Modelling focal mechanism errors of seismicity induced at Rittershoffen geothermal field (Alsace, France)

Authors: Kinnaert, X; Gaucher, E; Kohl, T; Achauer, U
Keywords: EGS, fault plane, inaccuracy, uncertainty, velocity model error
Conference: European Geothermal Congress Session: Science – Geophysics
Year: 2016 Language: English
Abstract: The determination of focal mechanisms of earthquakes induced in geothermal fields is of primary interest for reservoir characterization. For example, parameters like fault identification, faulting regime and stress field may be derived from it. The impact of a few parameters on the earthquake focal-mechanism determination is investigated using a three-step approach. First, synthetic waveforms are created for double-couple sources whose strike, dip and rake are consistent with the fault plane on which they are located. From the synthetic waveforms generated on the network, the polarity of P-waves and the amplitudes of P- and S-waves are measured to constitute the observed data. In the second step, the polarities and amplitude ratios are inverted to compute the focal mechanisms, once the earthquake was located. However, to represent the lack of knowledge or simplifications, the initial velocity model is replaced for this inversion. In the last step, the focal mechanisms obtained are compared with the original ones.
The method is applied to the deep geothermal site of Rittershoffen (France). In 2013, a massive water injection was performed at approximately 2.5 km depth to enhance the GRT1 well productivity. Induced seismicity was recorded by a surface network installed in several steps. After discussing the capabilities of the seismic network to estimate the focal mechanisms in a perfect case, we investigate the impact of approximating a 3D velocity model including a fault and a shifted block by a 1D velocity model. For each case, results using only polarities or polarities and amplitude ratios are compared.
The angular inaccuracies associated with the slip vector determination are usually smaller than the corresponding uncertainties or of the same order. The addition of amplitude ratios data only affect the focal mechanism of sources located in a low velocity zone and the impact of an incorrect velocity model can only be seen on focal mechanism uncertainties.
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