To veer back to the original question;
SHTF is any disaster that effects you in a bad way. Mall shooting, get fired, hurricane, personal injury, blizzard, whatever. It is something that forces YOU to fall back on your own resources. Can't get to the store, no electricity, no money for food or what have you.
TEOTWAWKI is just that The End Of The World A We Know It. Civilization no longer functions, you are forced to handle your problems by yourself or with the people in your immediate area. No more food out of season, no more access to cheap mass-produced products, no more petroleum products, nothing but what you have or can make.
Katrina and the rest are SHTF but not TEOTWAWKI, the people there might not have had access to the fruits of civilization but only temporarily. A major nuclear war, asteroid impact, total economic collapse, major pandemic and other things that prevent to wheels of civilization from turning are what you are talking about there.
Civilization is not cities and technology, it's millions of people you don't know getting up every morning and doing jobs you have never heard of. When people cannot do their jobs and thereby prevent you from doing yours, that is TEOTWAWKI.