Fluid Injection Experiments at the KTB drilling hole, Germany. I) Fault Plane Solutions and Stress Field Analysis

A 60 day long-term fluid injection experiment was performed at the 9.1 km deep KTB (Kontinentale Tiefbohrung) borehole in southeastern Germany. About 4000 cbm of fresh water were injected into the well head aiming to induce seismicity near the open-hole section in 9 km depth. Seismicity was monitored over a period of 90 days by a 40-element three component surface station network and a three component downhole sonde in the nearby pilot hole in a depth of 3.8 km. A total of about 2800 induced events were detected with magnitudes ranging from -2.4 to +0.7. Based on the hypocenter locations several leaks within the casing of the borehole were unexpectively found that were later confirmed by temperature- and flow-logs. The present dataset with distinct hypocentral depth levels in addition with that of a short-term injection experiment that was performed earlier at the same location forms an excellent data base. Different subsets were formed in order to investigate spatio-temporal variations of both fault mechanisms and local stress field. For more than 100 events fault plane solutions were determined based on recordings at the surface stations with the majority of them located in depth intervals of 5.2-5.4 and 8.8-9.1 km. The predominant mechanism was found to be strike-slip. This corresponds well with the regional stress field with a nearly vertical orientation of the intermediate principal stress and a NW-SE trend for the maximum horizontal principal stress. Based on the fault plane solutions the local stress field was investigated in terms of orientations of the principal stresses and the relative stress magnitude using the method of Gephart and Forsyth (1984).