The robustness of seismic moment and magnitudes estimated using spectral analysis

We present an assessment of how microseismic moment magnitude, MW, estimates vary with the method and parameters used to calculate seismic moment. This is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity. It is therefore imperative that these parties know and understand what errors exist in given magnitude values, something that is poorly reported. This study concentrates on spectral analysis techniques and compares MW computed in the time and frequency domains. Using recordings of MW > −1.5 events at Cotton Valley, east Texas, the maximum discrepancy between MW estimated using the different methods is 0.6 units, a significant variation. By adjusting parameters in the MW calculation we find that the radiation pattern correction term can have the most significant effect on MW, generally up to 0.8 units. Following this investigation we make a series of recommendations for estimating microseismic MW using spectral methods. Noise should be estimated and removed from recordings and an attenuation correction should be applied. The spectral level can be measured by spectral fitting or taken from the low frequency level. Significant factors in obtaining reliable microseismic MW estimates include using at least four receivers recording at ≥1000 Hz and making radiation pattern corrections based on focal mechanism solutions, not average values.