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

Abstract

[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 ±15°. 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.

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

“…They observe a direct relation between the size of earthquakes inverted (M>3) and homogeneity of the stress field determined. This phenomenon was also observed for fluid-injection induced seismicity at lower (-2<M<1) magnitude range (Bohnhoff et al, 2004). However, the data base of fault mechanisms from the Hellenic subduction zone give no indication for a similar relation which might be due to the smaller number of intermediate-sized (M~3) events which is explained by low seismic activity compared to other subduction zones.…”

“…We do not discuss the stress tensor inversion techniques here in detail but refer to the relevant articles where the methods themselves and their applications are entirely discussed (e.g. Gephart and Forsyth, 1984;Gephart, 1990;Michael, 1987;Michael, 1991;Hardebeck and Hauksson, 2001;Bohnhoff et al, 2004). The inversion result for the entire catalogue reveals a well-constrained NS orientation for the maximum principal stress (σ 1 ) at shallow dipping angle.…”

“…The stress field around Karpathos is very similar to that on Western Crete but rotated clockwise by ~20°. Inversion results for interplate seismicity should be interpreted carefully due to the comparatively small number of fault mechanisms contained in the subvolumes 5-7 (15, eight and seven events, see discussion in Bohnhoff et al, 2004). The best solutions for σ 1-3 are not as well constraint as for the subvolumes 1-4 discussed above.…”

Microseismicity induced during fluid-injection: A case study from the geothermal site at Groß Schönebeck, North German Basin

“…They also found that the static stress drop is increasing with seismic moment. In a similar experiment at the same site six years later (Baisch et al, 2002;Bohnhoff et al, 2004) the seismic network detected 2799 events (-1.2 <M L < 1.1). 237 events had a sufficient signal-to-noise ratio at the surface stations to determine reliable hypocenters.…”

Rock specific hydraulic fracturing and matrix acidizing to enhance a geothermal system — Concepts and field results

“…Borehole breakouts, borehole fractures (Brudy et al, 1997;Zoback et al, 2003;Haimson, 2007), microseismic events (Shapiro et al, 1997;Baisch & Harjes, 2003;Bohnhoff et al, 2004;Michelet & Toksöz, 2007) and stimulation pressures (Zoback & Harjes, 1997;Legarth et al, 2003;Legarth et al, 2005) have been evaluated to confine the orientation and amplitude of the principal stress components.…”