125 Fault plane solutions for microearthquakes induced during a long-term
fluid-injection experiment at the KTB boreholes (Germany) in 2000 are investigated. A
predominant strike-slip mechanism is observed, partly with components of normal but
also 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 NS
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 15°. However, the results of the stress
tensor inversion point to heterogeneities of second order.
Based on 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.
Keywords: Induced seismicity, fault mechanism data, stress field, fault structure, KTB