Fault geometry affecting spatial distribution and evolution of fracture zones

Niwa, Masakazu   

Larger fault zones are developed as a result of coalescence of multiple faults. To understand fracture zone development in such larger fault zones, we surveyed the spatial distribution and characteristics of fracture zones in areas around the Atotsugawa fault zone in central Japan. Within 500 m of the fault trace, the number of exposed fracture zones increases sharply. Though several concepts have been proposed that fracture zones are pervasively developed around a compressional step where two faults are opposed each other, the number and width of the exposed fracture zones display almost the same pattern between a compressional step and a long strand. Fracture zones less than 2 m width are sparsely but widely distributed in the study area. From a viewpoint of their spatial distribution and deformation structure, the small fracture zones possibly indicate an evidence of shallow earthquakes at a point distant from the fault trace.