Well, hold on...
This is Suggestions, where plausability takes a back seat to awesome.
Now, my basic argument for natural disasters is not that we need something rolling over the land, destroying everything; no, we need them as a motivation for co-operation and challenge.
Consider floods; let's say your town is on a riverbank for the trade advantages and access to water. However, rain clouds appear and it is clear the waters will rise. The whole town works together, shoveling earth to make waterbreaks and other defenses, until the town is safe, and in the aftermath, finds washed-up timber and good soil for farming. Or, the town splinters and no one tries to protect it, and thus, it's washed away.
The Flood is not a random instant-loose condition; it is a reward for co-operating and a penalty for failing to do so.
A less severe example is the lightning; it can harm and even kill Hearthlings, and may damage possessions- but only those possessions that are going to decay away anyway, and only those Hearthlings that don't seek cover. Imagine that you are exploring and get caught in a storm. You might run into a house for safety, and talk to the locals, whom you might not have met otherwise. Perhaps they'll be friendly- and of course, perhaps it might be safer in the storm- but the "disaster" drove interaction between the players, which is what an MMO is all about, right?
The Earthquakes are an even more apt example of how these disasters could cause interesting actions. Imagine if everyone on the multigrid gets knocked down by an earthquake- which means that somewhere in the grid, a new mine has appeared!
Quite possibly, earthquakes could cause cave-ins in mines, that might get teams of miners "trapped" and unable to travel anywhere, even home, until they've reached the surface.
However, this might also expose rich veins of ore otherwise hidden!