Earthquake prediction

Source: Today, 2/7/8, p.12

Headline: China quake unusual: Scientists

Quote1:
WASHINGTON: A team from the Massachusetts Institute of Technology (MIT) had studied the region around Sichuan province, where the 7.9 magnitude quake hit on May 12, for more than two decades but found nothing to warn them of a major quake. ... "This earthquake was quite unusual."

Comment1:
This is a tragic real-life reminder not to commit an Argumentum ad Ignorantiam (appeal to ignorance):

Reason: (no evidence of quake potential)
Conclusion: (no quake potential)

Or, more generally:

Reason: (no proof of P) [where P is a variable, standing for anything at all]
Conclusion: Therefore, (not-P)

That said, nor should we argue:

Reason: (no proof of P)
Conclusion: Therefore, P.

That would be to believe in something for no reason at all.

Quote2:
However, the scientists warned that because earthquakes can sometimes occur in clusters, there was a possibility of another large-scale tremor in the area.

Comment2:
How often is "sometimes"? How large is the "possibility"? Beware of the word "possibility" being used as a euphemism for "don't know".