1. A computer-implemented method for analyzing microseismic data from a subterranean zone, the method comprising: from a plurality of basic planes, each defined from a subset of the microseismic data and each having an orientation relative to a common axis, updating, by data processing apparatus, clusters of orientations of the basic planes identified adaptively based on the extent of variation in the orientations; and identifying the number of orientations associated with each of the clusters.