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Abstract Seismic arrays constrain local wave propagation that can be used to infer earthquake source characteristics. 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 ML 0.0–1.8 earthquakes induced by the 6 km deep 2018 Espoo/Helsinki geothermal stimulation to evaluate the performance and capabilities of beamforming and backprojection array methods. Using accurate travel-time-based event locations as a reference, we first show that miniarray beamforming is sensitive to medium heterogeneities and requires calibration to mitigate local systematic slowness biases. A catalog-based calibration significantly improves our multiarray beam raytracing estimates of source locations. Second, the application of the backprojection technique using P-wave signals with sufficient azimuthal coverage yields hypocenter estimates with generally good horizontal but poor vertical resolution. The short local source–receiver distances result in incomplete separation of P- and S-wave arrivals during backprojection. Numerical tests show that the relatively large S-wave amplitudes can influence coherent P-wave stacks, resulting in large location errors. Our combined P- and S-wave backprojection approach mitigates the influence of the large S-wave amplitude and improves the depth resolution significantly. The average depth offset to the reference catalog locations reduces from ≥1.4 km to ∼91 m. Third, 3D numerical simulations demonstrate that backprojection swimming patterns are not merely processing or configuration artifacts. We show that the swimming patterns correlate with and can resolve the source focal mechanism when the azimuthal wavefield sampling is sufficiently complete. Our work demonstrates that the backprojection techniques can help to better constrain important properties of local-scale microseismicity. 

ABSTRACT Seismic waves can couple with the atmosphere and generate sound waves. The influence of faulting mechanisms on earthquake sound patterns provides opportunities for earthquake source characterization. Sound radiated from earthquakes can be perceived as disturbing, even at low ground-shaking levels, which can negatively impact the social acceptance of geoengineering applications. Motivated by consistent reports of felt and heard disturbances associated with the weeks-long stimulation of a 6-km-deep geothermal system in 2018 below the Otaniemi district of Espoo, Helsinki, we conduct fully coupled 3D numerical simulations of wave propagation in the solid Earth and the atmosphere. We assess the sensitivity of the ground shaking and audible noise distributions to the source geometry of the induced earthquakes based on the properties of the largest local magnitude ML 1.8 event. Utilizing recent computational advances and the open-source software SeisSol, we model seismoacoustic frequencies up to 25 Hz, thereby reaching the lower limit of the human audible sound frequency range. We present synthetic distributions of shaking and audible sounds at the 50–100 m scale across a 12 km × 12 km area and discuss implications for better understanding seismic nuisances in metropolitan regions. In five 3D coupled elastic–acoustic scenario simulations that include data on topography and subsurface structure, we analyze the ground velocity and pressure levels of earthquake-generated seismic and acoustic waves. We show that S waves generate the strongest sound disturbance with sound pressure levels ≤0.04 Pa. We use statistical analysis to compare our noise distributions with commonly used empirical relationships. We find that our 3D synthetic amplitudes are generally smaller than the empirical predictions and that the interaction of the source mechanism-specific radiation pattern and topography can lead to significant nonlinear effects. Our study highlights the complexity and information content of spatially variable audible effects associated with small induced earthquakes on local scales. 

Seismic array processing is routinely used to infer detailed earthquakeproperties of intermediate and large events, however, the sourceproperties of microseismicity often remain elusive. In this study, weuse high signal-to-noise ratio seismograms of 204 earthquakes induced bythe 6 km deep 2018 Espoo/Helsinki geothermal stimulation to evaluate thecapabilities of beamforming and back-projection array methods. We showthat mini array beamforming is sensitive to medium heterogeneity andrequires calibration to mitigate systematic slowness biases.A combinedand wave back-projection approach significantly improves depthresolution, reducing offsets to catalogue locations from km to m.Supported by numerical experiments, we demonstrate that back-projectionswimming patterns can constrain focal mechanisms. Our results imply thatback-projection of data collected over a wide azimuthal range can beused to monitor and characterize local-scale microseismicity, whereasbeamforming calibration requires independently obtained referenceobservations.