Item

Abstract

The observation of ground motions excited by induced earthquakes near urban areas is of significant engineering importance. During the 2018 geothermal stimulation in the Otaniemi district of Helsinki, Finland, five seismic arrays consisting of 3 to 25 three-component geophones were deployed with interstation distances of 50 meters to record ground motions of 6-km deep induced events. This dataset provides an opportunity to study the ground motion patterns in the low-attenuation environment of the Fennoscandian Shield. The translational seismograms are used to calculate rotational motion for ~400 events with local magnitudes ranging from -0.5 to 1.8 using the seismogeodetic method. We evaluate the relationship between array-derived ground rocking rate (GRR) and vertical ground acceleration (VGA) associated with direct body waves. The robustness of the GRR estimates in the 2-15 Hz frequency range are assessed by comparing the VGA waveforms to the GRRs from the full arrays and to the GRRs from subarrays with different wavelength-to-aperture ratios. By rotating the two perpendicular components of the GRRs, we obtained the radial-transverse coordinate system that minimizes the GRR values on the radial component. We, find different P- and S-wave propagation directions that deviate from the theoretical back-azimuth of the earthquakes, and variations in the apparent body wave velocities beneath the arrays that we attribute to local propagation effects. While the deployment of broadband rotational sensors for wavefield gradiometry analyses is anticipated to become a common practice in the near future, this study provides a low-tech and band-limited experimental verification of the theoretical scaling models.