HR: 0800h
AN: S21B-0278    [Abstracts]
TI: Hydraulically Induced Seismicity at the KTB - Event Dislocation Caused by Anisotropy
AU: Sommer, K
EM: sommer@dkrz.de
AF: Inst. of Geophysics, Uni. of Hamburg, Bundesstr. 55, Hamburg, 20146 Germany
AU: * Gajewski, D J
EM: gajewski@dkrz.de
AF: Inst. of Geophysics, Uni. of Hamburg, Bundesstr. 55, Hamburg, 20146 Germany
AB: The environment of the German Continental Deep drilling site (KTB) is known to be anisotropic. In this study we have analyzed the seismic events generated by the KTB long term injection experiment. For a period of 60 days a total of more than 4000 $m^3$ of water were injected into the KTB generating more than 2500 seismic events. About 260 events were recorded not only by the borehole geophone but also by the 40 station surface array centered at the KTB. For this experiment we are in the favorable position to have a priory knowledge, where the acoustic emission should originate, i.e., they should start at the injection point slowly propagating away from it. Localizing the events recorded in the borehole and by the surface array using an isotropic model based on average velocities derived from check shots leads to a southward lateral shift of the center of the event cloud of about 500m away from the injection point. Since the total extension of the elongated event cluster is about 2 km in E-W direction and about 300-400 m in N-S direction this is a significant contribution. Using the 3-D isotropic velocity model obtained from the KTB reflection data, no significant change in the localization is observed and the lateral shift is reproduced. The application of station corrections is inappropriate since the geophones of the recording array are located on the same formation. Localizing the same events using an anisotropic model based on published data centers the event cloud around the injection point. The anisotropic localization not only removes the lateral shift of the events but it also significantly alters the shape of the event cloud from an elongated cluster to an almost circular distribution. This is important if the spatio-temporal evolution is interpreted with respect to the hydraulic properties of the rocks. Also interpretations with respect to event clustering due to tectonics or hydro fracing may be severely affected if the localization is based on the wrong model assumptions. The localization of the data with the simple anisotropic model lead to a physically reasonable distribution of events which is not obtained with isotropic models. In seismology most often there is no a priory information on the subsurface available and we are in a less favorable position than for the KTB injection experiment. A dislocation of the events due to anisotropy may be overlooked misleading the interpretation of the distribution of the events and its spatio-temporal evolution.
DE: 7230 Seismicity and seismotectonics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting