We consider various cases of seismicity, induced by artificial fluid injections in boreholes. Like many other authors, we support the hypothesis that to a large extent the triggering of this seismicity is caused by a diffusive process of the pore pressure relaxation in porous (or fractured), saturated rocks. We show that if this hypothesis is correct, then the spatio-temporal distributions of the seismic events must have several specific features related to the effective permeability of the rock. As a rule the fluid injectioninduced seismicity obeys such features. These features can be indications of the diffusive and even hydraulic nature of the seismicity triggering process.From this point of view we analyze the spatio-temporal distribution of the late aftershocks of the Antofagasta (northern Chile) 1995 earthquake. These aftershocks were concentrated in a plane, an approximately 3 km-thick spatial zone. This thin seismogenic layer is a part of the South American subduction zone. The time-distance distribution of the aftershocks along this layer indicates that they could be triggered by a diffusion-like process. Possibly, such a process is the relaxation of the pressure perturbation caused in the pore fluid by the main Antofagasta event. We estimated the permeability required to explain the spatio-temporal distribution of the aftershocks by such a triggering mechanism. The obtained value, 60 mD, is very large. However, it is realistic for a long-time existing and large-scale fault zone.

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Source localization is a fundamental problem in seismology. Current localization techniques often rely on homogeneous isotropic models, even if a survey region is known to be geologically complex or anisotropic. We investigated a model's influence on localization, using data from a hydraulic injection experiment at the Continental Deep Drilling ͑Kontinentale Tiefbohrung, or KTB͒ site in Germany. We performed the localization with a grid-search algorithm and two additional methods for verification. From previous work, a homogeneous isotropic model based on a borehole check shot and a heterogeneous 3D isotropic model were available. Vertical seismic profiling ͑VSP͒ and borehole data as well as laboratory data from rock samples provided homogeneous anisotropic models. Although localization with the isotropic and anisotropic models led to the same magnitude of residuals and therefore to a comparable quality of fit, the distribution of the event cloud differed significantly for isotropic and anisotropic media. The isotropic model leads to a southward lateral shift of the event cloud of about 800 m from the injection point; the cloud appears almost centered at the injection point for the anisotropic models. This implies the classical diffusion concept for event generation, whereas isotropic models suggest the existence of a fault system providing a migration path for injected fluid, generating seismicity at an offset from the injection point. These results indicate that meticulous model building is essential for seismic-event localization and subsequent interpretation.

[1] A temporary seismological network, which was located in the Antofagasta (northern Chile) region, recorded a large part of the aftershocks from a M W = 8.0 earthquake in 1995. Results from a tomographic inversion show that the aftershocks were concentrated in a plane, approximately 3 km thick spatial zone. Comparison of our results with earlier processed wide-angle reflection data, shows clearly that this zone is positioned at the top of the oceanic crust. Therefore, it represents the shearing boundary between the subducting oceanic and the continental plate, and gives an in-situ estimation of the thickness of the boundary. Outside of this zone, events were detected in the continental crust, as well as within the oceanic mantle.