Three-Dimensional Velocity Structure of the Earth’s Upper Mantle

Abstract

Global seismic tomography now has a history of more than 20 years, and it is possible to identify several trends in research and the continuing evolution towards more accurate and refined models of the Earth’s interior: (1) The incorporation of a variety of seismological observations. In the earliest global tomographic studies, only P wave arrival times from the ISC catalog were used. Many recent tomographic models are based on a variety of data, including complete long-period seismograms, observations of mode frequencies and splitting, extensive collections of surface wave dispersion measurements, and large sets of primary and secondary phase arrival times. (2) Parameterizations of Earth structure which allow for higher resolution images. While early studies used simple regionalizations of the Earth with a small number of regions, or low-order expansions in terms of spherical harmonics, recent trends are towards smaller grid sizes and higher order expansions. (3) Larger data sets. Increasingly, very large collections of seismograms, and large numbers of observations obtained from seismograms, are incorporated in tomographic studies. The dramatically improved geographical coverage of modern digital seismographs offered by various global networks (e.g. IRIS GSN, GEOSCOPE) and the availability of more powerful computers have made this development possible. (4) Greater complexity in the description of Earth structure. In addition to the imaging of lateral variations in P and S velocity, recent efforts have been made to map anelasticity, density, anisotropy, and the undulations of internal boundaries.