Would be a nuclear exchange triggered by a small asteroid or comet impact event - and here on Earth, less than a month ago we came about 5 hours and 10 meters from one of these things setting off a nuclear war between Pakistan and India -

THE REASON, and the only sufficient reason, for returning to the Moon is to build optical band and radar instruments to ensure that we get the best warning of the next impactor before it hits - and you can be certain that one of these will hit, and however long before it hits, it will hit, and it will hit all too soon -

The reason for return to the Moon is not for manufacturing or mining, or to create a "lifeboat" capable of saving a very few people at best - though these may follow as incidental benefits -

We don't need large launchers to build the necessary detection devices, just the launchers currently used for commercial purposes - the construction techniques and the docking techniques necessary for the construction of these warning systems are already in hand as well -

As far as manned flight to Mars goes, there are significant barriers to this which currently exist - chief among these is the problem of back contamination of the Earth by a Martian life forms - and we currently don't know if they exist. My opinion is that the best course of action, the best way of making use of the time available while this and other questions are being answered, is to proceed with the construction of necessary Moon based asteroid and comet detection systems -

Now when one of these asteroids or comets is detected on an earth bound course, it will be necessary to deflect it. Possibly this same technology might someday be used to turn Mars into a useful planet, but in any case it is certain that this technology is necssary to preserve the Earth as a useful planet -

As to your initial point, as far as tsunami go, while landslides can create deadly tsunami, the truly deadly ones, the mega-tsunami, have recently been caused solely by impact - the energy released by these hits is far more deadly than the energy released by landslide. And you may ask the world's leading tsunami expert, Ted Bryant, about this, if you do not believe that I am reporting his findings accurately -

ep