Abstract

Fault damage zones record the evolution of brittle deformation and play a key role in controlling rock strength and fluid flow. In this study, we quantitatively evaluate the width and internal architecture of the fault damage zone along the Median Tectonic Line (MTL) in southwest Japan using an integrated geological and geophysical approach. A 125-m-long drilled core penetrating the MTL was analyzed using detailed geological core observation, fracture analysis, X-ray computed tomography (X-CT), and borehole geophysical logging data. We developed and applied a new image-processing method to automatically extract open crack parameters, including crack density, intensity, and mean length, from X-CT images. Cumulative frequency curves of open cracks reveal a highly asymmetric open crack–based damage zone (OCDZ): ~24 m wide in the hanging wall and only ~2 m in the footwall. This asymmetry is further supported by variations in rock rigidity. A reanalysis of published displacement–damage zone width scaling shows that lithology exerts a stronger control than structural position, consistent with the mechanical contrast between the weaker Izumi Group sedimentary rocks in the hanging wall and the stronger Sambagawa metamorphic rocks in the footwall. Depth constraints indicate that the OCDZ formed at shallow crustal levels (<~3.5 km　depth) under a normal-faulting regime during 15–14 Ma, consistent with conditions inferred from clay mineral stability. Our integrated approach, combining geological and geophysical datasets from drilled cores, provides a framework for characterizing fault damage zones at shallow crustal levels. Such quantitative characterization is essential for constraining the conditions and timing of damage zone development in long-lived faults such as the MTL, where multiple deformation phases are superimposed.