Magnitude Scaling of Induced Earthquakes

Presented are the results of an earthquake magnitude homogenization exercise for several datasets of induced earthquakes. The result of this exercise is to show that homogeneous computation of earthquake momentand local-magnitude is useful in hazard assessment of Enhanced Geothermal Systems (EGSs). Data include records from EGSs in Basel (Switzerland), Soultz (France) and Cooper Basin (Australia); natural geothermal fields in Geysers (California) and Hengill (Iceland), and a gasfield in Roswinkel (Netherlands). Published catalogue magnitudes are shown to differ widely with respect to Mw, with up to a unit of magnitude difference. We explore the scaling between maximumamplitude and moment-related scales. We find that given a common magnitude definition for the respective types, the scaling between momentand local-magnitude of small earthquakes follows a second-order polynomial, consistent with previous studies of natural seismicity. Using both the Southern-California ML scale and a PGV-magnitude scale (Mequiv) determined in this study, we find that the datasets fall into two subsets with well-defined relation to Mw: Basel, Geysers and Hengill in one and Soultz and Roswinkel in another (Cooper Basin data were not considered for this part of the analysis because of the limited bandwidth of the instruments). Mequiv was shown to correlate 1:1 with ML, albeit with region-specific offsets, while the distinct subsets in the Mequiv to MW scaling led us to conclude that source and/or attenuation properties between the respective regions were different.