Geological data sheds light on past and future earthquakes in the Sea of ​​Japan

The Noto Peninsula earthquake, which struck the central-northern region of Japan on January 1, 2024, registered a maximum magnitude of 7 on the Japanese scale and caused the coastline and ocean floor to lift. Associate Professor Tatsuya Ishiyama of the Japan Arc Monitoring Research Center of the Earthquake Research Institute explains why a massive earthquake occurred on the Noto Peninsula and what data on geomorphological, geological, and crustal structures can tell us about past and future earthquakes in the Sea of ​​Japan area.

A photo of a 4.1-meter coastal surge taken at Ejisu Fishing Port in Wajima City, Ishikawa Prefecture, on January 4, 2024. (Source: Ishiyama, Hiroshi, Mata, Tateishi, and Yasui, 2024*1)

The Noto Peninsula in the Context of Japan's Geological History

– What can you tell us about geomorphology, which describes landforms – distinctive natural features on the Earth's surface – on the Noto Peninsula?

If we compare the coastal terrain before and after the Noto Peninsula earthquake, we can see that the coastal uplift caused by the earthquake has dried up beaches and fishing ports. We know that the coastline has advanced by hundreds of metres in a number of locations. Surveys conducted after the earthquake confirmed that there was an uplift of about 4-5 metres at the most extreme points.

Coastal elevation distribution map associated with the 2024 Noto Peninsula earthquake. This map shows an estimate of seismic coastal elevation based on algal species and community distribution at 75 sites in the northern Noto Peninsula. (Source: Yasui, Tateishi, Ishiyama, Mata, Hiroshi, & Shirahama, 2024*2).

The Sea of ​​Japan region, located on the west coast of Japan, which includes Okushiri Island, Sado Island, Oga Peninsula and Noto Peninsula, is known for its intense crustal and seismic activity. This is especially true of the Noto Peninsula, which is surrounded by several active faults close to the shore. On the peninsula, you can find landforms known as sea terraces that rise more than 100 meters above sea level; they depict coastal features that are more than 120,000 years old, meaning that the land has been rising at a rate of 1 millimeter per year for more than 100,000 years – very fast compared to other regions.

Although we still don’t fully understand what’s behind this intense crustal activity in the Sea of ​​Japan, we have to think about it in terms of the origin of the Japanese islands themselves. About fifteen to twenty million years ago, part of the Eurasian continent separated from the mainland by hundreds of kilometers, opening the Sea of ​​Japan and forming the Japanese islands. There are various theories about the mechanisms that led to the opening of the sea, but the enormous extensional forces exerted during this process nonetheless led to the concentration of numerous faults along the coast. Like ancient sea-opening wounds, these faults are thought to be active today because they are currently subject to east-west compression due, for example, to the subduction of the Pacific Plate beneath Japan’s main island of Honshu.

――Is there any evidence of a major earthquake occurring on the Noto Peninsula before?

Close observations of coastal terrain reveal evidence of intermittent uplift over a period of several thousand years, suggesting that large earthquakes have recurred in the past. We also know that earthquakes have occurred in the past because fossils of coastal marine worms can be found at higher than usual elevations. However, we have yet to find any clear evidence on the ground of a previous earthquake similar to the one on January 1.

Most previous inland earthquakes exceeding magnitude 7 in Japan were caused by either a single fault rupture or successive ruptures of several faults extending for tens of kilometers. By contrast, previous large earthquakes in the Sea of ​​Japan—such as the Shakotan (1940), Niigata (1964), Nihonkai-Chubo (1983), and Hokkaido Nansei-Oki (1993) earthquakes—typically measured in the upper half of magnitude 7, with ruptures longer than 100 kilometers. The epicenters of most of these earthquakes were also offshore. In other words, the main cause of the damage from these earthquakes was the resulting tsunami.

The fault line caused by the Noto Peninsula earthquake was about 150 kilometers long, but it was located on the border of land and sea, causing tsunamis and strong ground shaking at the same time, causing severe damage to populated areas on the ground.

Understanding Earthquakes at the Land-Sea Boundary

―― What are some characteristics of earthquakes that occur at the land-sea boundary?

Active internal faults leave behind some evidence of past large earthquakes, and using methods such as trench surveys and in-situ borehole drilling, we can determine when they occurred and how intense they were. We can also find information about past earthquakes written down in historical records. However, we have much less information about past earthquakes in the Sea of ​​Japan than we do about the giant earthquakes along the Nankai Trough, the Japan Trench, and active internal faults. The geomorphological and geological records of active faults are often incomplete, and there are many things we still don’t know. Combined with the fact that marine surveys are much more expensive than on land, getting the data we’re missing is no easy task.

Surveys at the land-sea boundary, which is typically used for commercial fishing, are more difficult. In shallow waters, even if seismometers can be installed, noise from tidal currents creates disturbances in the recording. Moreover, previous high-altitude earthquakes may be erased by wave erosion, reducing information about seismic activity and crustal deformation in that area.

– What were you looking for in your surveys immediately after the earthquake?

A photo of a raised wave-shaped bench and four-legged animals taken in Tatsugasaki in Wajima, Ishikawa Prefecture, on January 14, 2024. (Source: Ishiyama, Tateishi, and Yasui, 2024*3)

I had been visiting the Noto Peninsula since before the earthquake as part of a project involving infrastructure exploration and geomorphological surveys with researchers from the University of Toyama, Okayama University, Shinshu University and others. Following the January 1 earthquake, some areas were inaccessible due to snow and landslides, but members of the University of Toyama arranged transportation, and we arrived at the site on January 3. It was essential that we get to the area immediately after the disaster to investigate and record how the geomorphology of the coastline changed when the earthquake struck, before erosion and restoration work could hide or alter the evidence.

The Japanese government uses long-term earthquake forecasting to assess the likelihood of an earthquake occurring within the next 30 years. It does this by using geomorphological, geological, and historical data to estimate the timing and magnitude of past major earthquakes spanning several thousand years. But the information we can glean from this data is often incomplete, like a puzzle with missing pieces. It is important to try to fill in these gaps when an actual earthquake occurs by understanding how the coastline has changed so far and how it will continue to change hundreds of years from now. Understanding the different scales and patterns of seismic activity, such as the Noto Peninsula earthquake, allows us to estimate potential earthquakes more realistically. The January 1 earthquake will serve as an important case study for understanding other earthquakes and active faults that lie at the land-sea boundary.

We express our deepest condolences to the victims of the earthquake and all those affected by it, and wish them a speedy recovery. We hope that by collecting as much data as possible about earthquakes as they occur, we will be able to better understand seismic phenomena and prepare for future seismic disasters.

*1Ishiyama, Tatsuya, Daisuke Hiroshi, Nobuhisa Mata, Ryo Tateishi, and Kenichi Yasuo. (2024). “Seismic coastal uplift associated with the 2024 Noto Peninsula Earthquake (Preliminary Report 2).” Earthquake Research Institute, The University of Tokyo. *2Yasu, Kenichi, Ryo Tateishi, Tatsuya Ishiyama, Nobuhisa Mata, Daisuke Hiroshi, and Yoshiki Shirahama. (2024). “Seismic coastal uplift associated with the 2024 Noto Peninsula Earthquake”. Program and Proceedings of the Emergency Symposium on the 2024 Noto Peninsula Earthquake, p. 11. Noto Earthquake Disaster Response Headquarters of the Society of Japanese Geographers. *3Ishiyama, Tatsuya, Ryo Tateishi, and Kenichi Yasuo. (2024). “Coastal Seismic Upsurge Associated with the 2024 Noto Peninsula Earthquake (Preliminary Report 4).” Earthquake Research Institute, The University of Tokyo.

Tatsuya Ishiyama Associate Professor, Japan Arc Observation Research Center, Earthquake Research Institute

Ph.D. from the Department of Earth and Planetary Sciences, Graduate School of Science, Kyoto University. Prior to assuming his current position in 2024, he held the positions of Researcher at the Center for Active Fault Research at the National Institute of Advanced Industrial Science and Technology; Assistant Professor at the Department of Earth Sciences, Tohoku University; and Assistant and Associate Professor at the Center for Earthquake Prediction Research at the Earthquake Research Institute, The University of Tokyo. Contributing author of the edited volume Atlas of Quaternary Thrust Faults in Japan (University of Tokyo Press, 2002), and co-author of several journal articles, including “Structures and Active Tectonics of the Compression-Reactivated Back-Arc Failure Fault across the Toyama Basin in the Sea of ​​Japan Revealed by a Multiscale Seismic Profile” (Tectonophysics, July 25, 2017).

Interview Date: April 4, 2024 Interview: Yuki Terada, Hannah Dahlberg Dodd

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