Array-derived rotational motion provides local wave propagation properties of earthquakes induced by the 2018 geothermal stimulation experiment in Finland

Abstract:

A proportional relationship exists between the translational acceleration and the vertical rotation rate induced by horizontally-polarised S-waves. In an homogeneous elastic medium, the waveform of SH-waves recorded perpendicular to the propagation direction will differ from that of the rotation rate by a factor of 2c, where c is the local phase speed. The propagation direction will usually, though not always perfectly, align with the back-azimuth of the seismic source. In this study, we apply wavefield gradiometry at several arrays of translational seismometers to estimate vertical rotation rates for 204 induced earthquakes that occurred during the 2018 stimulation of the Espoo/Helsinki geothermal reservoir. The three 25- and three 4-station arrays are located at hypocentral distances of 7-10 km, with the events varying in magnitude from M0.0 to M1.8, at depths betweeen 4.8 and 6.3 km. Our estimates of vertical rotation rates vary between 10^-7-10^-9 rad s^-1, indicating that array-derived rotation estimates have a similar sensitivity as more sophisticated and expensive instruments, such as ring-lasers. Our subsequent estimates of earthquake back-azimuth consistently outperform conventional S-wave beamforming in predicting the true back-azimuth, with an average misfit improvement of 48.1%. Both approaches show consistent deviations from great-circle propagation, indicating heterogeneous velocity structure, with the array-derived measurements able to resolve velocity variations over the 100 m aperture of the seismic arrays. We measure apparent S-wave speeds of ~6 km s^-1, consistent with steep incidence angles and high propagation velocities in the crystalline rocks of the Fenno-Scandian Shield. The reliable measurement of rotation rates as small as 10^-9 rad s^-1 obtained from small and micro-earthquakes suggests that arrays of translational sensors can play an integral role in providing observations of rotational motion in weak-motion seismology.