Nonsystematic Rupture Directivity of Geothermal Energy Induced Microseismicity in Helsinki, Finland

The rupture behavior of microseismicity in fluid-injection settings with low fault stresses is generally believed to be controlled by the pore pressure, including a tendency of the larger induced earthquakes to rupture into the perturbed volume toward the injection well, implying a degree of predictability. Here, we examine directivity patterns to identify fault planes and rupture directions of the 21 largest earthquakes (local magnitudes, M-L 1.3-1.9) recorded during the 2018 St1 Deep Heat geothermal project near Helsinki, Finland. We use the Empirical Green's Function technique to retrieve per-seismic-station corner frequencies, earthquake durations, and directivity trends. After combining the directivity trends with focal mechanisms calculated using principle component analysis, we resolve rupture planes and rupture directions of 10 events. In contrast to studies of induced events at other sites, we find that one event rupture toward, two rupture away, and the remaining rupture parallel to the well. Furthermore, we find that the events prefer mode II failures rather than mode III. These observations provide new constraints for mechanical models of rupture growth in pore-pressure dominated settings.