You're assuming an attack made only when it is complete. Any attack prior to completion negates the argument. Second, people have two very strong reasons to blow up the elevator. First, the argument of being able to trace the nation assumes safeguards on the sale of nuclear weapons. One can easily imagine scenarios wherein blackmarket weapons from Russia, North Korea, Iran, Pakistan, et al. fall into the hands of terrorists, with no ability to trace effectively the nation or organization of origin. Second, you are assuming, either that the nation or organization in question cares (e.g. Al-Qaeda/ Taliban don't care, and actually want a worldwide economic collapse) about economic incentive or that the party in question won't benefit from the destruction involved (e.g. North Korea).
Taking out everything within a line of movement. JUst imagine the amount of damage that is caused on the sea floor when an anchor and chain are dragged, or better yet, when a oil rig drill is shaken in a hurricane. Just remember that in any disaster, the balance that you are hoping for will fail, and reattachment will not be possible.
Actually, the same amount of time to "eat away" can allow a potential terrorist to escape undetected. Just remember that if you have the technology to assemble carborn fibers on an atomic level, you also have technology that can take it apart. All it takes is malicious and patient programming, two things hackers and terrorists always have...
The North Koreans would have no incentive one way or another... aside from the fact that the origin of the bomb used can be traced back to them, and therefore doing something like that is committing national suicide when the US, China, India, South Korea, and Japan come gunning with blood in their eye. As for Al-Queda, the same constraints that prevent the DPRK from using a weapon prevent anyone else from giving them one, and they cannot build it themselves. Now, finally, if you blow up the cable near the ground, within a few or even a few dozen kilometers (which would be necessary as there is no delivery system available to terrorists that can get higher with any accuracy), only that part will fall, and the damage will just not be that catastrophic. To cut the entire thing and cause it to plummet, it would have to be cut at the spacial anchor. Not likely, assuming any means of scanning cargo for nuclear materials is employed.
As for the idea that you can attack it while it is under construction... you do realize that this is the point at which you will cause the
least damage, right? While under construction, everything is supported in such a way that cutting the cable won't matter because there is nothing holding it at the other end, anyway. Any engineer with a brain will build it in orbit from the anchor end down, then literally drop it into a pre-constructed base platform and anchor it there very, very tightly. So not only is an attack less effective while it is under construction, it is also much harder to pull off.
What makes you assume that clipping the cable will cause the balance of this thing to instantly fail catastrophically? Any engineer with a brain will make sure that the "roof" is capable of active maneuvers to aid in the relinking of a broken cable. You don't get to say "reattachment will not be possible," just because you want to win the argument. Give me some proof that clipping the cable of a precisely aligned space elevator with a center of mass as close to geosynchronous orbit as possible, and with significant active maneuver capability, is going to unbalance it such that it can't be repaired before the entire thing is a total loss.
Now, finally... as a terrorist group, you depend on stealth and surprise to sever this. The ABSOLUTE LAST THING you want to do is use a weapon that takes weeks or months to function and has easily noticeable effects in the meantime. What's more, it is easily countered, because to cleanse the cable of nanotechnology, all that is required is to sweep it with a focused pulse of x-rays. So I have about as much fear of nanites being used to bring the cable down as I do of termites being employed by terrorists to destroy the Capitol building. They're equally threatening and only a terrorist organization made up of idiots would try to use them to take out the elevator.
I really don't understand how you expect the people who built this thing, partly through the use of nanotechnology, to
not take precautions to make sure they can detect a nanotechnology attack on it long, long before it actually begins to cause crippling damage. They'll catch it the second the cable's surface starts degrading and then wipe the nanites out before any lasting damage is done.
EDIT: Just reread your post and realized that you missed the following: you clip the line, and the structure moves
upwards. Thus, your anchor dragging analogy doesn't work at all, because this cable will be a few dozen kilometers up, and not moving relative to the surface anyway.
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