Properties of the core‐mantle boundary and observations of PcP
Buchbinder, Goetz G. R.
Журнал:
Journal of Geophysical Research
Дата:
1968
Аннотация:
Study of PcP phases from eight explosions and three earthquakes shows that the initial direction of motion of PcP reverses at an epicentral distance of about 32° and that PcP amplitudes pass through a minimum at this distance. The effects are caused by properties of the core‐mantle boundary. Calculations of reflection coefficients at a plane solid‐liquid boundary show that a model with P and S velocities at the bottom of the mantle of 13.64 km/sec and 7.30 km/sec, respectively; with a P velocity at the top of the core of 7.5 km/sec; and with a ratio of core density to mantle density of 1.0 will satisfy the observations of amplitude and change of initial phase of PcP. A range of similar models with velocities at the top of the core down to 7.2 km/sec and density ratios as high as 1.05 will also satisfy the observations. Amplitude observations of PKKP phases also satisfy the model. The new structure is in sharp contrast to the structure of conventional models that have a core velocity of about 8.1 km/sec and a density ratio of about 1.7, but such models will not satisfy the new data. One way to reconcile the new structure near the core‐mantle boundary with other relevant information is to assume a model in which the lowermost mantle is inhomogeneous. The inhomogeneity is caused by an increase in iron or other heavy metal content with depth, which increases the mean atomic weight and the density with little change in P velocity. The drop in P velocity across the core‐mantle boundary may then be explained by a step increase in mean atomic weight with little change in density. The Adams‐Williamson equation will not be valid in the regions in which the changes in density and mean atomic weight are postulated. The incompressibility is not continuous across the core‐mantle boundary. PcP travel‐time observations do not permit resolution of irregularities at the core‐mantle boundary of less than about 5° in slope and about ±5 km in height. The scatter of the observed travel times from a least‐squares fit does not indicate slopes of over 6° or elevation differences much over 5 km on the core‐mantle boundary.
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