Source Characteristics of Tectonic and Induced Events near St. Gallen, Switzerland: Utilizing the Empirical Green’s Function Method and Generalized Inversion Technique

ABSTRACT Stress drop is a kinematic source parameter essential for understanding the physical mechanisms behind natural or human-induced seismic events, contributing to ground shaking and seismic hazards. This study aims to estimate the Brune stress drop of earthquakes across Switzerland for the years 2013–2014. The geothermal energy project in St. Gallen induced 347 events with a maximum magnitude of ML 3.5, providing an opportunity to quantitatively compare the physical differences between induced and tectonic earthquakes in the surrounding region. Using the S-wave Fourier amplitude spectra of 82 induced events and 83 tectonic earthquakes, we apply two spectral methods to isolate the source terms of the earthquakes: (1) empirical Green’s functions (EGFs) and (2) the nonparametric generalized inversion technique (GIT). GIT is applied to all the induced and tectonic earthquakes, whereas six of the induced events (ML>1.4) with appropriate EGF events are also analyzed using the EGF method. For these six events, both methods yield consistent average and median stress-drop values: 16.4 and 18.1 MPa for EGF, and 14.4 and 14.3 MPa for GIT, respectively, which are a factor of 10 larger than those of induced earthquakes. This discrepancy is possibly due to the alteration of the stress state under the influence of pore fluid in the geothermal setting. Interestingly, we observe that the stress drop increases with seismic moment for both induced and tectonic events, which is consistent with previous findings, though we note the narrow magnitude range in this study. Such nonself-similar scaling, if true, may be related to the depth dependence of the stress drops for tectonic events and the perturbation of pore pressure for induced events.