Microseismic data sets of excellent quality, such as the seismicity recorded in the Basel-1 enhanced geothermal system, Switzerland, in 2006–2007, provide the opportunity to analyse induced seismic events in greater detail. In this paper we demonstrate the feasibility of identifying directivity and rupture complexities for selected events (M ~ 1) from the Basel reservoir using an empirical Green’s function (EGF) approach. We show that the EGF method allows to resolve both directivity and complex rupture behaviour even if it is not directly identifiable in the seismograms. We find clear evidence of multi-phase ruptures in the analysed relative source time functions with time delays between consecutive subevents in the order of 10 ms. Amplitudes of the relative source time functions of connected subevents do not always show the same azimuthal dependence, indicating dissimilarity in the rupture direction of the subevents. Our observations support the assumption that heterogeneity on fault surfaces persists down to small scale (few tens of meters). For a given dataset with suitable station layout and data quality this approach could contribute to an improved understanding of rupture processes of microseismic events and to better assess small-scale heterogeneity of reservoir structure.


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