Abstract
Recent crustal and upper mantle seismic experiments such as the 1963 Lake Superior experiment and the Project Early Rise experiment showed that the Mohorovicic discontinuity may in general not be a plane layer boundary but rather a very irregular surface. In this report a simple model is presented to illustrate the effect of large-scale undulations along this discontinuity on P-wave amplitude and travel-time curves. The results show that the discontinuity in acting as a lens may (1) focus the seismic rays, in which case small regions of relatively large amplitudes are produced, (2) diverge the rays to produce large areas of weaker amplitudes. For a continental crust, the magnitude of this effect is significant when the angle of incidence of P waves at the base of the crust are relatively large (greater than 40°) and when the radius of curvature of the M discontinuity is less than 100 km. Under certain conditions seismic rays may cross below the surface of the earth to produce triplications of the travel-time graph, in which case multiple arrivals with different apparent velocities, separated from each other by fractions of a second, are to be expected. A comparison of the theory with actual amplitudes observed along the Superior-Churchill line of the Project Early Rise experiment is made.
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