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During an earthquake rupture, strained blocks of rock material rapidly move against each other. During this process, the immediate vicinity of the earthquake hypocenter, usually illustrated as an imaginary sphere of unit radius (focal sphere), is deformed in a quadrilobed pattern. Two quarters of this sphere (white) undergo compression while the two others (orange) are relaxed. The orientation of these quarter-spheres in space depends on the geometry of the earthquake fault and the direction of slip (vector ) on it. The orientation of maximum compression is called the axis while the direction of maximum dilatation is called the axis.