Local beamforming and back-projection of induced earthquakes in Helsinki, southern Finland

Seismic array processing is routinely used to infer detailed earthquake properties of intermediate and large events, however, the source properties of microseismicity often remain elusive. In this study, we use high signal-to-noise ratio seismograms of 204 earthquakes induced by the 6 km deep 2018 Espoo/Helsinki geothermal stimulation to evaluate the capabilities of beamforming and back-projection array methods. We show that mini array beamforming is sensitive to medium heterogeneity and requires calibration to mitigate systematic slowness biases.A combined and wave back-projection approach significantly improves depth resolution, reducing offsets to catalogue locations from km to m. Supported by numerical experiments, we demonstrate that back-projection swimming patterns can constrain focal mechanisms. Our results imply that back-projection of data collected over a wide azimuthal range can be used to monitor and characterize local-scale microseismicity, whereas beamforming calibration requires independently obtained reference observations.