[P-EM14] Study of coupling processes in solar-terrestrial system

[PEM14-P20] Developments in Fault-Agnostic Earthquake Modeling and Its Relations with the Sun and the Moon

*Matheus Henrique Junqueira Saldanha¹, Masanori Shiro², Yoshito Hirata¹ (1.University of Tsukuba, 2.National Institute of Advanced Industrial Science and Technology)

Keywords:earthquakes, dynamical systems, solar activity, moon, event series

Despite extensive research in earthquake forecasting, accurately predicting large seismic events remains a significant challenge. One reason for this is the existence of various other natural systems that could be interacting with the movement of tectonic plates, and we do not know which of them are in fact significant to seismogenesis, nor do we know how exactly the interaction between them takes place. Within the past few years, our research group has attempted to approach this question, and here we summarize our findings, focused in next-day fault-agnostic earthquake prediction. In [3], MHJS and YH proposed a forecasting model for next-day forecasting that leverages earthquake point patterns and radial basis functions to perform forecasts. With this model, we were able to perform various investigations concerning the influence of external systems on seismicity on Earth, especially those concerning the Sun and the Moon. An overview of how these two entities could affect earthquakes is shown in Fig. 1. Also in [3], MHJS and YH showed a cause-effect connection between the Sun and earthquake activity on Earth. The investigation employed mathematical techniques, namely the coupling detection methods of Andrzejak and Kreuz [1] and Hirata et al. [2], to assess the coupling between these two dynamic systems, and a significant unidirectional coupling from the Sun to seismic activity on Earth was found. It was also found that using solar activity data with our prediction model enhances forecasting accuracy, thus further reinforcing the argument that the Sun is coupled unidirectionally to earthquakes in Earth. In [4], the authors focused on the effects exerted by the Moon, in particular those related to the tidal force exerted on Earth. Tidal forces are hypothesized to affect earthquakes because it deforms not only the ocean levels, but also the solid Earth’s crust, which could be responsible for triggering fractures. Using similar methods as in [3], it was found that the Moon is also coupled to the seimogenic process on Earth. This served to strengthen the hypothesis that the gravity of the Moon has an influence on earthquake occurrence. Finally, since in [3] the authors established a causal link between the Sun and earthquakes on Earth, but did not provide insights on how exactly the Sun can actually provoke earthquakes to occur, we analyzed the problem further in [5]. In particular, since gravitational and magnetic mechanisms received far more attention in the literature, we were particularly worried that the solar heat mechanism might have been neglected thus far. By analyzing surface temperature records, sunspots data and earthquake catalogs, we managed to find various pieces of evidence supporting the significance of solar heat, such as: (i) there is a significant delay between solar activity and its effects on earthquakes; (ii) surface air temperatures can complement the sunspot numbers toward higher forecasting accuracy for the next day’s maximum magnitude; among others. With these results, we believe to have contributed to the problem of fault-agnostic next-day forecasting by proposing a novel forecasting model for the problem, and by investigating precursor data (namely data from the Sun, the Moon and surface temperatures), and successfully using these data to improve the forecasting accuracy achieved by our prediction model, when compared to using earthquake catalog data alone. Our findings should bring hope that the seismogenic process and all its external factors can be better understood, and ultimately result in forecasting models that can save numerous lives in the future. References [1] Ralph G Andrzejak and Thomas Kreuz. Characterizing unidirectional couplings between point processes and flows. Europhysics Letters, 96(5):50012, 2011. [2] Yoshito Hirata, José M. Amigó, Yoshiya Matsuzaka, et al. Detecting causality by combined use of multiple methods: Climate and brain examples. PLOS ONE, 11(7):e0158572, 2016. [3] Matheus Henrique Junqueira Saldanha and Yoshito Hirata. Solar activity facilitates daily forecasts of large earthquakes. Chaos, 32(6):061107, 2022. [4] Matheus Henrique Junqueira Saldanha and Yoshito Hirata. Next-day largest earthquake magnitude forecasting with the aid of moon tidal force and sunspot data. Journal of Physics: Complexity, 5(2):025015, 2024. [5] Matheus Henrique Junqueira Saldanha, Shiro Masanori, Yagi Yuji, and Yoshito Hirata. The role of solar heat in earthquake activity. Chaos, 2025. Forthcoming