1. A computer-implemented method for identifying dominant fracture orientations in a subterranean zone, the method comprising: receiving microseismic event data associated with a fracture treatment of a subterranean zone; identifying coplanar subsets of microseismic events in the received microseismic event data, each of the coplanar subsets comprising a non-collinear triplet of microseismic events in the received microseismic event data; determining a basic plane orientation for each of a plurality of basic planes defined by the coplanar subsets, each basic plane orientation being defined by the non-collinear triplet of microseismic events in a respective one of the coplanar subsets; calculating, by data processing apparatus, the quantity of the basic plane orientations in each of a plurality of orientation ranges; identifying a dominant fracture orientation for the subterranean zone based on one or more of the quantities; identifying a cluster of microseismic events associated with the identified dominant fracture orientation; generating a fracture plane based on the identified dominant fracture orientation and the identified cluster of microseismic events associated with the identified dominant fracture orientation; and displaying a histogram of the basic plane orientations, wherein the histogram indicates the quantity of basic plane orientations in each of the orientation ranges.