What is probabilistic forecasting?

Probabilistic forecasting involves the computation of a spatial probability density function together with a temporal probability, leading to a  conditional probability.  The latter is the probability that an earthquake of a specified magnitude will occur, conditioned on the observation that no earthquake has occurred in the recent past.  An example of a forecast probability statement might be that "there is a 40% probability that an earthquake having a magnitude between 6.5 and 7.0 will occur within a 20 km radius around location X  during the next 3 months."