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DEPARTMENT OF GEOLOGY AND GEOPHYSICS GEOPHYSICAL AND POLAR RESEARCH CENTER UNIVERSITY OF WISCONSIN, MADISON, WISCONSIN
Abstract
Observations of seismic waves generated by explosions in Lake Superior during the Early Rise Experiment have been made westward to a range of 2600 km. A phase filtering technique involving the vertical and horizontal motion, used to enhance the body phases, is applied to both the compressional and shear wave portions of the seismograms, presented in record section form.
Our interpretation of these data implies that the P velocity increases in a series of discontinuities to a depth of 500 km (the extent of our data); the most important of these being increases from 8.2 km/sec to 8.47 km/sec at 126 km, and from 8.9 km/sec to 9.5 km/sec at 450 km. Q for P waves appears to decrease below 126 km.
Although the shear-wave data is not as convincing as the P-wave data, it is probable that the increase in P velocity at 126 km is accompanied by a decrease in shear velocity from about 4.7 km/sec to 3.9 km/sec, with a corresponding increase of Poisson's ratio, bulk modulus and density, and decrease in rigidity. The shear velocity then increases to 5.0 km/sec at 220 km. The lowest value of the shear velocity is necessitated by the relatively small ranges at which the turning point of the cusp associated with this low-velocity channel seems to occur.
A comparison of the Rayleigh wave phase velocities computed for this model (using Birch's relationship between density and P velocity) with the Canadian Shield data of Brune and Dorman and the midwest United States data of McEvilly is favorable.
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