** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.
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Research
Solid earth sciences
202109202109
Quantitative determination of the lowest density domain in major fault zones via medical X-ray computed tomography
Iwamori A, Takagi H, Asahi N, Sugimori T, Nakata E, Nohara S, Ueta K
medical X-ray CT, CT number, beam hardening, bulk density, effective atomic number, rock/protolith density ratio, the youngest active domain of a fault zone
X-ray CT image analysis results for fault rock samples (example of MTL Awanodabiki outcrop).
Determination of the youngest active domains in fault zones that are not overlain by Quaternary sedimentary cover is critical for evaluating recent fault activity, determining the current local stress field, and mitigating the impacts of future earthquakes. Considering the exhumation of a fault zone, the youngest active domain in a fault zone is supposed to correspond to the activity at the minimum fault depth of a buried fault, such that the most vulnerable area, which possesses the lowest rock/protolith density ratio, is assumed to be indicative of this recent fault activity. However, it is difficult to measure the density of fault rocks and map the rock/protolith density ratio across a given fault zone. Here, we utilize medical X-ray computed tomography (CT), a non-destructive technique for observing and analyzing materials, to investigate the fault characteristics of several fault zones and their surrounding regions in Japan, and attempt to determine the lowest density domain of a given fault zone based on its CT numbers, which are a function of the density and effective atomic number of the fault rock and protolith. We first investigate the density, void ratio, and effective atomic number of active and inactive fault rocks, and their respective protoliths. We then calculate the CT numbers after reducing the beam-hardening effects on the rock samples and study the relationships among the CT number, density, and effective atomic number. We demonstrate that the density, effective atomic number, and CT number of the fault rock decrease as the youngest active zone, identified by outcrop observation, are approached, such that the region with the lowest CT number and rock/protolith density ratio defines the lowest density domain of a given fault zone. We also discuss the relationship between the lowest density domain and the youngest active domain in major fault zones and investigate the points to be considered when the youngest active domain is identified from the lowest density domain determined by the CT number.
