Variability of seismicity rates and maximum magnitude for adjacent hydraulic stimulations

We hindcasted the seismicity rates and the next largest earthquake magnitude using seismic and hydraulic data from two hydraulic stimulation campaigns carried out in adjacent (500 m apart) ultra-deep wells in Finland. The two campaigns performed in 2018 and 2020 took place in the frame of the St1 Helsinki project producing stable, pressure-controlled induced seismic activity with the maximum magnitudes of M (sub w) 1.7 and 1.2, respectively. The seismicity rates were modeled using simplified physics-based approaches tailored to varying injection rates. This is the first time that this framework was applied to a cyclical injection protocol. The next largest earthquake magnitude was estimated using several existing models from the literature. Despite the close proximity of the two hydraulic stimulations and associated seismicity, we obtained strongly different parameterizations of the critical model components, questioning the usefulness of a priori seismic hazard modeling parameters for neighboring stimulation. The differences in parameterization were attributed to the contrasting hydraulic energy rates observed in each stimulation, small differences in the fracture network characteristics of the reservoir and resulting seismic injection efficiency, and potentially to variations in the injection protocol itself. As far as the seismicity rate model is concerned, despite a good performance during the 2018 campaign, the fit during the 2020 stimulation was suboptimal. Forecasting the next largest magnitude using different models led to a very wide range of outcomes. Moreover, their relative ranking across stimulations was inconsistent, including the situation when the best-performing model in the 2018 stimulation turned out to be the worst one in the 2020 stimulation.