1858-1936

Irish geologist and seismologist who established the existence of Earth's core. Based on his landmark survey of the 1897 Assam earthquake, Oldham identified three types of seismic waves—primary (P-waves), secondary (S-waves), and surface waves—predicted earlier by Siméon Poisson. Oldham noted that during an earthquake, seismographs located on the opposite side of Earth detect P-waves later than expected, leading him to conclude that Earth has a core less dense and rigid than its mantle.