The importance of accurate and consistent microseismic event magnitudes

Seismicity induced by fluid injection during hydraulic fracturing; carbon capture and storage (CCS); and geothermal developments is of concern to industry and the public. However, the magnitudes of such events are often reported without a clear understanding of the meaning or implications. Using microseismic events recorded at injection sites we show how moment magnitude estimates depend on the monitoring set-up and the calculation, in the time or frequency domain. Magnitude estimates, and in particular moment magnitude estimates, vary by tenths of a magnitude unit depending on the geophone array configuration and how the calculation is made. This raises important questions for the monitoring of industrial activities, such as hydraulic fracturing, if the regulation of such activities is to be based on the magnitude of seismic events induced by industrial projects.