The Seismology of Japan

Japan’s active seismic landscape

Caption_right

Shaded relief map of the East Asia – Pacific region, labeled with geographic (italicized) and tectonic features discussed in this paper. Red lines show the block boundaries used in a reference model of the greater Japan region, referred to as JB1. From Loveless and Meade (2010) ¹.

Japan is renowned for its high seismic activity, a consequence of its unique position at the convergence of four tectonic plates: the Pacific, Philippine Sea, North American, and Eurasian plates ²${}^{,}$ ¹. These dynamic interactions have shaped the islands’ geography and culture but have also exposed the nation to frequent earthquakes, some of which have been devastating. Central to understanding Japan’s seismic activity is the subduction of the Pacific Plate beneath the North American Plate along the Japan Trench in the northeast and the Philippine Sea Plate beneath the Eurasian Plate along the Nankai Trough and Sagami Trough in the southwest ²${}^{,}$ ³.

Japan’s seismicity is also influenced by the complex fault systems across the islands, such as the Itoigawa-Shizuoka Tectonic Line and the Niigata-Kobe Tectonic Zone, which accommodate significant crustal deformation ⁴. Recent geodetic studies utilizing the GEONET GPS network reveal the extent of strain accumulation and fault motion, providing insights into earthquake cycles and the segmentation of subduction zones ⁴${}^{,}$ ¹. These studies emphasize Japan’s seismic complexity, with oblique convergence resulting in both dip-slip and strike-slip faulting ¹. Additionally, seafloor borehole tiltmeters have been deployed to measure geodetic and seismic phenomena in real-time, which help to improve our understanding of tectonic strain and early earthquake signals ⁵.

Nankai Trough: Past, Present, and Future

Caption_left

Yellow dots indicate epicenters of very low frequency earthquakes ⁶${}^{,}$ ⁷${}^{,}$ ⁸${}^{,}$ ⁹${}^{,}$ ¹⁰ and green dots represent tremors ¹¹${}^{,}$ ¹². Blue and red stars indicate the epicenters of the 1944 Tonankai and 1946 Nankai earthquakes, respectively. The rupture areas of these earthquakes are represented by the red and blue dashed lines ¹³${}^{,}$ ¹⁴. From Flores et al. (2024)¹⁵.

Among the various subduction zones, the Nankai Trough stands out due to its regular seismicity and significant societal risks. Historical records dating back to 684 AD document cycles of great earthquakes along the trough, typically occurring in pairs or sequences separated by intervals of approximately 90 to 150 years ³. For example, the Hoei Earthquake of 1707 ruptured all segments of the Nankai Trough, generating a massive tsunami that devastated coastal communities ³. More recent events, such as the 1944 Tonankai and 1946 Nankaido earthquakes, ruptured the eastern and western segments, respectively, providing modern research with valuable data on fault mechanics and tsunami generation ¹⁶${}^{,}$³.

Research on the Nankai Trough has revealed that the seismic cycle is driven by elastic strain accumulation at the plate interface, where the subducting Philippine Sea Plate drags the overriding Eurasian Plate downward. This strain is eventually released in catastrophic ruptures, with fault slip vectors and seismic waveforms indicating low-angle thrust faulting along the plate boundary ¹⁷${}^{,}$¹⁶${}^{,}$³. Studies in geodetic imaging have shown variations in slip deficit that correlate with regions of past earthquake ruptures ¹⁸${}^{,}$¹. Additionally, slow-slip events (SSEs) have been observed along the Nankai Trough, which might indicate that transient aseismic slip may be altering stress distribution and influencing the likelihood of future megathrust events ¹⁹.

Caption_right

Tectonic setting of the Nankai Trough megathrust. The grey area indicates the Philippine Sea Plate. The thick arrow indicates the plate convergence direction. The grey arrows indicate ground displacements. The light orange area corresponds to the maximum-possible fault area published by the Japanese Government. From Fukushima et al. (2023)²⁰.

Currently, the Nankai Trough is the subject of intense monitoring and research due to warnings of a potential once-in-a-century earthquake. Studies have identified a high probability of a megathrust event occurring within the next few decades, with simulations predicting that such an earthquake could reach a magnitude of 8.0 to 9.0 and cause widespread damage across western Japan ³${}^{,}$ ¹⁸. The likelihood of successive M8 twin ruptures within a short time frame has been estimated at 4.3–96% ²⁰. Mechanical coupling studies suggest that stress accumulation in specific regions may determine the rupture behavior of future megathrust earthquakes ²¹. The anticipated rupture could involve multiple segments of the trough, similar to the Hoei Earthquake, and generate tsunamis capable of flooding densely populated coastal areas ³.

Solutions and Future Directions

Efforts to understand and reducing the risks associated with the Nankai Trough have led to initiatives such as the Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis (N-net), which aims to fill gaps in seismic monitoring through an expanded offshore observation network ²². Additionally, offshore drilling projects, such as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), have provided insights into the physical properties of the fault zone, including temperature, pressure, and fluid dynamics, which influence earthquake nucleation ²³.

The improvement of seismic and tsunami early warning systems like Japan’s Earthquake Early Warning (EEW) system, operated by the Japan Meteorological Agency (JMA), has already proven effective in providing seconds to minutes of warning before ground shaking begins ²⁴. Kawaguchi et al. (2014)²⁵ discussed the development of the DONET (Dense Ocean-floor Network System) for earthquake and tsunami monitoring in Japan. Another priority is the reinforcement of infrastructure, particularly in regions at high risk of ground shaking and tsunami inundation. Urban planning should prioritize the relocation of critical facilities, such as hospitals and emergency response centers, to higher ground or earthquake-resistant structures. Modifying existing buildings with advanced seismic isolation systems can reduce casualties and economic losses ²⁶${}^{,}$ ²⁷.

Public education and community engagement play an important role in disaster preparation. Regular drills, clear evacuation routes, and accessible information about earthquake and tsunami risks can encourage residents to respond effectively during an emergency ²⁸${}^{,}$ ²⁹. Initiatives which emphasize individual decision-making and swift evacuation, have been credited with saving lives during the 2011 Tōhoku Earthquake and Tsunami and could be adapted for the Nankai Trough region ³⁰${}^{,}$ ³¹.

On the scientific front, continued research into the mechanics of subduction zones and earthquake prediction is essential. Machine learning algorithms and dense sensor networks offer new opportunities for detecting precursory signals and understanding subduction zone mechanics or estimating the probability of earthquake occurrences ³²${}^{,}$ ²⁰.

Footnotes

1. Loveless, J. P. and Meade, B. J. (2010). Geodetic imaging of plate motions, slip rates, and partitioning of deformation in japan. Journal of Geophysical Research: Solid Earth, 115(B2). ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Seno, T., Stein, S., and Gripp, A. E. (1993). A model for the motion of the philippine sea plate consistent with nuvel-1 and geological data. Journal of Geophysical Research: Solid Earth, 98:17941–17948. ↩ ↩²

3. Ando, M. (1975). Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan. Tectonophysics, 27(2):119–140. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

4. Sagiya, T., Miyazaki, S., and Tada, T. (2000). Continuous gps array and present-day crustal deformation of japan. Pure and Applied Geophysics, 157:2303–2322. ↩ ↩²

5. Tsuji, S., Araki, E., Yokobiki, T., et al. (2023). Precise tilt measurement by seafloor borehole tiltmeters at the nankai trough subduction zone. Earth, Planets and Space, 75:188. ↩

6. Nakano, M., Hori, T., Araki, E., Kodaira, S., and Ide, S. (2018). Shallow very-low-frequency earthquakes accompany slow slip events in the nankai subduction zone. Nature Communications, 9(1):984. ↩

7. Takemura, S., Baba, S., Yabe, S., Emoto, K., Shiomi, K., and Matsuzawa, T. (2022a). Source characteristics and along-strike variations of shallow very low frequency earthquake swarms on the nankai trough shallow plate boundary. Geophysical Research Letters, 49(11). ↩

8. Takemura, S., Matsuzawa, T., Noda, A., Tonegawa, T., Asano, Y., Kimura, T., and Shiomi, K. (2019a). Structural characteristics of the nankai trough shallow plate boundary inferred from shallow very low frequency earthquakes. Geophysical Research Letters, 46(8):4192–4201. ↩

9. Takemura, S., Noda, A., Kubota, T., Asano, Y., Matsuzawa, T., and Shiomi, K. (2019b). Migrations and clusters of shallow very low frequency earthquakes in the regions surrounding shear stress accumulation peaks along the nankai trough. Geophysical Research Letters, 46(21):11830–11840. ↩

10. Takemura, S., Obara, K., Shiomi, K., and Baba, S. (2022b). Spatiotemporal variations of shallow very low frequency earthquake activity southeast off the kii peninsula, along the nankai trough, japan. Journal of Geophysical Research: Solid Earth, 127(3). ↩

11. Ogiso, M. and Tamaribuchi, K. (2022). Spatiotemporal evolution of tremor activity near the nankai trough trench axis inferred from the spatial distribution of seismic amplitudes. Earth, Planets and Space, 74(1):49. ↩

12. Tamaribuchi, K., Ogiso, M., and Noda, A. (2022). Spatiotemporal distribution of shallow tremors along the nankai trough, southwest japan, as determined from waveform amplitudes and cross-correlations. Journal of Geophysical Research: Solid Earth, 127(8). ↩

13. Baba, T., Tanioka, Y., Cummins, P. R., and Uhira, K. (2002). The slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model. Physics of the Earth and Planetary Interiors, 132(1–3):59–73. ↩

14. Tanioka, Y. and Satake, K. (2001). Detailed coseismic slip distribution of the 1944 tonankai earthquake estimated from tsunami waveforms. Geophysical Research Letters, 28(6):1075–1078. ↩

15. Flores, P. C. M., Kodaira, S., Kimura, G., Shiraishi, K., Nakamura, Y., Fujie, G., et al. (2024). Link between geometrical and physical property changes along nankai trough with slow earthquake activity revealed by dense reflection survey. Geophysical Research Letters, 51:e2023GL106662. ↩

16. Kanamori, H. (1972). Tectonic implications of the 1944 tonankai and the 1946 nankaido earthquakes. Physics of the Earth and Planetary Interiors, 5:129–139. ↩ ↩²

17. Imamura, A. (1929). On the Chronic and Acute Earth-Tiltings in the Kii Peninsula. Japanese Journal of Astronomy and Geophysics, 7:31. ↩

18. Hirose, F., Nakajima, J., and Hasegawa, A. (2008). Three-dimensional seismic velocity structure and configuration of the philippine sea slab in southwestern japan estimated by double-difference tomography. Journal of Geophysical Research: Solid Earth, 113:1–26. ↩ ↩²

19. Ozawa, S., Muneakane, H., and Suito, H. (2024). Time-dependent modeling of slow-slip events along the nankai trough subduction zone, japan, within the 2018–2023 period. Earth, Planets and Space, 76:23. ↩

20. Fukushima, Y., Nishikawa, T., and Kano, Y. (2023). High probability of successive occurrence of nankai megathrust earthquakes. Scientific Reports, 13:63. ↩ ↩² ↩³

21. Saito, T. and Noda, A. (2022). Mechanically coupled areas on the plate interface in the nankai trough, japan and a possible seismic and aseismic rupture scenario for megathrust earthquakes. Journal of Geophysical Research: Solid Earth, 127:e2022JB023992. ↩

22. Aoi, S., Takeda, T., Kunugi, T., Shinohara, M., Miyoshi, T., Uehira, K., Mochizuki, M., and Takahashi, N. (2023). Development and construction of nankai trough seafloor observation network for earthquakes and tsunamis: N-net. In 2023 IEEE Underwater Technology (UT), pages 1–5. ↩

23. Tobin, H. J. and Kinoshita, M. (2006). Nantroseize: The iodp nankai trough seismogenic zone experiment. Scientific Drilling, 2:23–27. ↩

24. Kodera, Y., Hayashimoto, N., Tamaribuchi, K., Noguchi, K., Moriwaki, K., Takahashi, R., Morimoto, M., Okamoto, K., and Hoshiba, M. (2021). Developments of the nationwide earthquake early warning system in japan after the 2011 mw9.0 tohoku-oki earthquake. Frontiers in Earth Science, 9. ↩

25. Kawaguchi, K., Araki, E., Hoshino, M., Yokobiki, T., Matsumoto, H., Nishida, S., and Kaneda, Y. (2014). Decision-making on seafloor surveillance infrastructure site for earthquake and tsunami monitoring in western japan. In OCEANS 2014 - TAIPEI. ↩

26. Lekkas, E., Emmanuel, A., Alexoudi, V., Kapourani, E., and Kostaki, I. (2012). The mw=9.0 tohoku japan earthquake (march 11, 2011) tsunami impact on structures and infrastructure. ↩

27. Reza, N., Opdyke, A., and Ochiai, C. (2024). Disrupted sense of place and infrastructure reconstruction after the great east japan earthquake and tsunami. Progress in Disaster Science, 22:100322. ↩

28. Sakurai, A., Sato, T., and Murayama, Y. (2020). Impact evaluation of a school-based disaster education program in a city affected by the 2011 great east japan earthquake and tsunami disaster. International Journal of Disaster Risk Reduction, 47:101632. ↩

29. Nakai, F. and Nakano, G. (2023). Community-mediated individual disaster preparedness practices: A case study in kochi, japan. International Journal of Disaster Risk Reduction, 86:103532. ↩

30. Grau Vila, C. (2024). Tohoku resilient schools and youth engagement in memory after 2011 tsunami. Disaster Prevention and Management, 33(5):646–662. ↩

31. Katayama, A., Hase, A., and Miyatake, N. (2021). Disaster prevention education along with weekly exercise improves self-efficacy in community-dwelling japanese people-a randomized control trial. Medicina (Kaunas, Lithuania), 57(3):231. ↩

32. Fayaz, J. and Galasso, C. (2024). Interpretability and spatial efficacy of a deep-learning-based on-site early warning framework using explainable artificial intelligence and geographically weighted random forests. Geoscience Frontiers, 15(5):101839. ↩