As far as I'm concerned, any natural event which is possible given known geology in an area and is equivalent to one known to have happened in the history of the earth is not ASB. It is a legitimate PoD. Thus, the comet strikes which created the alternate history in Stirling's Peshwar Lancers is legitimate AH, while the events creating the ISOT story in his Islands on the Sea of Time are ASB. The key to me is if the PoD is one which is possible and reasonable according to current scientific and historic theory, it is just as legitimate as one based on a WI if certain people weren't born or made different decisions. A PoD which has parts of the earth inexplicably flood, switch places, or changes the earth's basic structure, rotation rate, attittude, or distance from the sun is ASB. If natural events are reasonable or possible in the area under consideration, it is entirely legitimate to change history based on creating or removing volcanic eruptions, earthquakes, hurricanes, etc.. At least that's my take.
Given these limitations, I would consider an PoD which creates an entirely new land mass like Tarrantry ASB, while one which postulates the essential destruction of a real one like Britain or Japan by a meteor strike (Britain), or massive earthquake/series of volcanic eruptions (Japan) can be legitimate AH. I have an AH predicated on an alternate WW2 with several PoD's, the basic one being a masive 1940 earthquake in Japan resulting in the Japanese needing to devote almost all their energy to reconstruction of the home islands, hence a disengagement in China and no Pearl Harbor attack. I consider this a legitimate AH.