Seismic evidence for a new layered structure of the Earth's core

Abstract

Observed travel times and amplitude-distance curves based on new data from the World Wide Standardized Seismograph Network (WWSSN) are presented for a number of earthquakes. These data are used to deduce a detailed structure of the earth's core and lower mantle. To obtain PKP2 travel times near 180° and beyond, lower velocity is needed at the base of the mantle. This reduction in velocity also causes the direct P to propagate up to 130° and beyond. Thus the short-period, small-amplitude P waves observed in the shadow zone as far as 136° are direct P waves. Observed PKP2 travel-time curves consist of two branches, with the second branch 3–6 sec later than the first. The second branch is interpreted as the result of a decrease in velocity at 4015 km. Two other branches similar to those of Bolt and of Adams and Randall are a result of two discontinuous increases in velocity at 4500 km and 4685 km, respectively. Delays in PKIKP travel times observed at 153° and at 162° are interpreted as the result of two low-velocity layers within the inner core. Based on amplitude variations, the beginning of the PKIKP branch is put at around 125°. Waves observed along the DF branch at distances shorter than 125°, down to 106°, are interpreted as ordinary reflections from the inner core boundary. The exact location and characteristics of the caustic at 142° are discussed. New velocity values suggest that between 4015 and 5140 km there is a layered transition zone where the general level of velocity is lower than the values obtained by extrapolation of the velocities in the outer part of the outer core.