“Dialogue with the Earth, Dialogue with Society”
February 9, 2011
Dr. Satoko Oki
Assistant Professor，
Outreach and Public Relations Office,
Earthquake Research Institute, The University of Tokyo
Seismology has twofold missions: one is to pursue the scientific truth and the other is to make use of such results for the betterment of human lives.
The 1995 Great Hanshin-Awaji Earthquake claimed 6,434 lives, of which 80% were crushed to death and 10% were burnt to death. The powerful quake right above the epicenter lasted for 10-15 seconds and damaged the lifeline utilities. We realized anew the importance of disaster prevention. Earthquakes and volcanic eruptions make us witness the enormous forces of nature. Let me explain their mechanism and various countermeasures that could be taken.
1. Why does Japan have so many earthquakes and volcanoes?
Only limited places on the earth suffer from damages caused by earthquakes and volcanoes. What makes Japan an especially disaster-prone country? This is because Japan is located where the Pacific Plate and the Philippines Sea Plate collide. The map indicating all the earthquakes, in the past 30 years, with magnitudes of 5 or more on the Richter scale, unveils the mechanism of plate tectonics. There are mainly two types of earthquakes, the “plate-boundary earthquake” and the “crustal earthquake (or inland earthquake),” which triggered the Great Hanshin-Awaji Earthquake. Although Japan accounts for less than 1% of the earth’s surface area, 10% of all earthquakes do occur in this country. 300 tremors occur per day in Japan, including unfelt ones.
Volcanic chains form along the submarine ridges, where the plates emerge in the ocean. Thanks to the technological advancement of various observation equipments as well as progress made on rock structure analysis, we have a clearer picture on how volcanoes are formed where the old plates collide. It takes as long as 200 million years for an oceanic plate to reach Japan and collide, during which time much water is soaked into its rocks. As the plate collides beneath the Japanese Islands, its massive pressure heats up the water in the rocks, forms the magma and eventually gives birth to volcanoes. The currently-erupting Mt. Shinmoe and Mt. Asama were both formed by such mechanism.
2. About the Earthquake Research Institute
Dr. Torahiko Terada and Dr. Hantaro Nagaoka established the Earthquake Research Institute in 1925, two years after the Great Kanto Earthquake. Dr. Terada stipulated the mission of this institute to be “conducting scientific research on various phenomena of earthquakes as well as taking disaster preventive measures to mitigate damages caused by earthquakes both directly and indirectly.” We have been making research under this spirit.
Our institute is based in the Hongo Campus of the University of Tokyo. We also have our observatories in various parts of Japan, dedicated to catch the warning phenomena of volcanic activities.
Mt. Asama is still an active volcano. The Asama Volcano Observatory is said to be the birthplace of modern volcanic observations in Japan. The 1911 eruption of Mt. Asama gave birth to the modern volcanology in Japan. This observatory is currently under renovation to reopen as a museum for the general public.
3. Countermeasures for a severe environment of earthquakes and volcanoes in Japan
It is relatively easier to predict volcanic eruptions than earthquakes. Volcanic eruptions are somewhat predictable by installing various observation equipments, including seismometers, barometers and clinometers, and by analyzing the rock structure. Yet, it is difficult to forecast when the eruption ends or how it will progress.
For example, as we try to grasp the next warning signs of the erupting Mt. Shinmoe, we can learn from its former eruption in 1716-17, when the eruption changed its style from phreatic explosion to magmatic eruption 8 months later, and then to a great eruption a further 7 months later.
4. Countermeasures against earthquakes
The actual-size experiment on the strength of a 30-year-old two-story wooden house against the earthquake of Hansin-Awaji earthquake scale proved that the first floor would be smashed flat. The aftershock would then destroy the second floor. Let me advise you to sleep on the second floor.
Although earthquake prediction is difficult, it is possible to make a long-term prediction on earthquake occurrence over the coming 30 years. There are 99% chances of Miyagi-oki earthquake (around M 7.5) to strike in the coming 30 years, 87% for Tokai earthquake (around M 8.0), 60-70% for Tonankai earthquake (around M 8.1) and 60% for Nankai earthquake (M 8.4). We also drew the seismic vibration prediction map by taking into account the ground information. For example, Hibiya in the heart of Tokyo is predicted with 15.5% chances of seismic intensity of six minor, or 99.6% chances of seismic intensity of five minor. I am sure most of your houses are insured against fire, which has the occurrence probability of 2%. I wonder how many of you have your home insured against earthquake or how many of you have already discussed with your family about what to do should a great earthquake strike.
Let me close my speech by emphasizing the necessity of disaster-prevention education to mitigate damages caused by natural disasters in our daily lives. We have prepared the disaster prevention maps for elementary school children, so please contact us if you need them.