The 1-A clears for the US are 640, 650, 660, 670, 700, 720, 750, 760, 770, 780, 820, 830, 840, 870, 880, 890, 1020, 1030, 1040, 1100, 1120, 1160, 1180, 1200, 1210.
There are some decent 1-B (former) clear channel stations that are non-directional such as 680 in San Francisco, 1070 in LA and 810 in Schenectady. There are also some very directional ones such as those that were the result of the breakdown of the clears in the 70's... three in Nevada and ones like Roswell on 1020 and Lexington on 880 or Boise on 670 that could possibly run non-directional in a pinch; they are too far from large metros that might be affected by a disaster to be useful, though.
A good example might be Seattle. There are a number of stations that are 50 kw at least in the daytime, and some might be able to run non-directional with some engineering work. But if the Seattle area were so affected by the possible monumental earthquake that no station was standing, then there is no AM close enough and powerful enough to put a signal into the market day and night... the closest thing would be to use a Vancouver, BC station.
And both of those stations are in areas with such horrible ground conductivity that they do not really cover the outskirts of their own metros well.
The 1-A clears fought the FCC for nearly 30 years and even had an association to promote high power. The FCC kept pushing them back and finally the Commission started allowing additional 50 kw stations on the 1-A clears, making futher efforts impossible.
And any effort to increase power on an existing non-directional clear will cause overlap with adjacent channel stations that have come on the air since the effort to upgrade to 500 to 750 kw was finally put to rest in the late 60's. So if WLW wanted to go to 100 kw or 250 kw, they would find that newer stations on 690 and 710 make that impossible.
