Abstract

A 60-day, long-term fluid-injection experiment was performed at the 9.1-km-deep Kontinentale Tiefbohrung, Germany (KTB), borehole. About 4000 m3 of water were injected into the well head to induce seismicity near the open-hole section at 9-km depth. Because of several leaks in the borehole casing (unknown before), seismicity occurred at distinct depth levels between 3-km and 9-km depth. Two events occurred at 10-km and 15-km depth. The combination of a temporary, 40-element, three-component surface network of seismometers and a three-component downhole sonde at 3.8-km depth in the nearby pilot hole enabled us to determine absolute hypocenter locations by using a velocity model that was calibrated from several downhole shots at depths of 5.4 km and 8.5 km. Of a total of 2799 induced events, hypocenter locations were obtained for 237 events having good signal-to-noise ratio at surface stations. The spatiotemporal distribution of hypocenters at each depth level exhibits complex structures extending several hundred meters from the injection points, with strong spatial and temporal clustering. Regions that were seismically active at a certain time often showed reduced or no activity at later times, indicating local shear-stress relaxation. A similar “memory” effect (Kaiser effect) is observed by comparing hypocenter locations of the present experiment with those obtained for a previous injection experiment at KTB. The limitation of hypocentral depths to 9.1 km for events near the borehole suggests changes in rheological properties of the upper crust and thus supports a transition from the regime of brittle failure to ductile deformation at this depth. Large fluid-level changes observed in the nearby pilot hole demonstrate that fluid flow occurs over distances greater than 1.5 km and that major flow zones are not mapped by the induced seismicity. This might also explain the occurence of isolated events at greater distances and depths. Brittle failure at depths greater than 10 km indicates the existence of critically stressed fractures even at temperature over 300°C.

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