Induced earthquake source parameters, attenuation, and site effects from waveform envelopes in the Fennoscandian Shield

TL;DR

This study analyzes waveform envelopes from induced earthquakes in the Fennoscandian Shield to determine source parameters, attenuation, and site effects, providing insights into seismic hazard assessment and wavefield imaging.

Contribution

It introduces a comprehensive analysis of induced earthquake waveforms to separate source spectra and site effects, revealing low intrinsic attenuation and scattering properties in the Fennoscandian Shield.

Findings

Intrinsic attenuation is exceptionally low, enabling observation of deep reflections.

Site effects vary significantly across stations, especially at higher frequencies.

The Qopen analysis tool proves effective for seismic monitoring and hazard assessment.

Abstract

We analyze envelopes of 233 and 22 $M_{\text L}$0.0 to $M_{\text L}$1.8 earthquakes induced by two geothermal stimulations in the Helsinki, Finland, metropolitan area. We separate source spectra and site terms and determine intrinsic attenuation and the scattering strength of shear waves in the 3 Hz to 200 Hz frequency range using radiative transfer based synthetic envelopes. Displacement spectra yield scaling relations with a general deviation from self-similarity, with a stronger albeit more controversial signal from the weaker 2020 stimulation. The 2020 earthquakes also tend to have a smaller local magnitude compared to 2018 earthquakes with the same moment magnitude. We discuss these connections in the context of fluid effects on rupture speed or medium properties. Site terms demonstrate that the spectral amplification relative to two reference borehole sites is not neutral at the other sensors; largest variations are observed at surface stations at frequencies larger than 30 Hz. Intrinsic attenuation is exceptionally low with $Q_{\text i}^{-1}$ values down to $2.4\times 10^{-5}$ at 20 Hz, which allows the observation of a diffuse reflection at the ${\sim}$50 km deep Moho. Scattering strength is in the range of globally observed data with $Q_{\text{sc}}^{-1}$ between $10^{-3}$ and $10^{-4}$. The application of the employed Qopen analysis program to the 2020 data in a retrospective monitoring mode demonstrates its versatility as a seismicity processing tool. The diverse results have implications for scaling relations, hazard assessment and ground motion modeling, and imaging and monitoring using ballistic and scattered wavefields in the crystalline Fennoscandian Shield environment.