Seismology in the U.S. 1979–82

The years 1979 to 1982 have been marked by a continuing expansion and development of seismology in the United States. In this introduction, we summarize some of the highlights of this period, as a prelude to the more detailed papers that follow. These papers cover both improvements in our understanding of earth processes together with improvements in the various geophysical techniques available to study them. Perhaps the most traditional area of seismology is the study of the structure of the earth's interior, though recent developments are following most untraditional directions. Highlights of this area include significant new information on lateral heterogeneity, anisotropy and the mechanisms of scattering and attenuation. Using a variety of techniques, we are beginning to identify the regions of the earth that are the most heterogeneous, and to characterize the heterogeneity in those regions. The upper mantle has long been known to contain lateral variations, particularly in the uppermost part. New data are beginning to suggest substantial heterogeneity in the transition region between 400 and 670 km depth. In addition, more evidence is confirming lateral variations in the lower 200 km of the mantle. Attempts to relate these various heterogeneities to tectonic processes at the earth's surface and motions in the earth's core are continuing.