Structure Beneath Continents: A Critical Review of Explosion Studies 1960-1965

Some of the significant results, techniques, and ideas that have emerged from explosion studies of the continental crust between 1960 and 1965 are brought together and evaluated. The results of explosion refraction studies in Russia, Japan, Europe, and the United States are outlined and discussed. Later sections deal with velocity-depth relations, information derived from observations of deep crustal near-vertical reflections, and the accuracy and precision of explosion seismology. A consideration of the results available leads to the following conclusions: (1) Uppermost mantle velocities vary from area to area by as much as 10%. Observed crustal velocities and weighted mean velocities vary significantly laterally. A consequence of these observations is the fact that no simple, direct relationship exists between gravity anomalies, elevation, and crustal thickness. (2) Although the evidence is inconclusive as yet, there is some indication that small velocity reversals may occur in the lower parts of the crust. (3) Near-vertical reflections from discontinuities within and at the base of the crust, definitely observed in some areas, indicate crustal structures of greater complexity than that suggested by refraction results. (4) Despite the large quantity of work done, there still is no agreement among investigators about the nature of intracrustal discontinuities, or even whether a crustal model made up of layers has any relevance to the actual velocity-depth function. Some evidence has been produced for continuous velocity variation with depth and for abrupt velocity changes with depth. These observations are not necessarily contradictory. The current progress and interest in seismology suggest that effort in the near future should be directed toward comprehensive three-dimensional studies of velocity distributions in the crust and upper mantle and toward investigations designed to reveal the nature of discontinuities. Studies of the S-wave velocity structure, Q values, and Poisson's ratio within the crust are also needed.