Resolving the issue with what map to make fell to the people of facepunch. There, they had a nice democratic vote, with no bribes from myself or any third parties, and the votes passed on the idea of a seismic post-apocalypse.
Now...i watch a lot of "core" simply because channel 4 always show it when i want something fairly mindless to watch. The basic synopsis is that the core of the earth stops working and some people are found to pilot a craft down to restart it. Suffice to say, that film, added with a level from No One Lives Forever and Fallout has been part of the inspiration.
The map in question will be based around natures war on humanity. In the end, humanity would eke out a life of either survival or optimism, people would have mixed reactions to the situation.
There would be a few forgotten nuggets of technology remaining. A base similar to those volcanic lairs that evil genius' always seem to build would fit in nicely with the theme. Ruined military bases with screens full of data on satellites. Maybe even a wrecked missile silo, the missiles trapped in a semi launched position, never able to fire again.
More noticeable damage would be evident too, buildings torn apart by violent quakes, subway systems or storm drains full of molten lava.
However, where you find people, you find towns. Some living in huts, others behind electric fences and patrolling guards. I'll probably add a few towns of differing technological levels. Outside these towns, people would not want to dwell alone, for fear of the animals that have adapted to the harsh environments.
To finish, and to quote Jurrassic Park;
"Life will find a way"