2.1. Damage zone characterization
In order to characterize the damage zone along the Atera Fault, the
occurrence and characteristics of fractures reported by previous studies
(Oshima and Yoshida, 2004; Nagatomo and Yoshida, 2009; Yoshida
et al., 2009) were compiled. These characteristics include fracture
density, fracture length and filling minerals from more than 100 outcrops
identified in an area of several square kmalong the fault. Detailed
fracture mapping was also carried out at major outcrops using a square
metre grid-mappingmethod and all fractureswith lengths ofmore than
0.5mwere traced. The traceability of fractures in the damage zonewith
lengths less than 0.5 m is not clear due to the dense fracture network.
The ‘density’ was defined by the number of fractures counted over
every metre of a grid line. The average was calculated by dividing the
total number by the number of grids at each outcrop. The density is
therefore an ‘apparent’ density observed at the surface in one dimension
rather than three dimensions. These outcrop observations have been
compiledwith data obtained at larger spatial scales than individual outcrops
including trace length of fractures, fracture frequency and fracture
filling minerals (Oshima and Yoshida, 2004).
2.1. Damage zone characterizationIn order to characterize the damage zone along the Atera Fault, theoccurrence and characteristics of fractures reported by previous studies(Oshima and Yoshida, 2004; Nagatomo and Yoshida, 2009; Yoshidaet al., 2009) were compiled. These characteristics include fracturedensity, fracture length and filling minerals from more than 100 outcropsidentified in an area of several square kmalong the fault. Detailedfracture mapping was also carried out at major outcrops using a squaremetre grid-mappingmethod and all fractureswith lengths ofmore than0.5mwere traced. The traceability of fractures in the damage zonewithlengths less than 0.5 m is not clear due to the dense fracture network.The ‘density’ was defined by the number of fractures counted overevery metre of a grid line. The average was calculated by dividing thetotal number by the number of grids at each outcrop. The density istherefore an ‘apparent’ density observed at the surface in one dimensionrather than three dimensions. These outcrop observations have beencompiledwith data obtained at larger spatial scales than individual outcropsincluding trace length of fractures, fracture frequency and fracturefilling minerals (Oshima and Yoshida, 2004).
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