ABSTRACT
Data on more than 22,000 fracture traces within the Upper Devonian and Lower Mississippian strata exposed in northeastern Ohio reveal two pervasive fracture sets. One set, the master shear set, is a double set of fractures intersecting at an angle of 27°, with an average median orientation of N51°W. The smooth, clean fracture planes with occasional rib markings indicate that this set resulted from shear stresses. The other set, the subsidiary tensional set, consistently trends at 90° to the master shears.
These two sets may be extended as far south as central Pennsylvania and as far east as southeastern New York state, thus defining the fracture pattern of the northern Appalachian basin. The regional fracture pattern is one of a radial set of fractures (master shears) accompanied by a concentric pattern of perpendicular fractures (subsidiary tensional set).
Because of the stress conditions necessary to produce this fracture system, it is believed that the fractures were formed during two distinct structural episodes: the master shears developed during an earlier period of compressional basin formation, and the subsidiary tensional set developed during a later period of tensional relaxation.
Other major fracture sets are well developed in isolated areas of Cuyahoga County. Superimposed on the major fracture sets is a random, fine-scale fracture network which constitutes 50.8% of all fractures.
The major fracture sets conceivably provide pathways for the migration of natural gas. In northeastern Ohio, the only commercial production of natural gas is in areas of apparently increased fracture intensity.
