Oldham, Richard Dixon
OLDHAM, RICHARD DIXON
(b. Dublin, Ireland, 31 July 1858; d. Llandrindod Wells, Wales, 15 July 1936)
Oldham was the third son of Thomas Oldham, a distinguished geologist who was professor of geology at Trinity College, Dublin, and then a director of the geological surveys of Ireland and India. He was educated in England, first at Rugby and then at the Royal School of Mines. He followed in his father’s footsteps by joining the staff of the Geological Survey of India in 1879. He devoted much energy to completing the unfinished work of his father, who died in 1878, notably an extensive investigation of a great earthquake in Cachar in 1869.
Oldham became superintendent of the Geological Survey of India and wrote some forty of its publications, chiefly on earthquakes in India, the hot springs of India, the geology of the Son Valley, and the structure of the Himalayas and the Ganges plain, taking account of geodetic observations. He developed a great interest in the then emerging science of seismology and is now noted more for his contributions to seismology than to geology. He left India in 1903, partly because of ill health, and returned to England, spending some time working with the seismologist John Milne on the Isle of Wight. Later, for health reasons, he lived in the Rhone Valley and then in Wales; but he remained an active contributor to science until about eight years before his death. He was awarded the Lyell Medal of the Geological Society of London in 1908 and was elected to the Royal Society in 1911.
Oldham became famous for his report on the great Assam earthquake of 12 June 1897, one of the most violent of modern times, which caused complete devastation over 9,000 square miles and was felt over 1.75 million square miles. It far surpassed in quality all reports on previous earthquakes, describing the remarkable Chedrang fault, with a thirty-five-foot uplift at one point; gave evidence of the occurrence of fractures without apparent rock displacement; showed that in some places accelerations of the ground motion had exceeded the vertical acceleration of gravity; and reported the results of the first resurvey ever carried out after a large earthquake. From the point of view of seismology, the most far-reaching result was the first clear identification on seismograms of the onsets of the primary (P), secondary (S), and tertiary (surface) waves, previously predicted in longstanding mathematical theory. This identification showed that the earth could be treated as perfectly elastic to good approximation in studying seismic waves, a result of supreme importance to the further development of seismology.
Oldham also supplied the first clear evidence that the earth has a central core (1906). Others had suspected its existence but had not succeeded in obtaining direct evidence. In the course of analyzing some of Milne’s records of large earthquakes, Oldham invariably found delays in the arrival of P waves at points on the earth diametrically opposite to earthquake sources; and he showed that the delays could be interpreted only in terms of the presence of a sizable core inside which the average P velocity is substantially less than in the surrounding shell.
Oldham was an original and independent thinker whose writings, whatever the subject, were always interesting and suggestive. One account describes him as “a little too independent sometimes for those in authority.” There is a suggestion that he was impatient with the red tape of administrators less brilliant than himself. Above all, he is noted as a pioneer in the application of seismology to the study of the interior of the earth.
I. Original Works. Oldham’s “Report on the Great Earthquake of 12th June 1897” was published in Memoirs of the Geological Survey of India, 29 (1899), i–xxx, 1–379, along with a supplementary report in 30 (1900), 1–102. “On the Propagation of Earthquake Motion to Great Distances,1” in Philosophical Transactions of the Royal Society, 194A (1900), 135–174, includes his work on P, S, and surface waves. Most of his other papers were published in Memoirs of the Geological Survey of India.
II. Secondary Literature. See the accounts of Oldham’s life by C. Davison, in Obituary Notices of Fellows of the Royal Society of London, 2 (1936–1938), 111–113; and by P. L., in Nature, 138 (Aug. 1936), 316–317.
K. E. Bullen