Understanding the source characteristics of hydraulic‐fracturing‐induced microearthquakes is expected to provide better understanding of both the fracturing process and the influence of pre‐existing structures on the distribution of events. However, details of the source characteristics of microearthquakes remain largely unknown. One controversial issue is whether the mechanism of such events differs from natural tectonic events, that is, whether significant reductions of effective stress and/or volumetric change occur because of the stimulation. Herein, to address this question, we estimated the apparent stress of the 61 Mw<−0.6 microearthquakes observed during the hydraulic fracturing in the Carthage Cotton Valley gas field. We first obtained the apparent stress by estimating the radiated seismic energy from the duration of the relative moment rate function and then adjusted the values, considering the possibility of seismic energy underestimation. The initial value range of 0.001–0.02 MPa is consistent with other past studies of hydraulic‐fracturing‐induced microearthquakes. Although the adjustment increases the original value by a factor of 10, it is still within the lower bound of the range of constant apparent stress commonly observed for natural tectonic events. Furthermore, relatively lower apparent stress was observed for the events striking subparallel to the maximum horizontal compressional stress axis. This implies the mechanism of these events could be different from that of events optimally oriented for failure, although the uncertainty of their measurements was expected to be large compared with other events because of their small magnitude.
Online Material: Table with information on microearthquakes observed during the hydraulic stimulation in the Carthage Cotton Valley gas field.