If a "very large event" ( disaster or emergency) involves thousands; then what term should I use for one affecting a million plus? I'm doing some writing and this has never come up.

Off the top of my head:

cataclysm
catastrophe
widespread devastation
mass casualty event
mass destruction
mass displacement

Probably too strong: Armageddon, holocaust

BTW, http://thesaurus.com/ is useful, just don't roll over anything (annoying ads).

See http://dictionary.reference.com/browse/disaster for suggested uses of these synonyms (again, avoid rollover ads).

Perhaps step outside the strictly "size" box:

If confined geographically, perhaps: citywide, countywide, watershed-wide, peninsula-wide, everyone downstream of, everyone downwind of, in the X canyon, etcetera.

If defined by infrastructure failure, perhaps: system-wide, everyone dependent for [resouce] from X utility, beyond X's capability to respond, etcetera.

By comparison to specifically defined governmental unit current census known to or identified for the target audience [city, county, state, country], perhaps: San Franciso level, Sacramento County level, California level, mainland United States level, etcetera.

By comparison to specifically defined historical experience numbers, perhaps: United States polio epidemic, Spanish flu pandemic of 19??, European Black Plague of ????, etcetera.

By referencing animal population threat status terms, perhaps: at-risk, endangered, extinction-level, etcetra.

Numerically, perhaps: literal one-in-ten decimation,

Grammatically, perhaps: "extremely" large event, use of the words like "beyond, exceeding," etcetera.

I would tend to use geographic area as a descriptor. Terms like: national, regional, statewide come to mind. Simply noting the number of square miles affected, especially when compared to well understood standards, big as the state of Texas or what-have-you, can be quite effective. A few years back we had wildfires in Florida so bad most of a county was evacuated and declared a disaster. That is pretty big.

Its simply called 'The Event'

http://www.youtube.com/watch?v=zCnCRj5-bC4

http://training.fema.gov/EMIWeb/edu/docs/terms%20and%20definitions/Terms%20and%20Definitions.pdf

has a list of definitions. For example, a catastropic disaster is one affecting tens or hundreds of thousands of people in an event which overwhelms local, state, and federal resources. I think that would cover a million plus.

One of my instructors called such events "training opportunities". I'm guessing he would call a really large event a "really good training opportunity".

Really large events are termed by the actusl event. Ex: the Challenger Disaster, the Kennedy Assasination, etc.

Fictitious events: Since The Day, Warday, ww III, etc.

Would not a "very large EVENT" be one that changes normal everyday life onto a new direction from that point forward. AKA: The End of The World As We Know (TEOTWAWKI).

I have to respectfully disagree. TEOTWAWKI is way too generic and stereotypically doomy and gloomy. Except for a limited audience, you'd feed into fears, or open yourself up to accusations of fear mongoring or ridicule. Lots of the people with brighter minds than me around here have come up with some great suggestions. i'd pick one of those.

In addition to amplitude, perhaps an event's cause[s], effect[s], duration, and frequency might be useful descriptors.

Causes, such as: manmade, weather-caused, seasonal-hazard, known geological, as-yet-unknown, etc.

Effects, such as: inconvenience, permanently altering, as-yet-unknown, public health hazard,

Duration, such as: matter of seconds, minutes, hours, days, weeks, months, as-yet-unknown,etc.

Frequency, such as: annually, once in a decade, 100-year event, etcetera.

I guess the earthquake/ Tsumani in Japan would qualify

The Japan situation merits an f-bomb preface.

It's not just a single catastrophe, it's a triple crown catastrophe: Mega-quake, Tsunami, Nuclear emergency

Is there precedent for that?

Godzilla.
-Blast

The FEMA guide has the following terms:

Calamity: “A massive or extreme catastrophic disaster that extends over time and space.” Notes the Black Death of the 14th century as an example.

Catastrophe: (from the 11 pages giving various definitions)
An event in which a society incurs, or is threatened to incur, such losses to persons and/or property that the entire society is affected and extraordinary resources and skills are required, some of which must come from other nations.
and
“A catastrophic disaster is one that so overwhelms response agencies that local, state, and federal resources combined are insufficient to meet the needs of the affected public.”
and
“A catastrophic disaster is one that so overwhelms response agencies that local, state, and federal resources combined are insufficient to meet the needs of the affected public.”

There are also 6+ pages defining "disaster".

I like the idea of naming it if it's really freakin' huge.

One thing is for sure, if you're discussing the proper terminology to call a really freakin' huge disaster, then you're definitely not in one.