The transition from the Earth's solid inner core to liquid outer core is the location where the inner core grows and from which compositional convection in the outer core originates. Most seismological models of the Earth describe the inner-core boundary as sharp and simple, although experimental data requiring the presence of a thin transition layer at the bottom of the outer core have been reported. The density jump at the inner-core boundary--an important parameter determining gravitational energy release and constraining the compositional difference between the inner and outer core-is also not well known. Estimates of this density jump obtained using free-oscillation eigenfrequencies give low values of 0.25-1.0 g cm(-3), whereas a method using the amplitude ratio of core-reflected phases yielded values of 0.6-1.8 g cm(-3) (refs 14, 15, 16-17). Here we analyse properties of waves precritically reflected from the Earth's inner core (PKiKP phases) that show significant variability in amplitude, consistent high-frequency content and stable travel times with respect to a standard Earth model. We infer that the data are best explained by a mosaic structure of the inner core's surface. Such a mosaic may be composed of patches in which the transition from solid inner to liquid outer core includes a thin partially liquid layer interspersed with patches containing a sharp transition.

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DISCLAIMERThis document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of CaIifosnia nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefthss of any information, apparatus, product, or process disclosed, or represents that its use woufd not infringe privately owned rights. Reference b e i n to any specific commercial product, process, or service by trade m e , trademark, manufacturer, or otherwise, does not necesSariiy constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California The views and opinions of authots expressed herein do not necessarily state or rdect those of the United Shts ABSTRACTThe report is carried out by the Institute for Dynamics of the Geospheres, Russian Academy of Sciences under contract NB280344 with Lawrence Livermore National Laboratory, University of California. The work includes investigation of seismic waves generation and propagation from Soviet peaceful underground nuclear explosions in salt based on the data from temporary and permanent seismic stations. The explosions were conducted at the sites Azgir and Vega within the territory of the Caspian depression of the Russian platform. The data used were obtained in the following conditions of conduction : epicentral distance range from 0 to 60 degrees, yields from 1 to 65 kt and depths of burial from 160 to 1500 m. One of the goals of the work was to create a data bank of peaceful nuclear explosions including original data on explosions' conduction and materials of seismic measurements from the explosions. Original data include coordinates, origin times, yields, and depths of burial as well as geological conditions. Seismic data include digitized waveforms recorded in local zone in the range from 0 to 260 km and Bulletines of Soviet seismic network in the range from 4 t o 60 degrees. The seismic data from five individual explosions A -I , A-2, A-3, A-4, and A-5 fired at the site Azgir and from four similar explosions 7T, 6T, 5T, and 3T fired at the site Vega with time delay of about 5 minutes were' generally studied. All the 2 the site Azgir (official title: Site Galite) conducted in boreholes A-1,2,3,4,5 with yields ranging from 1 to 65 kt and depths ranging from 160 to 1500 m are used. There were also 12 explosions conducted at this scientific test site: 6 explosions in the water-filled cavity of the A-2 borehole, one explosion in the air-filled cavity of the A-3 borehole, and four group explosions in the A-7,8,40,11 boreholes in which a set of two or three (A-11) charges was placed. At the Vega site situated within Astrakhan gas condensate field, five series with total of 15 explosions were conducted in the period from 1980 to 1984 (one series per year). The series Vega-1 contained one explosion in the borehole IT (1980). The series Vega-2 included two explosions (...

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