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.