HR: 0800h
AN: S41A-0935    [Abstracts]
TI: Pressure drop in a borehole intersecting an active fault
AU: * Doan, M
EM: doan@ipgp.jussieu.fr
AF: Departement de sismologie, Institut de Physique du Globe de Paris, 4 place Jussieu, PARIS, 75005 France
AU: Cornet, F H
EM: cornet@ipgp.jussieu.fr
AF: Departement de sismologie, Institut de Physique du Globe de Paris, 4 place Jussieu, PARIS, 75005 France
AB: The Corinth Rift, in western Greece, is one of the most active continental Rift in the world, with an opening rate of 1.5cm/yr. Its deformation process is being monitored with a broad range of sensors dispatched across the rift, near the city of Aigio, some 40km east of Patras. In particular, a set of pressure transducers has been set in a 1000m-deep borehole that intersects the active 10km long Aigio fault at a depth of 760m. Below its upper 700m deep cased section, the well has been left open and intersects two artesian aquifers. The upper aquifer is fully hydraulically decoupled from surface aquifers and is developed in tectonized platy limestone, with a 0.5MPa original pressure. Below the fault, the limestone is heavily karstified and the artesian overpressure reaches about 0.85MPa. Hence the fault supports a 0.35MPa differential pressure through the 5m thick radiolarite clay layer that has been smeared along the 150m fault offset. In September 2003, the borehole was let produce water and then was plugged with a packer set at the top of the casing resulting in a direct connection between both aquifers. The pressure is monitored by sensors set just below the packer. Tidal waves are recorded with a resolution better than 1/100. In addition a variety of pressure anomalies have been observed. A 60Pa drop in pore pressure has been recorded at the onset of the S waves generated by the Mw=7.8 Rat Island Earthquake of November, 17th 2003. It is followed by a slow recovery which lasted about 30 minutes. This anomaly, compatible with a minor movement along the fault with a seismic moment of $10^9$Nm, is one of the farthest local effects induced by teleseismic waves ever recorded. A 80Pa pressure drop has been detected 15 minutes before a ML=4.2 earthquake that occured about 15km west of the well. It is much sharper than the coseismic drop. This precursory event exhibits a 2-step recovery that lasted 10 minutes. As seismic sensors located near the well detected no major vibration, we assume the pressure anomaly is transmitted through the karstic conduits and give insight to events close to the hypocenter.
DE: 7299 General or miscellaneous
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting