Oldham, Richard Dixon (1858–1936)

Irish seismologist who discovered the earth's core and first distinguished between primary and secondary seismic waves.

During his tenure at the Geological Survey of India (1879–1903), Oldham investigated an earthquake in Assam in June 1897 and recognized that there were two phases of seismic waves recorded by the seismograph, primary waves (P-waves) and secondary waves (S-waves), and that these should propagate through the whole earth.

In 1906, while analysing seismic records, Oldham noticed an area on the globe in which P-waves were not detected. Every time an earthquake occurred, this P-wave ‘shadow zone’ appeared on the opposite side of the globe. Oldham demonstrated that the earth had a core that was causing the primary waves to refract (bend) away, leaving a seismic shadow. Oldham further realized that the material of the core was significantly different from the rest of the earth, since the strength of the secondary waves was greatly reduced, creating another shadow zone. In 1919, after the theory of seismic waves had been developed by Wiechert, Oldham suggested that the core may be liquid.

Oldham was born in Dublin, Ireland, and was educated at the Royal School of Mines.