Temporal Changes in the Earth's Outer Core From Nuclear‐Test PKP Waves

Abstract

We investigate temporal changes in P‐wave velocity in the outer core using short‐period PKP waves generated by underground nuclear tests conducted in French Polynesia and recorded at seismic station BRVK in Kazakhstan. The observed first arriving PKP phase cannot be explained by ray theory. Finite‐frequency sensitivity analysis of this phase identifies it as an internal reflection from the outer core. The phase shift associated with the internal caustics, together with its dispersive nature, makes its waveform characteristics distinctly different from PKIKP or PKiKP. We refer to this phase as PKrKP. We measure PKrKP‐PP and PKiKP‐PP double differential travel times, which quantify changes in the relative arrival times between PKrKP (PKiKP) and PP waves, and solve for the evolution of the P‐wave travel time in the outer core from 1977 to 1995 based on least‐square inversions. The PKP waves in the outer core are found to be about 0.1–0.15 s faster during 1982–1983 and 1988–1990, and approximately 0.15–0.2 s slower in 1995.