Bilinear Magnitude-Frequency Distributions and Characteristic Earthquakes During Hydraulic Fracturing

Abstract

Previous studies show that a temporal drop in the b value (slope) of magnitude-frequency distributions observed during hydraulic-fracturing operations could signify the activation of a preexisting fault system. Based on a new data set from Alberta, Canada, we provide a case study wherein events induced during hydraulic fracturing are localized within spatial clusters with a range of b values from ~1.0 to ~2.5. The distribution in b values is related to the orientation and depth distribution of these clusters. As a consequence of the superposition of spatially varying clusters, the catalog for the entire data set yields a bilinear magnitude distribution with exceptionally low apparent b value at larger magnitude levels. Several clusters are compatible (at 95% confidence level) with the characteristic-earthquake hypothesis, a controversial model for some fault systems wherein episodic large ruptures occur significantly above the maximum-likelihood Gutenberg-Richter relationship.