Fusakichi Omori

Seismologist

• Born: October 30, 1868
• Birthplace: Fukui, Japan
• Died: November 8, 1923
• Place of death: Tokyo, Japan

Significance: Fusakichi Omori was a Japanese seismologist who was a pioneer in the scientific study of earthquakes. He was especially known for his study of the frequency of aftershocks, from which he developed a law that bears his name.

Background

Fusakichi Omori was born on October 30, 1868, in Fukui, a coastal city in northern Japan. He attended college at the Imperial University of Tokyo where he studied under noted British geologist John Miline. Miline was one of the founding fathers of modern seismology and invented the horizontal seismometer. He encouraged the young Omori to focus his attention on the study of aftershocks, the smaller earthquakes that follow the main seismic jolt. Omori also studied under geologist Seikei Sekiya who became the first professor of seismology at the Imperial University in 1880. Omori was named an assistant to Sekiya in 1891 and a professor of seismology at the university two years later. He later served as president of the Imperial Earthquake Investigation Committee of Japan.

Life’s Work

Japan sits at the boundaries of four major tectonic plates, making it one of the most seismically active nations in the world. The country is prone to earthquakes and is home to more than 110 active volcanoes. Using Miline’s seismometer, Omori studied three of these earthquakes, destructive temblors that occurred in 1889, 1891, and 1893, to develop his scientific theory about aftershocks. He published his work in 1894 at the age of twenty-six. Omori found that the frequency and severity of aftershocks after a main earthquake decreases with time and follows a mathematical formula. Based on his observations, Omori found that the occurrence of aftershocks on the first day of a seismic event determines the frequency of aftershocks in the days that follow. Regardless of the number of aftershocks that occur on the first day, the second day after the main quake will cut in half the probability of aftershocks occurring. That number continues to decrease so that on the tenth day after the event, the probability of aftershocks falls to one-tenth of the first day. The formula was later named Omori’s Law.

Omori’s discovery made him one of the preeminent seismologists of his time. In 1910, Omori was using a seismometer to observe Mount Usu, an active volcano on Japan’s northern island of Hokkaido. Based on his readings, Omori noticed that the volcano was becoming increasingly unstable and gave a lecture to officials from a nearby town. The town was evacuated just days before the volcano erupted.

Omori traveled the world studying earthquakes and other seismic events. He studied earthquakes in Taiwan, Italy, and the United States, including the Great San Francisco Earthquake of 1906. Omori’s observations were some of the first to chronicle a region in the Pacific known for its seismic activity. He found that the Pacific Ocean is the most geologically active region on the planet and that the earthquakes in the area did not happen randomly but struck along weak points in Earth’s crust. His pioneering work later led geologists to identify a seismically active region they labeled the “Ring of Fire.” This region stretches from the southern tip of South America, up the western coast of North America, down the eastern coast of Asia, and into the island chains of Indonesia and New Zealand.

Impact

Omori has a profound influence on the early field of seismology. He collaborated with scientists from around the world and assisted in the establishment of the Hawaiian Volcano Observatory in 1912. He improved on John Miline’s seismometer and, along with refinements from German company J&A Bosch, developed the Bosch-Omori Seismograph, which served as state-of-the-art equipment for decades. He also developed the Omori Seismic Intensity Scale, a seven-point scale that measures ground acceleration and structural damage to gauge an earthquake’s intensity. A modified version of his scale continues to be used by Japanese scientists in the twenty-first century.

Although scientists have not succeeded in explaining just how Omori’s Law works, more than a century of observations still show that it provides correct results. The law continues to be used by modern seismologists although it was modified by seismologist Tokuji Utsu in 1961. Today the law is known as the Omori-Utsu Law.

Death

In 1923, Omori was in Australia for a scientific conference. During his trip, he began complaining of severe headaches and experiencing hallucinations. On September 1, a 7.9 magnitude earthquake struck the main island of Honshu, destroying Tokyo and killing more than 142,000 people. It remains the deadliest earthquake in the nation’s history. Omori quickly headed home to study the earthquake. Along the way, his health worsened.

He was admitted to a Tokyo hospital where he was diagnosed with a brain tumor. He died on November 8, 1923, at the age of fifty-five.

Bibliography

Faraoni, Valerio. “Lagrangian Formulation of Omori’s Law and Analogy with the Cosmic Big Rip.” The European Physical Journal, vol. 80, no. 5, 19 May 2020, p. 445, www.ncbi.nlm.nih.gov/pmc/articles/PMC7319289/. Accessed 26 June 2023.

“Fusakichi Omori.” Pennsylvania State University, 2022, www.e-education.psu.edu/earth520/content/l2‗p16.html. Accessed 26 June 2023.

Guglielmia, Anatol, Alexey Zavyalov, Oleg Zotov, and Boris Klain. “About the Aftershocks, the Omori Law, and the Utsu Formula.” arXiv, 30 May 2023, arxiv.org/abs/2305.18883v1. Accessed 26 June 2023.

Guglielmia, A. V. and A. D. Zavyalov. “The Omori Law: The 150-Year Birthday Jubilee of Fusakichi Omori.” Volcanology and Seismology, vol. 12, no. 5, Sept. 2018, pp. 353–358, www.researchgate.net/publication/328766522‗The‗Omori‗Law‗The‗150-Year‗Birthday‗Jubilee‗of‗Fusakichi‗Omori. Accessed 26 June 2023.

“Volcano Watch — Professor Fusakichi Omori—An Instrumental Person at HVO.” United States Geological Survey, 15 Mar. 2001, www.usgs.gov/news/volcano-watch-professor-fusakichi-omori-instrumental-person-hvo. Accessed 26 June 2023.