FIG. F-10. First motion (focal mechanism) in an earthquake. ‘‘Beach balls’’ (Sykes diagrams) are map views showing where the first motion is compressional (black) and dilatational (white); the epicenter being at the center of each diagram. (a) Map view of a strike-slip fault and its beach ball; (b) vertical section of a reverse fault and its beach-ball map; (c) vertical section of a normal fault and its beach-ball map; (d) Map of earthquakes in the Mediterranean region. (After McKenzie, 1972).