Assuming you're not just yanking my chain, using the phrase logarithmically in this context is supposed to convey that it isn't a straight line progression over time. The further in time you go from the event the MUCH further out the odds are of something happening. In this case you have a pretty good chance of a bigger quake within a few hours, but that dwindles to almost nothing over a short period of time. A linear progression would be much more straight forward, where the likelyhood goes down, say, by half in an hour, then another half of that in another hour and so on.