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Source parameters of injection-induced microearthquakes at 9 km depth at the KTB Deep Drilling site, Germany

Institute of Geophysics Ruhr-University Bochum, D-44780 Bochum, Germany

Abstract

A fluid injection-induced seismicity experiment was undertaken in the KTB (German Continental Deep Drilling Program) main borehole at 9 km depth. Several hundred microearthquakes were recorded by a three-component geophone at 4 km depth in the pilot hole of the KTB about 200 m west of the main hole. More than 100 of these events were also recorded with good signal-to-noise ratio by a 73-element temporary network at the surface.

Several different clusters of microearthquakes with distinct waveforms were defined. Compound fault-plane solutions for the two most prominent clusters of seismic events were determined: a strike-slip mechanism for cluster 1 at an average depth of about 8.9 km and a strike-slip/reverse mechanism for cluster 4 (with the "main" ML = 1.2 event) at an average depth of 8.6 km. For both fault-plane solutions, the P axis is subhorizontal and oriented NNW-SSE, similar to the N160°E direction of maximum horizontal stress observed in the well bore. Both clusters were analyzed using an empirical Green's function method to derive the relative source time function (RSTF). Azimuthal variations of the RSTF were used to determine rupture directions and velocities. By combining the information about rupture directions with fault-plane solutions, it was possible to identify the active fault planes (NE striking nodal planes) for both clusters. Although injection-induced events are supposed to exhibit a dilatational component due to the tensile character of the source, the moment tensor inversion for both microearthquake clusters resulted in a double-couple contribution of about 90% and P axes similar to the direction of maximum horizontal stress observed in the borehole. The isotropic components of the moment tensors are insignificant due to the size of the location uncertainties.

From records of the sensor at 4 km depth, we found seismic moments of the microearthquakes ranging from 10⁷ to 10¹¹ N-m. The spectra were corrected for Q [Q(f) = 420 f^0.5 for P, and Q(f) = 230 f^0.5 for S-waves, which were determined assuming an ² model]. Following Brune (1970, 1971), we found source radii between 12 and 28 m and stress drops between 0.01 and 6 MPa. The average ratio of S- to P-wave energy was determined as 14.2. Our relation between seismic moment and ML is log M₀ = 1.01 ML + 9.68, and between energy and seismic moment, log E = 2.0 log M₀ - 15.35. These seismic scaling relations suggest that stress drop increases with seismic moment for this data set. However, it cannot be precluded that our data, covering only somewhat more than three orders of magnitude, fall in a larger trend of constant-stress-drop scaling over many orders of magnitude due to the large scatter observed over several orders of magnitude.

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