Abstract
AbstractWe analyze slip distribution and rupture kinematics of a Mw3.3 induced event that occurred in the St. Gallen geothermal reservoir (NE Switzerland) in 2013. We carry out a two‐step procedure: (1) path effects are deconvolved from the seismograms using an empirical Green's function, resulting in relative source time functions at all seismic stations; (2) the relative source time functions are back‐projected to the corresponding isochrones on the fault plane. Results reveal that the mainshock rupture propagates toward NNE from the hypocenter with an average velocity of 2,000 m/s. Spatiotemporal organization of foreshocks and aftershocks shows that the mainshock broke a previously less active portion of the fault and suggests that the aftershock sequence could be mainly driven by stress transfer. Applying this method in an operational environment could enable fast retrieval of seismic slip, allowing assessment of fault asperities and structures involved in the reservoir creation process.
Highlights
We carry out a two-step procedure: (1) path effects are deconvolved from the seismograms using an empirical Green's function, resulting in relative source time functions at all seismic stations; (2) the relative source time functions are back-projected to the corresponding isochrones on the fault plane
Our results show that a detailed description of the rupture process of such a small earthquake can be obtained: The rupture propagates from the hypocenter in NNE direction for 150 m, with an average velocity of 2 km/s, breaking into a less active portion of the fault, where no earthquake was previously recorded
Our results suggest the plausibility of the latter hypothesis: it seems probable that the updip and NNE propagating MSH released a previously less active segment of the fault where significant seismicity occurred after the main event
Summary
Induced seismicity (i.e., earthquakes caused by human activity) is an increasingly debated topic in scientific literature and in public media (Ellsworth, 2013; Grigoli et al, 2017). Among the industrial activities that can produce earthquakes, geothermal projects receive particular attention due to the rising importance of alternative energy. Economical exploitation of geothermal energy requires sufficient fluid that circulates through connected channels in a hot medium (Hirschberg et al, 2015). It becomes necessary to enhance the permeability of the target rock formation, which is one of the main challenges. As induced earthquakes are the best indicators of permeability creation to date, geothermal projects are generally monitored by close dedicated seismic networks. Microseismicity is routinely detected and located (e.g., Albaric et al, 2014; Atkinson et al, 2016; Baisch et al, 2009; Deichmann et al, 2014; Diehl et al, 2017; Schoenball et al, 2018), characterized (i.e., magnitudes, focal mechanisms, moment tensors, stress drops, and source areas; e.g., Deichmann & Giardini, 2009; Edwards & Douglas, 2014; Edwards et al, 2015; Goertz-Allmann et al, 2011; Guilhem & Walter, 2015; Huang et al, 2017; Kraft & Deichmann, 2014), and modeled (e.g., Bachmann et al, 2011; Catalli et al, 2016; Gischig & Wiemer, 2013; Karvounis & Wiemer, 2015; Langenbruch & Shapiro, 2010; Langenbruch & Zoback, 2016; Mena et al, 2013; Rutqvist, 2011; Segall & Lu, 2015)
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