Microseismic activity and fluid fault interactions: some results from the Corinth Rift Laboratory (CRL), Greece

The Gulf of Corinth, in western-central Greece, is one of the fastest continental rifts in the world. In its western section near the city of Aigion, the previous work has outlined the existence of a shallow dipping seismogenic zone between 5 and 12 km. This seismic activity has been monitored with a network of 12 three-component stations for the period 2000-2007. Three, few months long, seismic swarms have been observed. They mobilize a complex structural fault system that associates both shallow dipping elements and subvertical structures with very different azimuths, some of which extend to depths greater than that of the shallow dipping zone. The swarm activity associates intensely active, short crises (a few days) with more quiescent periods. The long-term growth velocity of the seismically activated domains is compatible with a fluid diffusion process. Its characteristics are discussed in the context of the results from the 1000 m deep AIG10 well that intersects the Aigion Fault at 760 m. The vertical growth directions of the seismically activated volumes outline two different sources for the fluid and imply non-steady pressure conditions within the seismic domain. The diffusivity along the cataclastic zone of the faults is in the order of 1 m(2) s(-1), while faults act as hydraulic barrier in the direction perpendicular to their strike. If the vertical direction is a principal stress component, the high pore pressure values that must be reached to induce slip on the shallowly dipping planes can result only from transitory dynamic conditions. It is argued that the shallow dipping active seismic zone is only local and does not correspond to a 100 km scale decollement zone. We propose to associate the localization process with deep fluid fluxes that have progressively modified the local stress field and may be the cause for the quiescence of the West Heliki Fault presently observed.