Frontiers of Science

From a new discovery arises a paradigm shift in seismology

IDE Satoshi

Professor, Department of Earth and Planetary Science

January 11, 2022


Seismology is facing a time of change.
The framework for understanding earthquakes is about to expand drastically as the result of a new discovery in the 21st century

◎Why do earthquakes occur?

Japan sits in the Ring of Fire. About 10 percent of all earthquakes that jolt the world occur in the periphery of the Japanese archipelago. More than 100,000 earthquakes are recorded every year. They include small ones we don’t feel, but this works out to about one quake every five minutes.

But how do earthquakes happen? Prof. Satoshi Ide of the Department of Earth and Planetary Physics is trying to unlock that mystery.

“Earthquakes are basically caused by fractures in bedrock underneath the Earth’s surface accompanied by frictional slip. When the accumulated strain energy in the bedrock at depth is released, the bedrock is ruptured and slips while creating frictional heat. The portion of the accumulated strain energy that is not used in the rupture of the bedrock and in friction spreads as seismic waves. I am doing research to understand and explain the natural phenomenon of an earthquake, including the process of friction and seismic rupture and how the resulting seismic waves propagate.”

How, then, does the strain energy which causes earthquakes accumulate? The key lies primarily in the movement of tectonic plates.

“The Earth’s surface is comprised of multiple plates. There are two different kinds of plates, the oceanic plate and thecontinental plate. They behave differently. The former is created in volcanic regions under the sea called ridges. It moves and subducts into the mantle at a trench while the latter remains on the Earth’s surface for long time. In a boundary between the two, the denser oceanic plate subducts beneath the continental plate. This is how strain energy accumulates in the bedrock and is why many earthquakes occur in boundary regions.

The theory of plate tectonics, established in the second half of the 1960s, clearly explains the mechanism of earthquakes. The Japanese archipelago lies on top of the boundary of multiple plates so it is only natural that we frequently have quakes in the country.

Many people in Japan are interested to know if and when a major earthquake may strike the country. But what is it that distinguishes major earthquakes from small ones?

“We are talking more or less about the same phenomenon. The scale of energy is different, but the energy is released in just about the same way. In other words, we can say that a minor earthquake is a miniature version of a huge quake. The Great East Japan Earthquake registered a magnitude of 9, but we are unable to distinguish its origin from that of a small earthquake.”

A big earthquake does not start out as a big earthquake from the outset. When a small-scale seismic rupture in bedrock grows like a snowball, the scale of an earthquake grows correspondingly. When such a chain process occurs, it unleashes a huge earthquake but when it does not occur, only a small earthquake is generated.

“The difference is often coincidental in many cases and it is impossible to predict in today’s science. But it is necessary to quantify the degree of randomness in order to make risk evaluations more precise.”

Seismologists continue to rise up to challenging issues in order to understand the natural phenomenon of earthquakes and to respond to the needs of society.

◎A new 21st century discovery that changes established seismological concepts

At the beginning of the 21st century, a new seismological phenomenon was discovered, called a “slow earthquake.”

“In regular earthquakes, bedrock moves at a speed of 1 meter per second. In slow quakes, the speed is about several centimeters per second. The speed of the slip corresponds to the amplitude of seismic waves. We don’t feel slow earthquakes and seismometers pick up only imperceptible tremors. They are so faint, as a matter of fact, that they were once considered to be just noise on seismometers.”

It was reported in 2002 that weak tremors observed near the Nankai Trough on the Pacific side of the western Japanese archipelago were caused by shear slip in bedrock beneath the surface. This discovery was a seismological feat by Japanese researchers. The Nankai Trough is a zone where tectonic plates are subducting, triggering giant earthquakes many times in the past.

“We understand many things now–that slow earthquakes are happening in areas of Japan and the world where huge quakes usually occur, that there is a strong possibility that a massive earthquake is ready to occur, and that slow earthquakes are controlled by physical laws that are different from those that govern ordinary quakes. This is a major discovery that expands the scope of seismology. There are still things we do not understand, but many seismologists are working hard to clarify them. They are also actively conducting research to explain how slow earthquakes are related to ordinary quakes.

The powerful earthquake that struck southern Hyogo Prefecture in 1995 (the Kobe Earthquake) is closely related to the discovery of slow earthquakes. Following this disaster, the earthquake monitoring networks in Japan were greatly strengthened and became the best in the world. From these huge amounts of data, we are beginning to see the Earth’s movements from a new perspective.

◎The power of science that survives through changing times

Prof. Ide started studying earthquakes in earnest in 1992 when he enrolled in graduate school.

“I was good at natural sciences and especially interested in physics. The study of things like elementary particles and the universe was popular, but I decided to specialize in geophysics. I wanted to unravel the mystery of various phenomena that are happening around us.”

Prof. Ide was attracted especially to the study of earthquakes as a “purely natural phenomenon.” At that time, no serious disaster damage had been reported since 1984, when a big earthquake ripped through western Nagano Prefecture, and people were not as interested in earthquakes as they are today. But that changed drastically in 1993, the year Okushiri Island sustained devastating damage in a quake that occurred off the southwest coast of Hokkaido. Another struck the Kobe area in 1995 and social demands for earthquake research rapidly grew.

“I also went to Okushiri Island for research. Since then, I have been anxious to meet the needs of society but I feel frustrated because I haven’t been able to. But it is also meaningful to make the mechanism of earthquakes clear. People fear what they don’t understand. When they do understand, they can reduce needless fears. To shed light on what we are unable to understand–that’s what science is all about.”

Prof. Ide says what is needed in science is the attitude “to doubt things through and through.” Scientists produce hypotheses and see them knocked down. Only those hypotheses that withstand strict scrutiny survive as theories. Learning develops by going beyond the existing common sense and academic theories. Plate tectonics is now an established theory but only after having overcome traditional theories. And today, slow earthquakes are about to change the paradigm of seismology.

“If we just follow existing concepts, we immediately slip into local minima and change becomes intolerable. In order to secure diversity, it is necessary to doubt common sense and present different points of view. In the Faculty of Science, we train students and researchers to develop such an ability. I hope that people who are interested in this will study science.”

Science is truly a force through which we can survive the changing times.

Interview and text: Masatsugu Kayahara
​Photography: Junichi Kaizuka

Originally published in The School of Science Brochure 2018

IDE Satoshi
Professor, Department of Earth and Planetary Science
Graduated from the Department of Geophysics, Faculty of Science, The University of Tokyo, in 1992 and completed the doctoral course in the Department of Earth and Planetary Physics, Graduate School of Science of the University of Tokyo, in 1997. After serving as research assistant in the Earthquake Research Institute and then lecturer and associate professor in the Graduate School of Science, assumed the present position in 2013. '''The author of


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