Amplitude studies of core phases

Numerical modeling of long-period core phases from five earthquakes and comparison with records from long-period instruments in the Worldwide Network of Standardized Seismographs suggest that the P velocity-depth function in the earth's core is smooth down to the inner core boundary. There are apparently no first-order discontinuities in the transition region from the outer to the inner core, as proposed earlier by other authors. The P velocity increase at the inner core boundary is 0.6–0.7 km/sec and thus considerably less than that in standard core models. In the outermost part of the inner core, there is a pronounced P velocity gradient. The S velocity below the inner core boundary is 3–4 km/sec, and thus this study gives another piece of purely seismological evidence for solidity of the inner core. Precursors to the main core phases in short-period records, which are frequently observed, are not explained by the proposed core model. Scattering hypotheses for the origin of these arrivals are compatible with the results of this study. This appendix also includes record sections of 4 of the 5 earthquakes, tables of the earth models investigated, and short-period theoretical seismograms for several core models proposed in the literature. Order from the American Geophysical Union, 1707 L St., N.W., Washington, D.C. 20036. Document J73-001; $1.00. Payment must accompany order.