic observations and in Earth structure and as a reference in seismic-data inversions. PREM (released in 1981) and AK135 (1995) have served the community well but have well-known imperfections, including the seismic-velocity jump at the 220-km depth in PREM (a complexity not required by the data) and the probably unrealistic upper-mantle density profile in AK135. This has motivated the proliferation of non-standard versions of the reference models. The recent growth in the number of seismic stations around the world has yielded a dramatic increase in the global sampling of the Earth with seismic data. Here, we assemble a very large set of fundamental mode, phase-velocity measurements, performed with multimode waveform inversion using all available broadband data since 1990s. We compute phase-velocity maps at densely spaced periods in a broad, 17-310 s range. We then invert the Love and Rayleigh phase velocity curves averaged across the globe and, alternatively, across type lithospheric regions for best fitting, radially anisotropic 1D models of the upper mantle. We define 4 lithospheric types in the continents: Archean cratons, stable platforms, recently active continents, and active rift zones. In the oceans, t he 4 types are old, intermediate oceans, ridges and backarc regions. In the inversions, S velocities are the parameters constrained directly by surface waves and the other important parameters (P velocity, density, Q, temperature) are determined using computational petrology and thermodynamic databases. For most lithospheric types, the data can be fit to within 0.1- 0.2 percent by simple, smooth 1D models. The global-average dispersion curves, by contrast, cannot be fit with the same accuracy by a single, radially anisotropic 1D model, due to the non-linearity of the S velocity - phase velocity relationship and the strong lateral heterogeneity of the Earth's crust. Older lithospheres in both continents and oceans show greater thicknesses, lower temperatures and higher seismic velocities. Radial anisotropy also displays systematic patterns. We aim for the set of the 1D models to become a useful reference in regional and global upper-mantle studies.