Worldwide, the two most common types of seismographs at the time, the Milne-type and Bosch-Omori seismographs recorded the San Francisco earthquake. Īt the 1908 Messina earthquake, Omori noted the large loss of life, perhaps 75,000 and said that of those 99 percent had died because their houses were not built to withstand earthquakes. Omori later continued this research and is recognized in earthquake engineering as the first to research the effects of earthquakes on man-made structures through implementing the usage of shaking tables and comparing experimental results with measurements during actual earthquakes. For many years the modernization of Japan during the Meiji Restoration, through replacement of traditional light wooden structures resting on boulders, with red brick buildings and iron bridges, had been a major source of concern or Milne. Previously, in 1889 Omori had worked with John Milne to record experiments carried out at the Engineering college at the University of Tokyo to investigate the overturning and fracturing of brick and other columns by horizontally applied motion. He ascribed the high number of casualties due to structure collapse of the dominant local building type: sun-dried brick walls loosely cemented with mud and overlaid by heavy roof beams.
Omori arrived in Japanese Formosa (Taiwan) shortly after the 17 March 1906 Meishan earthquake, later describing soil liquefaction and the complete destruction of Meishan town. Omori conducted measurements of the three principal phases of earthquake motion originally described by Milne: the preliminary tremors, the main portion and the end portion, and visited areas after major earthquakes to ground verify the data collected by his instruments. This earthquake provided an initial data set which, when correlated with other earthquakes, revealed that aftershock frequency decreases by approximately the reciprocal of time after the main shock, a mathematical formula now called "Omori's law". Some areas had 18 to 20 foot high scarps, others looked like a linear mole had been at work. He found the strike-slip fault cut the surface for at least 40 miles and that the north-east side had shifted relative to the other side a distance of one to two meters. On 28 October 1891 Mino and Owari provinces were devastated by earthquakes their fault lines were traced by Bunjiro Koto (1856–1935), another professor at Imperial University. The last operating Bosch-Omori seismograph, now operating independently of the seismographic network, is exhibited at the Ferndale Museum (California). Distributed worldwide, Bosch-Omori seismographs formed the backbone of the world seismographic network until after World War II. In 1899, Omori described his horizontal recording pendulum, later called an Omori seismometer and with minor modifications by the J&A Bosch Company of Strassburg, the "Bosch-Omori Seismometer". He could read English, German, Italian and Japanese and maintained correspondence with many seismologists as well as writing papers in all four languages. Omori became chair of seismology at the university and secretary of the Imperial Earthquake Investigation Committee following Sekya's death on 9 January 1896. In 1895, he was sent to Germany and Italy for additional study and visited England briefly on his way home in September 1896.
In 1891 Omori was appointed assistant to Sekiya and in 1893 lecturer on seismology at the Imperial University. In 1886 Sekiya was made chair of seismology and secretary to the Imperial Earthquake Investigation Committee and by the time of his death a decade later, Japan had nearly 1000 seismological recording stations to study seismicity in Japan. Sekiya and Omori published the first clear record of a destructive earthquake, obtained by their measuring devices at the university. Omori studied physics with the initial British foreign advisors serving as professors at the Imperial University of Tokyo, especially John Milne until he left Japan in 1895, as well as Japanese colleagues including Seikei Sekiya who in 1880 became the first professor of seismology at Tokyo Imperial University. Omori is also known for his observation describing the aftershock rate of earthquakes, now known as Omori's law.
Order of the Sacred Treasure from the Imperial Court of Japanįusakichi Omori ( 大森 房吉, Ōmori Fusakichi, 30 October 1868 – 8 November 1923) was a pioneer Japanese seismologist, second chairman of seismology at the Imperial University of Tokyo and president of the Japanese Imperial Earthquake Investigation Committee.
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