资源环境科技发展态势分析平台

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 similar to 1.0 to similar 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.

Plain Language Summary The b value, or slope, of the semilogarithmic magnitude-frequency distribution quantifies the relative distribution of large and small events. Variability in the b value over space and time provides insight into the changes of behavior within the subsurface. We show that there is significant variability in b values at both regional and local scales in Alberta, Canada. Using a new hydraulic-fracturing data set, we show that there is an apparent bilinear magnitude distribution. By partitioning the seismic events into spatial and temporal clusters, we show they have varying b values and their superposition explains the bilinear behavior. Furthermore, we show that b value is strongly influenced by the orientation and depth expression of the faults. A number of moderate-magnitude events fall above the 95% confidence bounds for maximum-likelihood Gutenberg-Richter distributions, which is consistent with the characteristic-earthquake hypothesis. This hypothesis states that over long timescales, episodic rupture occurs over large fault segments such that magnitude-frequency distributions observed during the interseismic period between characteristic events underestimate the probability of future large events. This could have important implications for quantifying hazard from injection-induced earthquakes.