Evidence for temporal variation of seismic velocity within the upper continental crust

Observations of systematic temporal variations of seismic anisotropy are pr esented for an induced-seismicity experiment at 9 km depth. These observati ons were made under particularly well-controlled conditions in the German C ontinental Deep Drilling Program (KTB) borehole, using shear wave splitting from similar events recorded at a three-component instrument located at 4 km depth from a hydraulic fracturing experiment at 9 km depth. In at large set of seismic events recorded during the experiment, many can be associate d with multiplets exhibiting essentially identical waveforms. Since they mu st have approximately the same source location and source radiation pattern , these events are particularly useful for testing the hypothesis of time-d ependent anisotropy. Anisotropy itself is clearly a very prominent feature in the data. A. simple approach for waveform matching of split shear waves allows unprecedented resolution of variations in shear wave splitting. Impo rtantly, the variation of shear wave splitting with time is a relative meas urement, which can be performed with higher accuracy than the associated ab solute measurement. In particular, the relative measurement is not affected by timing errors nor by event distance variations. During the experiment t he difference between shear wave velocities decreases by similar to 2% with in similar to 12 hours. After that, the medium apparently approaches a stat e which is stable fbr at least 5 hours. We suggest that the temporal variat ion is due to the tectonic str ess release from seismic events caused by th e fluid injection. This model requires the presence of fluid-filled cracks at depths larger than 4 km.