Source Properties of the Induced ML 0.0–1.8 Earthquakes from Local Beamforming and Backprojection in the Helsinki Area, Southern Finland

Abstract

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.