  In order to monitor the hydrological strain forces of the karst micro fissure
networks and local fault activities, six capacitive extensometers were
installed inside a karstic cave near the midi-fault in Belgium. From 2004 to
2008, the nearby Lomme River experienced several heavy rains, leading to
flooding inside the Rochefort cave. The highest water level rose more than
thirteen meters, the karstic fissure networks were filled with water, which
altered the pore pressure of the cave. The strain response to the hydrological
induced pore pressure changes are separately deduced from fifteen events when
the water level exceeded six meters. The strain measured from the extensometer
show a linear contraction during the water recharge and a nonlinear exponential
extension releasing during the water discharge. The sensitivity and stability
of the sensor are constrained by comparing continuously observed tidal strain
waves with a theoretical model. Finally, a local fault deformation rate around
$0.03 \pm 0.002$mm/yr is estimated from more than four years' records.
