Explain the mechanisms of huge earthquakes

There is a 70-80% chance that the Nankai Trough earthquake will occur within the next 30 years. It is expected to cause more extensive damage than the Great East Japan Earthquake of 2011, and the death toll could exceed 320,000. It is believed that this powerful earthquake will occur when the stress energy that builds up at the plate interface due to the subduction of the Philippine Sea plate beneath the Eurasian plate (or Amur plate) on the land side exceeds a certain limit, which will trigger a strong earthquake. The Eurasian continental plate is emerging. In recent years, the relationship between slow-slip events and large-thrust earthquakes has received much attention. Slow slip events involve gradual sliding at the plate interface over an extended period of time. What progress has been made in predicting and elucidating the basic mechanisms behind large earthquakes? We asked Dr. Yoshioka Shoichi, a professor at the Urban Safety and Security Research Center who uses computer data analysis and numerical simulations to study earthquake mechanisms, about the current state of the research.

Interest was sparked by the film “Japan Is Sinking” (Nippon Shinbutsu, 1973).

What inspired you to study earthquakes?

Professor Yoshioka: Looking back, the impetus was the movie “Japan Is Sinking,” which my grandmother took me to see when I was in elementary school. I was amazed by the dynamic images of magma erupting from volcanoes and the sinking of the Japanese islands. This movie sparked my interest in solid geophysics. When I was studying at the Faculty of Science at Kobe University, I worked in a laboratory studying rock mineralogy, which was part of the field of solid geophysics. Solid geophysics covered earthquakes in the broad sense, so I was studying aspects related to earthquakes, not earthquakes themselves. I then went to graduate school at Kyoto University, where I began researching seismology in the Department of Earthquake Prediction and Measurement Research.

Did the Great Hanshin-Awaji Earthquake of 1995 (known as the 1995 Kobe Earthquake outside Japan) influence your research?

Professor Yoshioka: At the time of the Great Hanshin-Awaji Earthquake, I was an assistant professor at Ehime University. Although I lived in Matsuyama City, Ehime Prefecture, more than 200 kilometers from the epicenter, the movement caused by the seismic waves was so intense that I initially thought the so-called Tokai earthquake had occurred. About three weeks after the Hanshin-Awaji earthquake, I visited the affected areas, saw the Nojima Fault on Awaji Island, and walked around Kobe City. I remember that the building I lived in when I was a student had disappeared, and I had many valuable experiences at Ehime University, but I was still studying solid geophysics as a pure science because there were no seismologists there. Therefore, I did not feel that I was dealing with earthquakes directly.

Analyze monitoring data using computers

Your current research topic is earthquake prediction and their occurrence mechanisms. What drew you to this topic and when did you start researching it?

Professor Yoshioka: In 2009, I was appointed to the Urban Safety and Security Research Center at Kobe University. The Center was conducting research that would be beneficial to people under the principles of “Protection of Human Lives” and “Disaster Mitigation and Reduction”. Many of the researchers at the center were working in the engineering field, so I decided to get involved in earthquake prediction and research into the mechanisms of earthquake occurrence. Up to this point, my research has focused on the deep regions of the Earth. However, because major earthquakes occur in shallow areas, I moved my research to shallower areas of about 50 km depth. I decided to advance my research by using my own methods and originality, which I had developed up to that point.

What do you mean by originality exactly?

Professor Yoshioka: Many researchers study earthquakes through observations. I didn't want to do observational studies. Instead, I wanted to focus exclusively on computer-based data analysis and numerical modeling. I value originality in my research, so I did not want to replicate other people's research. I aim to foster highly original research as a whole laboratory through collaboration with students; – Creating ideas myself and discussing them with students.

It seems difficult to clarify the mechanism of occurrence of large earthquakes by analyzing observational data. Your research has been approved under the Kobe University International Strategic Collaborative Research Grant Program* and you will work to conduct joint research with researchers in Mexico and Chile. Can you tell us more about this exciting collaboration?

Professor Yoshioka: I am interested in mathematical and physical approaches using observational data from Japan, Mexico and Chile to understand and predict earthquake mechanisms. The three countries are located on the edge of the Pacific Ocean, where strong earthquakes are likely to occur when oceanic plates are subducted beneath continental plates. Japan experienced the Great East Japan Earthquake in 2011, and Chile witnessed the world's largest earthquake in 1960; The 9.5 magnitude Valdivia earthquake, then the 2010 Maule earthquake with a magnitude of 8.8, and Mexico witnessed the 2017 Tehuantepec earthquake with a magnitude of 8.2. Moreover, these countries have seismic gap areas, where earthquakes do not occur for long periods of time. For example, the Guerrero region of Mexico has not experienced an earthquake in more than 100 years. Likewise, Chile also has a seismic gap zone, and Japan has the Nankai Basin.

Previously, I invited current research colleagues from Mexico and Chile to Kobe under the Urban Safety and Security Research Center program. I thought it would be interesting to join forces because we were already in touch and our research interests aligned.

Detection of slow slides after the Great Hanshin-Awaji Earthquake

Why has the relationship between slow slip events (or slow earthquakes) at plate boundaries and large earthquakes recently received so much attention?

Professor Yoshioka: A slow slip event is a phenomenon discovered in Japan around the year 2000 where two plates move slowly in opposite directions at plate boundaries. After the Great Hanshin-Awaji Earthquake, the National Research Institute for Earth Science and Disaster Resilience (NIED) established a highly sensitive seismic monitoring network called “Hi-NET” throughout the Japanese islands. This network detected tectonic tremors, which were previously thought to be noise such as the vibrations of trains and trucks. However, a detailed investigation of this noise found that it was precisely arranged in a belt-like shape across the deep extension of the hypothetical source zone of the Nankai Trough earthquake (from the northern part of Shikoku to the Tokai area through the central part of the Kii Peninsula). In addition, the Japan Geospatial Information Authority has established a high-performance GPS monitoring network at approximately 1,300 locations across Japan to monitor how the Earth's surface moves over time. This network identified slow slip events in the Bungo Channel between the islands of Kyushu and Shikoku. Later, slow slide events were confirmed at deeper levels in the Pacific Ocean, including California and Alaska in the United States, as well as Canada and New Zealand. These deeper levels are extensions of plate boundaries found in seismic gap zones for large earthquakes.

I believe that computer simulations of such slow events should help us understand the mechanism behind large earthquakes, predict earthquakes with some degree of accuracy, and explain the results. Since it is now known that slow earthquakes can cause fast earthquakes (normal earthquakes), it is worth a try.

What are your future research plans and goals?

Professor Yoshioka: Although it is very difficult to elucidate the basic occurrence mechanism behind large-thrust earthquakes, I am determined to steadily develop our collaborative research and publish our efforts as joint international research papers. Japan has an abundance of high-quality observational data and has developed unique such techniques. Mexican researchers often create mathematical models to explain the occurrence of earthquakes, while some Chilean researchers are experts in artificial intelligence. Therefore, our cooperation will achieve certain results. We believe we can get as close as possible to the true nature of large-thrust earthquakes by developing a temperature structure model, which relates the relationship between temperature and dehydration (where plates tend to slip when dehydrated due to increased temperature and pressure) to the actual earthquake. Events.

It is said that accurate prediction of earthquakes is difficult. Will this be possible in the future?

Professor Yoshioka: Statistically speaking, Nankai Basin earthquakes are likely to occur once every 90-150 years based on past earthquakes. The biggest problem is that these forecasts do not use any existing observational data from highly sensitive seismometers or the Global Positioning System (GPS). In the case of the 2011 Great East Japan Earthquake, data showed that slow slip events occurred in undersea areas just before the earthquake. We hope to combine such data to find a relationship between slow slip events and large earthquakes to improve forecasts. Robust earthquake predictions must accurately extrapolate three factors: the location of the earthquake, the time of its occurrence, and the magnitude of the earthquake. Among these three factors, the timing of an earthquake is particularly difficult to predict. Within a few years, I hope to be able to predict large-thrust earthquakes with a lower error rate. To this end, I strive to pass on the results of my research to the next generation in the hope that they will contribute to future achievements.

Introductory account

He joined the Department of Geophysics, Graduate School of Science, Kyoto University in November 1990; Research assistant at Ehime University from April 1994 to July 1997; Associate Professor at Kyushu University from August 1997 to September 2009; Current position since October 2009; He served as Director of the Urban Safety and Security Research Center from April 2020 to March 2022.

Messages from foreign researchers Dr. Marina Mania, Senior Researcher, National Autonomous University of Mexico Professor Vlad Constantin Mania, Senior Researcher, National Autonomous University of Mexico

Although research exchanges such as data sharing can take place even when we are far apart, the physical residence at Kobe University accelerated the research exchange process between us as students from both universities were able to actively participate. Initially, each of us had our own vision of how to explain the mechanism of a giant earthquake. Through frequent discussions, we began to understand each other's way of thinking and generate new ideas. By analyzing and measuring seismic data from Japan and Mexico, we aim to elucidate the mechanism of occurrence of large earthquakes and ultimately provide accurate earthquake predictions. We plan to proactively publish our research results in academic journals.

*NB

International Strategic Collaborative Research Grant | Kobe University

Professor Yoshioka's project has been approved as a Type B project under the Kobe University International Strategic Collaborative Research Grant Program.

Kobe University research highlights: Shedding light on how massive earthquakes occur in Japan, Mexico and Chile

Kobe University is a national university whose roots go back to the Kobe Commercial School founded in 1902. It is now one of Japan's leading comprehensive research universities with approximately 16,000 students and nearly 1,700 faculty members in 10 colleges and schools and 15 graduate schools. By combining social and natural sciences to develop leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society's challenges.

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