Surface Wave Tomography

Surface waves are now being used by several groups to map lateral heterogeneity [Nakanishi and Anderson, 1982, 1983, 1984a, b; Woodhouse and Dziewonski, 1984] and anisotropy [Tanimoto and Anderson, 1984a, b; Nataf et al. 1984] of the upper mantle on a global basis. The method involves measuring the phase and/or group velocity over hundreds of small arcs and long arcs connecting earthquakes and seismic stations. These averages are then converted to three-dimensional images of the seismic velocity structure, and, hence, this is a form of tomography. The large amount of data processing required is made feasible by the existance of long-period digital seismic networks including IDA (International Deployment of Accelerometers), SRO (Seismic Research Observatories), and GDSN (Global Digital Seismic Network). These instruments are operated by a variety of university and government groups including University of California, San Diego, U.S. Geological Survey, D.A.R.P.A., and U.S. Department of Energy with the cooperation of many countries. The global coverage is still very sparse compared to the analog W.W.S.S.N. (World Wide Standardized Seismic Network), but preliminary results are very encouraging. The possibility of an expanded global digital network of broadband seismic stations is now being pursued actively by the United States and several other countries. Because of the sparseness of the present network, mantle structure can only be mapped with fairly low resolving power. Only features with half wavelength of the order of 2,000 km can be detected.