Natural occurrence of a nuclear fission chain reaction, similar to the ones occurring in nuclear weaponry, is extremely unlikely. It is actually very difficult to get fission material to explode, much more difficult than most schematics you can find on internet would suggest :)
Natural fission reactors do exist, though the power output (around 100kW as stated on the wiki) is pretty insignificant compared to even a small Earthquake. And the conditions under which these reactors will happen are pretty uncommon already (there's only one place in the world known to have had natural reactors).
Regarding an Earthquake hitting nuclear waste -- it's waste for a good reason: there is not much energy left in it. Certainly not compared to the vast amounts of energy associated with an Earthquake. What you need to worry about is not so much the energy release, but the breach of the containers, spilling and spread, etc. of the waste due to the Earthquake. Although not suited for energy production, it can be pretty unhealthy to touch, be near, inhale, etc. the bare unprotected stuff.
Here's a nice populist chart to put things in perspective. Not the most reliable source, so do your homework before basing life-or-death decisions on this chart :)