World of Scientific Discovery on Richard Dixon Oldham
Richard Dixon Oldham is credited with discovering the two main types of body waves -- elastic shock waves that travel through the Earth during earthquakes. The first of these waves is called a P , or Primary wave, because, due to its rate of propagation, it is the first wave to arrive at a seismograph. The second variety is called the S, or Secondary wave, which arrives after the inital P waves and vibrates at right angles to the direction of the travel. Oldham's interest in seismology developed from a distinguished background in geology.
Oldham was born in Ireland in 1858, and his father, who was a well-known geologist and professor at Trinity College in Dublin, encouraged Oldham's interest in geology. Oldham was educated in England, first at Rugby and later at the Royal School of Mines. In 1879 he traveled to India to resume the work of his father, who had died just one year earlier. While working for the Geological Survey of India, his father had started an extensive investigation of the great earthquake in Cachar in 1869. For the next twenty-four years, Oldham continued to work in India and became superintendent of the Geological Survey of India. His primary focus of study remained earthquakes, but he wrote approximately forty publications on other topics, ranging from studies of hot springs to the geology of Son Valley as well as the structure of the Himalayas and the Ganges plain. During this time, his interest in seismology, specifically body waves, heightened.
Starting in 1897, Oldham reported extensively on the great Assam earthquake, one of most violent in modern times. It caused havoc and destruction for over 9,000 square miles (2.3 million ha) and was felt by people over an area of 1.75 million square miles (453 million ha). This event allowed Oldham to gain detailed knowledge of important and unrecorded phenomena. He produced a report in which he compiled the results of his survey of the earthquake's effects. He described the great Chedrang fault which rose up 35 feet (10.7 m) in one area and revealed evidence of ground openings that did not show any noticeable rock displacement.
Most significant, however, was Oldham's study of ground vibrations produced by the large earthquake. This study provided evidence of the occurrence of ground accelerations that exceeded the vertical acceleration of gravity. An even more far-reaching outcome was his identification on seismograms of the three most significant types of seismic waves: the primary, secondary (both body waves), and surface waves. These compressional and distortional waves had been predicted in 1829 by mathematician Siméon-Denis Poisson and in other historical mathematical theory, but his records verified the vibration types.
When Oldham returned to England in 1903, he spent time working with the noted seismologist John Milne (1850-1913), who developed the first accurate modern seismograph. Milne's records of large earthquakes provided Oldham with enough data to study the Earth's interior through P and S waves. In 1906, he proved the existence of the Earth's core by using these waves. Oldham's work earned him the Lyell Medal of the Geological Society of London in 1908. His career was also honored when he was elected to the Royal Society in 1911. Oldham's pioneering work set the stage for other seismologists to pursue studies of the Earth's interior.
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