It will be finished in 2010 and is the deepest ever immersed tunnel crossing.
I have a question: what happens in case of a seismic event during the life of the tunnel? (North Anatolian fault line)

The tunnel has been designed to handle a major earthquake, mostly by being flexible enough to withstand shaking and at least some displacement. The BART tunnels between San Francisco and Oakland are of the same type and faced the same issues.

There are also automatic flood doors in the tunnel to contain the flooding in case of a breach but these would be small comfort to anyone in a flooded section.

I've talked to men who were in deep mine tunnels during earthquakes and they said the experience was frightening but turned out not to be especially dangerous. As long as a tunnel isn't too stiff, it can handle a lot of flex.

I think the Japanese have been involved in the construction ... and they have a lot of experience in this area!

Thank you DaveT and Marion!