Bokelmann, G.H.R., H.P. Harjes, 2000, Evidence for temporal variation of
seismic velocity within the upper continental crust, Journal of Geophysical
Research, 105, 23879-23894
Observations of systematic temporal variations
of seismic anisotropy are presented for an induced-seismicity
experiment at 9 km depth.
These observations were made under particularly
well-controlled conditions in the German Continental 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 a large set of seismic events recorded during the experiment, many
can be associated with multiplets exhibiting essentially identical waveforms.
Since they must have approximately the same source location and source radiation
pattern,
these events are particularly useful for testing the hypothesis
of time-dependent 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.
Importantly, the variation of shear wave splitting with time is
a relative measurement, which
can be performed with higher accuracy than the associated absolute
measurement. In particular, the relative measurement is not affected
by timing errors nor by event distance variations.
During the experiment the difference between shear wave velocities
decreases by approximately 2% within about 12 hours. After that, the medium
apparently approaches a state which is stable for at least 5 hours.
We suggest that the temporal variation is due to the
tectonic stress release from seismic events caused by the fluid injection.
This model requires the presence of fluid-filled cracks at depths larger
than 4 km.
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