A method of characterizing a subterranean geologic formation's response to hydraulic fracturing is presented. The method includes acquiring data representing a plurality of seismic events caused by hydraulic fracturing of the subterranean geologic formation, determining, by at least one electronic processor, a plurality of seismic event moment magnitudes for respective seismic events in the plurality of seismic events, estimating, by at least one electronic processor, a first parameter representing a negative slope of a line relating seismic event frequency to seismic event magnitude for at least some of the plurality of seismic events, estimating, by at least one electronic processor, a second parameter representing an axis intercept of the line, and characterizing the subterranean geologic formation based at least in part on the first parameter and the second parameter