Plate Tectonics and People

Fusakichi Omori

Print Print

Biographical Information

Fusakichi Omori was born in Japan in 1868. At a young age Omori studied under John Miline. Miline is credited as the inventor of the horizontal pendulum seismometer, marking the beginning of modern seismology, and as the founder of the Seismology Society of Japan. Miline encouraged Omori to concentrate his work on the study of aftershocks. Soon after Miline left Japan Omori became a professor of seismology at the Imperial University of Tokyo. This appointment was soon followed by an offer to become the secretary of the Imperial Earthquake Investigation Committee of Japan. After a several years of dedicated serve to this organization he was selected for president the committee. Omori’s first publication was a memoir published in 1894 on aftershocks, creating the well known Omori’s Law. He personally investigated some of the most powerful earthquakes of his time including the San Francisco earthquake of 1906. Omori was considered to be Japan’s leading seismological researcher during the Meiji and Taisho periods.

There have been some interesting stories surrounding the death of this scientific giant. Joshua Hammer's book Yokohama Burning describes the events well. Not long before his death Omori was attending a scientific conference in Australia. During his trip he began to develop some health problems, described as a series of intense headaches. While at the conference Omori missed what was described by Hammer as "the seismological event of the century." Aboard the ship back to Japan his health deteriorated, he began telling the crew that the he had envisioned this catastrophe taking place.His illness, a brain tumor, caused him to have intense hallucinations. He spend the rest of his time in a hospital until his death on November 8,1923. He was only fifty-five.

Specific contributions to the theory of plate tectonics and/or our modern view of the solid Earth

Fusakichi Omori’s work was mainly concentrated in the area of seismology. Most of his findings were created/discovered through his extensive work with early seismographs, and through field studies of earthquake prone areas in Japan and around the world. One of his earlier and most famous contributions is centered around his first area of study, aftershocks. He created what is called Omori’s Law. This law was created to explain the observed pattern and timing of aftershocks through the means of a mathematical formula. The formula for Omori’s law and the breakdown of its components can be seen below. This law states that there is sequence of aftershocks following the main earthquake event whose frequency decays with time. Today a modified version of this law is more commonly used which was created in by Utsu in 1961. The modified version of this law added an exponent to the equation (seen below) intended to modify the decay rate.  These laws explain that no matter the odds of an aftershock on the first day of the event, the second day will have half the odd of the first day, and the tenth day will have one tenth the odds of occurrence. These laws are considered to be empirical, they are only observable situations. To this day the scientific community still does not understand what actually makes this law work.

Omori's Law:

n= the rate earthquakes measure in time(t) after the main earthquake

K = amplitude

C = time offset parameter

Modified Omori's Law:

n(t) = \frac {k} {(c+t)^p}

This particular law helped us to understand the mechanics of earthquakes, and helped to develop a more modern view of the earth. In his work with earthquakes and volcanoes he made some significant contributions to the creation of the theory of plate tectonics, unfortunately this theory would come well after his death. In his paper titled Papers on Seismology: Earthquakes Zones of the Pacific, he laid the groundwork for what we know today as the "Ring of Fire'. This paper was printed as a part of the First Pan-Pacific Scientific Conference. He noted in his paper that the areas around the Pacific Ocean are the most active earthquake zones on earth. He described the great earthquakes that have occurred here were not random, but that they occurred along weak points in the earth's crust. He observed that in these locations that they had similar surface gradients which account for the earthquakes in these areas. The areas he named included the Pacific Coasts of North America, the Aleutian Islands, Japanese Islands Sumatra, and South America. He also noted the Himalayas in is work as well. Although today we know the type of movement here is a bit different than the previously mentioned locations. He wrote in his paper that "the Japan and other arc systems as well as the whole border of the Pacific may be regarded as forming a volcanic chain or an earthquake district" (Omori 370). He created a map to show these areas which can be seen below.

He referred to these areas or weak points around the Pacific as having two "sides", a convex and concave side. He identified the convex side as the tension side and the concave side as the compression side. The concave side is also the location where volcanoes will form. Omori also noted that the volcanoes that are found on the islands of the Pacific including Japan, the Aleutian Islands, and Sumatra experience submarine earthquakes which originated from the convex side. He also found that these submarine quakes were similar to the volcanoes and earthquakes occurring in Italy.

His work started a track which would lead more well known "giants of science" to the creation of plate tectonics. Omori's terminology was much different than was is commonly used today, but most of his ideas were spot on. His map of the Pacific mimics modern day maps of plate boundaries around the Ring of Fire.

Other important scientific contributions

Seismic Scale:

In 1894 Fusakichi Omori created the Omori Seismic Intensity Scale. His scale was created prior to the Mercalli Scale though they were both similar in nature. The Omori Scale is a 7 point scale which uses ground acceleration, damage to infrastructure, and other observable phenomena during an earthquake event.  This scale was eventually modified and  is still used in Japan as a measure of earthquake intensity. It is administered by the Japanese Meteorological Society.

Volcanic Quakes

Fusakichi Omori's work was not limited to earthquakes he also made great strides in the the study of earthquakes related to volcanic eruptions. He was considered to be the founder of volcanic seismology. He was the first person to use a seismometer to study a volcano. The first volcano he studied with this equipment was Japan's Mount Usu. After his investigation of this unstable mountain he gave a lecture to some members of the town of which he warned them of the danger of the volcano and ordered the residents to evacuate the area. Soon after the volcano had its main eruptions  and  his actions saved the lives of the people living around the mountain.

Professor Omori also had a large influence in the establishment of the Hawaiian Volcano Observatory. Thomas Jaggar, the founder, traveled to Japan to study with Omori to learn about the seismological methods he used to study volcanoes. Jaggar implemented this knowledge at the HVO. Omori even donated the first instrument installed at the observatory, an Omori seismograph. These seismographs were similar in nature to the horizontal pendulum seismometer created by Omori's mentor, John Miline. The Omori Seismography was updated with the Bosch-Omori Seismographph.


Other cool stuff you should know

- Though Omori was Japanese most of his papers were written in English, over 4000 pages worth.  He also spoke German and Italian. He was fluent enough in Italian to even write a few of his papers in this language.

- Omori did some work with in the field of meteorology to try and trace a relationship between variations in earthquake frequency and barometric pressure

- There is a working Bosch-Omori seismographph at the Ferndale Museum in California. The owners must attend the seismography once every 24 hours to change the record on the machine. Pictured below is an image of the seismograph at Ferndale.



Alden, Andrew. "Obituary Prof. Fusakichi Omori." Nature 113.2830 (1924): 133. Fusakichi Omori (1868-1923). Web. 28 Jan. 2011. <>.

Christensen, Kim. "Research: Unified Scaling Law for Earthquakes." Imperial College Condensed Matter Theory Group. Web. 01 Feb. 2011.

Hammer, Joshua. Yokohama Burning: the Deadly 1923 Earthquake and Fire That Helped Forge the Path to World War II. New York: Free, 2006

Omori, Fushakichi. "Papers on Seismology: Earthquakes Zones of the Pacific." Proceedings of the First Pan-Pacific Scientific Conference 2 (1920): 369-77.

"Omori Seismic Intensity Scale." About Geology - The Complete Guide to Earth Science and Geology. Web. 01 Feb. 2011.

"Professor Fusakichi Omori - an Instrumental Person at HVO Ago." USGS Hawaiian Volcano Observatory (HVO). 15 Mar. 2001. Web. 29 Jan. 2011.

"Recent Eruptions after the 19th Century (Volcano Science Museum)." 2007. Web. 29 Jan. 2011.

Zobin, V. M. Introduction to Volcanic Seismology. Amsterdam: Elsevier, 2003.