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Estimating the energy of past earthquakes from fracture in the fault zone

Estimating the energy of past earthquakes from fracture in the fault zone
Estimating the energy of past earthquakes from fracture in the fault zone


During a 2017 field research trip to the Ichinokawa Mine (Ehime Prefecture), famous for its beautiful sword-shaped stibnite crystals, Noriyoshi Tsuchiya discovered something unexpected. Although most people were dazzled by the sparkling crystals, it was a bundle of sedimentary rocks called breccia that caught his attention.

“I couldn't stop thinking about the breccia,” explains Tsuchiya (a professor at the Graduate School of Environmental Studies and Hachinohe National College of Technology). “We visited the mine several times and discovered that the breccia records traces of earthquakes and tremors.” “It provides valuable evidence for estimating the energy of past earthquakes.”

In the same way that the number of a tree's rings can tell us about its age, the characteristics of rocks like breccia can tell us about the history of an area. The Ichinokawa breccia is unique in that it can preserve a record of recurring seismic activity that occurs along the mesotectonic line (MTL). The MTL is a fault line that runs about 1,000 kilometers along the southwestern region of Japan. This makes the breccia formed in this area of ​​special interest to researchers.

In this study, Tsuchiya and their team evaluated fragmented rocks in the field and in the laboratory by extracting sections (slices so thin that they allow light to penetrate) from the collected rocks to observe them under a microscope. The energy dissipated by the past earthquake was successfully estimated on the basis of statistical and fractal analyzes of the angular deformation and crushed texture of the rocks from micro to macro scale.

He found that the surface energy required to explain the normal distribution of fractured rocks was much greater (about 100 times greater) than the surface energy required from a single impact fracture experiment on a rock performed in the laboratory.

Many factors were evaluated in order to calculate the energy and nature of the earthquake. For example, close observation of breccia has revealed the formation of carbonates such as CaMg(CO3)2. Since the host rocks that were fractured did not contain any carbonates, it was concluded that this mineral must have formed after the formation of breccia. They concluded that the earthquake did not occur just once, but repeatedly, crushing fine particles into smaller particles. Their findings indicate that the Ichinokawa breccia was formed by 10-100 earthquakes with an instantaneous magnitude (an index of seismic energy) of 5.8-8.3 Mw.

Further analysis revealed that the breccia had a very unique pattern of fragmentation (or crushing). “Previous models designed to explain the history of earthquakes used a different theory, mainly based on hydraulic fracturing. However, we adopted an interdisciplinary approach, so our analysis can be used to propose a new model that takes more factors into account,” Tsuchiya adds.

This study, which was in collaboration with the National Institute of Technology and Hachinohe College, may redefine our understanding of the cosmic energy budget in this region.

These results were published in the journal Scientific Reports on May 27, 2024.




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