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APPENDIX A RESEARCH DIRECTIONS What kinds of global seismological studies are foreseen over the next decade? What could be the effect of a readily available, global, high-quality digital-data base? What research needs may still not be met by an integrated global system? This appendix attempts to answer these questions and to develop a rationale, based on research potential, for an optimum global data-acquisition and distribution system. Data from the integrated global network should be of sufficiently high quality to permit refined resolution of both global and regional earth structures. Well-calibrated instruments would enable the use of amplitudes of long- period waves and oscillations and of short-period pulses. The amplitude data are valuable for the study of both source mechanisms and structures and offer the possibility of improving our knowledge of seismic-energy dissipation. Further improvement in the identification of normal modes will be made possible by the expanding network of broad- band stations. The short-period data could be important in reducing the reading error in travel-time studies and in the identification of depth phases. High-quality data of wide dynamic range could be essential in the identifi- cation of multiple events accompanying large earthquakes and could lead to a better understanding of the propaga- tion of rupture along a fault. Data from the global network would be used primarily in studies of earthquakes from far-field displacements and in investigations of earth structure. In general, these two objectives cannot be separated, since our ability to infer properties of a source from seismic recordings is limited by our knowledge of the mechanical properties of the medium (earth structure). In recent years, significant progress in the theoretical and numerical aspect of 65
64 S/N - Signal-to-noise ratio SRO - Seismic Research Observatories T - Period (time domain) USGS - U.S. Geological Survey WDC - World Data Center WWSSN - Worldwide Standardized Seismograph Network (or Worldwide Network of Standardized Seismograph Stations)