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The Rigakubu News
The Rigakubu News Sep. 2025
Advancing Science >

~ Message from a graduate student~.
What Does the Earth’s Whisper Tell Us About Giant Earthquakes?

Seiya Yano

Department of Earth and Planetary Science (GR) Doctoral Student

Place of birth Saitama, Japan

Department of Physics Faculty of Science, The Tokyo University of Science

Earthquakes—when people hear this word, many imagine a violent phenomenon accompanied by strong shaking. Others may think of the small rattling of the initial tremor (P-waves) followed by the heavy shaking of the main tremor (S-waves). Both devastating large earthquakes and imperceptible microearthquakes in daily life are, in most cases, composed of P-waves and S-waves (Figure A). However, only about two decades ago, researchers discovered a new seismic phenomenon distinct from these typical earthquakes. This is tectonic tremor. The waveform of tectonic tremor shows characteristics that are clearly different from ordinary earthquakes (Figure B). In most cases, clear P-waves cannot be identified, and the shaking associated with S-waves gradually intensifies over time and then slowly subsides.

Tectonic tremor is thought to occur when deep rock at plate boundaries slips and fractures slowly. Its shaking is so weak that humans cannot perceive it at all. Even with highly sensitive seismometers, it can barely be detected, if at all. Nevertheless, in recent years, this subtle tremor has attracted significant attention. The reason is that tectonic tremor may hold important clues for understanding the mechanisms of giant earthquakes and predicting their occurrence.

Tectonic tremors are mainly observed around regions where giant earthquakes have repeatedly occurred. Their occurrence is not entirely random but shows a certain periodicity. In addition, seismic sources sometimes migrate along plate boundaries (Figure C). Such behavior is thought to reflect, in real time, changes in the way stress is applied at plate boundaries. In particular, when tectonic tremor exhibits “anomalous” behavior different from its usual pattern, it may indicate large-scale changes in the state of the plate boundary, and perhaps even the preparatory process of a giant earthquake.

At present, I am conducting research to detect tectonic tremor with high accuracy and to identify changes in the state of plate boundaries through analysis of occurrence patterns. By utilizing machine learning, a rapidly developing branch of artificial intelligence (AI), I am attempting to comprehensively detect tectonic tremors that were overlooked by conventional methods. Furthermore, I am developing techniques to extract the “anomalous” behavior of tectonic tremor by mathematically modeling its temporal and spatial patterns. Ultimately, my goal is to establish a method for real-time assessment of the risk of giant earthquakes through tectonic tremor observation.

Great discoveries sometimes lie hidden in what appears to be insignificant or inconspicuous. By listening closely to tectonic tremor—the whispers of the Earth that almost no one notices—I am striving to gradually reveal the invisible world deep underground. To me, seismology, and more broadly natural science, is about the pursuit of the essence of nature: probing what is happening inside the Earth using indirect clues such as seismic waves. This pursuit is, for me, the greatest fascination of science.

(A) Waveform of a typical earthquake. (B) Waveform of tectonic microseismic activity. (C) Movement of the epicenter from southwest to northeast over approximately one month.