The preferred orientation pattern of minor fractures in a geologic body is influenced by the stress condition in the earlier stage of the fracture formation process.
In experiments, tension fractures were observed at first on the surface of the clay-cake mounted on the two iron plates sliding each other. The tension fractures increased in number and lined on the conjugate zones which made an angle of about 30 degrees with the tension fractures (Fig. 3). Namely they reveal en echelon structures. The tension fractures on a zone were connected with secondary fractures into a shear fracture. The shear fractures had zigzag or feather stuctures. The earlier fractures remained as minor fractures.
The preferred orientation patterns of minor fractures were presumed in the following stress conditions.
In case the principal stresses except the maximum stress are equal (tension is positive): The planes of tension fractures are perpendicular to the maximum stress axis. Shear fractures make an angle of about 60 degrees with the maximum stress axis. Using Wulf's net, the poles of the shear fractures lie on a small circle girdle whose pole coincides with the direction of the maximum stress and whose mean radius is about 30 degrees.
Due to uncertainties and errors, poles of shear fractures and tension fractures may fill up a circle whose pole is the direction of the maximum stress and whose radius is a little larger than 30 degrees.
In case the principal stresses except the minimum stress are equal: Shear fractures make an angle of about 30 degrees with the minimum stress axis. The fabric pattern is a small circle girdle whose pole coincide with the direction of the minimum stress and whose mean radius is 60 degrees (Fig. 11).
These patterns show axial symmetric. But, if the rock body has parallel planes of weakness, the symmetry of pattern may be monoclinic.
In case the principal stresses are different each others: In is assumed that fractures may take place only on weak planes in a rock body. A weak plane slips if the shearing stress on the plane gets over its resistance. If the stresses are statistically homogeneous in the rock body, an apparent Mohr's circle may be assumed as a whole. Statistically, fractures take place on the weak planes whose poles on the apparent Mohr's diagram reach their own Mohr's envelopes. Each plane has one's own Mohr's envelope. The apparent Mohr's circle, as a whole, is cut by the Mohr's envelopes on weak planes. The apparent Mohr's circle and the envelopes on weak planes give possible orientation patterns of fractures (Fig. 12).

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