Oldham made two enormous contributions to plate tectonic theory and geophysics in our understanding of what lies at the interior of the earth. In his report on the 1897 Assam earthquake he was able to identify three distinct seismic waves emanating from an earthquake epicenter on a seismic record. We know his discovery as P-waves, S-Waves and Surface Waves. He also discovered that these three waves all travel at different speeds and paths as they move across or through the earth, which led to his second major discovery.
The second of Oldham’s great findings was the discovery of Earth’s core. He studied the propagation of seismic wave paths from Earthquakes originating all around the world and found that certain waves were refracted and changed speeds as they pass through the Earth’s interior while others were not. Oldham found that earthquake waves increased in speed as they moved deeper into the Earth, up to a certain point. At that point, below a certain depth, they suddenly move much slower. This indicated to him that the wave must be moving through a distinctly different material. By analyzing this evidence Oldham was not only able to show that the Earth has a core, but was also able to estimate its size using measurements from thousands of globally distributed earthquakes which showed the depth at which the waves changed speeds.