Geological and hydrogeological conditions of the Aigion seismic active fault zone (Deep Geodynamic Laboratory Corinth) based on borehole data and hydraulic tests

The Gulf of Corinth and the northern part of the Peloponnesus/Greece, an area of asymmetric graben structure, step faults and tilted blocks, is one of the most active seismic zones in the world. Six major faults are known to be most responsible for the historic and present seismic activities in the area of Aigion. Our study focuses preliminarily on the area around the Aigion fault, whose trace runs E-W through the harbour of Aigion. Investigations of the stratigraphic sequence, tectonic structure and hydrogeologic conditions of the southern Corinth graben shoulder and first drilling activities there, have started in summer 2001. From July until September 2002 the International Continental Deep Drilling Project (ICDP) and the EU Project DGLab-Gulf of Corinth drilled the AIG10 borehole in the harbour of Aigion to a total depth of 1001 m. Our investigations in this ICDP/EU framework are aimed at studying the thermal-hydraulic conditions on the southern graben shoulder. Here we report the first results on sampling and hydraulic testing. The deep AIG10 borehole has successfully cored in approx. 760 m depth the fault plane, which separates fractured radiolarite in the hanging wall from highly fractured and karstified platy, micritic limestone (Olonos-Pindos Unit) in the footwall. A complete lithologic section is now available through the monitoring of cuttings and cores, which built a major cornerstone for defining an integrated regional tectonic and geologic model. Several pumping tests and hydrochemical investigations made in the region of Aigion and especially in the AIG10 borehole deliver together with geophysical borehole logging the database for a thermo-hydraulic heat flow model. The pumping test AIG10C in the conglomerates of the graben sediments show a hydraulic conductivity of about 2 x 10E-5 m/s - 3 x 10E-4 m/s at a depth of approximately 211 m. The result was a residual drawdown, which indicates a closed hydraulic system between the semi-permeable Aigion fault zone and other faults farther north. The second pumping test AIG10L at 708-750 m depth shows artesian conditions with a fluid pressure of 5 bar and a flow of approx. 40 l/min. The hydraulic conductivity was about 1 x 10E-7 m/s. By drilling deeper an increase of pressure and flux immediately after crossing the fault zone was observed. The cores from 773-786 m depth and the hydraulic behaviour of the deeper intervals up to 1001-m depth suggest karstic water-flow conditions. The water-pressure difference of more than 5 bar between the hanging wall and the footwall provides additional evidence that the Aigion fault zone has a water-blocking capability. An artesian production test showed that the pressure of > 10 bar and the flow of 900 l/min did not decrease for more than four days. An average hydraulic conductivity of 1 x 10E-6 m/s was determined. From this test and the second pumping test we conclude that the Aigion fault zone behaves as a semi-permeable zone or even as a hydraulic barrier not only in the conglomerates but also in the platy limestones.