(b. Fukui, Japan, 30 October 1868; d. Tokyo, Japan, 8 November 1923)
Omori entered the College of Science of the Imperial University, Tokyo, in 1886. After graduating in physics in 1890, he turned his attention to the then rapidly emerging science of seismology. He became a lecturer at the university in 1893; and after some further study in Italy and Germany he became professor of seismology in 1897, a post which he held until his death. During this period he was secretary of the Japanese Committee for the Prevention of Earthquake Disasters, becoming noted as Japan’s foremost seismologist of the time and one of the world’s great early seismologists.
Omori’s work was inspired by Seikei Sekiya and by John Milne, who in Tokyo had become one of the great pioneers of modern seismology. Under Milne’s encouragement Omori made the first precise studies of earthquake aftershocks and published an important memoir on this subject in 1894. His studies, principally of a great earthquake in the Japanese provinces of Mino and Owari in 1891, led him to evolve a formula, still quoted, for the rate of falloff of aftershocks following major earthquakes.
Omori is probably most noted today for his work in designing seismological instruments. One of these, a horizontal—pendulum—type seismograph, was used in many countries and, with certain modifications, is still in use in some observatories. Omori was the first to experiment with the tiltmeter, an instrument designed to measure small tilting of geological blocks before, during, and after large earthquakes. An important innovation, this instrument led to the gathering of much information useful in predicting earthquakes.
Omori carried out pioneering work on earthquake zoning— the division of a region into areas of greater and less earthquake risk. He showed, incidentally, that destructive Japanese earthquakes were centered predominantly under the steeply sloping ocean floors on the Pacific side of Japan, a result of some importance to modern theories of earthquake occurrence.
Omion’s contributions touched on practically all aspects of seismology, and his published papers are numerous. Further topics treated by him include the characteristics of earthquake motions as recorded on seismograms; detailed measurements of periods, displacements, and accelerations of the motions; the location of earthquake sources from seismograph records; the evolution of earthquake intensity scales based on acceleration measurements; experiments on the overturning of brick columns on shaking tables designed to simulate earthquakes; measurements of vibrations of buildings, bridges, chimneys, and towers during earthquakes; and the compilation of earthquake catalogs. He was also interested in the mechanism of volcanoes and used seismic methods in studying them. Omori also applied his ideas in investigations of large earthquakes in India, California, Sicily, and Formosa.
Omori’s approach was that of the practical physicist. It has been stated that his achievements, important as they are, could have been greatly enhanced had he been more mathematically minded. But that judgment does not detract from his central importance in maintaining unbroken the distinguished reputation of Japanese seismological research since Milne’s time.
On 1 September 1923 Omori, who had gone to Australia to attend a Pan-Pacific Science Congress, visited the Riverview Observatory in Sydney. While he was there, the seismographs started to trace out records of a large distant earthquake. This event proved to be a great earthquake in the province of Kanto, Japan, which caused the loss of 140,000 lives and left Tokyo in ruins. During Omori’s return by sea to Japan, his health declined sharply. He died shortly after his return in the university hospital close by the wrecked buildings where he had carried out his lifework.
Omori’s many seismological papers, some of which were written in English, appeared mainly in Japanese journals, especially Publications of the Imperial Earthquake Investigation Committee, Transactions of the Seismological Society of Japan, and Journal of the College of Science, Imperial University of Tokyo. Other papers were published in Bollettino della Societa sismologica italiana. Omori’s major papers include “On the Aftershocks of Earthquakes,” in Journal of the College of Science, Imperial University of Tokyo, 7 (1895), 111–200; and “Materials for the Earthquake History of Japan From the Earliest Times Down to 1866,” which is Publications of the Imperial Earthquake Investigation Committee, 46, nos. 1 and 2 (1904), written in Japanese with S. Sekiya.
On Omori’s life and work, see Charles Davison, The Founders of Seismology (Cambridge, 1927), ch. 11; publication details (without titles) of about 100 of Omori’s papers are given.
See also Who’s Who in Japan (Tokyo, 1912), 691.
K. E. Bullen