Fault mechanisms of induced seismicity at the superdeep German Continental Deep Drilling Program (KTB) borehole and their relation to fault structure and stress field

[1] One hundred twenty-five fault plane solutions for microearthquakes induced during a long-term fluid injection experiment at the German Continental Deep Drilling Program (KTB) boreholes ( Germany) in 2000 are investigated. A predominant strike-slip mechanism is observed, partly with components of normal but also with reverse faulting. Adding 54 fault plane solutions of an earlier injection experiment at the KTB, we determine the local stress field and find a subhorizontal north-south orientation for the maximum principal stress and a near-vertical orientation for the intermediate principal stress. The stress field exhibits no temporal or spatial variations within the resolved accuracy of +/- 15degrees. However, the results of the stress tensor inversion point to heterogeneities of second order. On the basis of the hypocentral distribution of the induced microearthquakes and the similarity of fault mechanisms, we relate our data to the fault structure at the KTB. We find that the larger faults act as pathways for the injected fluid, whereas the brittle failure occurs on fault asperities of the larger mapped faults and nearby smaller faults, both in agreement with the local stress field. Applying a thorough error analysis of the individual fault plane solutions, we correlate the diversity of mechanisms with their strength and find that the strongest events tend to a representative mechanism that is in good correspondence with the stress field. In contrast, the diversity of fault mechanisms is larger for the smaller events, indicating local stress perturbations.