Everyone has been saddened by the recent earthquake in Haiti and the subsequent loss of life. Earthquakes are extremely unpredictable and there is not much in the way of a warning as the data for the last week indicates. If one adds up the energy released by all the earthquakes worldwide during an interval of time and converts this sum into a magnitude one gets a plot similar to the one below. Time is given in days from 00:00 hours on 1 Jan 2010 UTC. The highest dot indicates the time at which the Haiti earthquake occurred. The horizontal gap in the data at about 2.0 seems to be due to a gap in the magnitude of earthquakes at about M4.0. Seismic activity seems to switch off and on. It may be that the 4s are blocked with any stress just being absorbed or they are promoted by a rupture to a higher magnitude. It acts like stress is accumulated and the faults suddenly break down. The crackling of an electric discharge caused by the breakdown of air seems to be just as volatile. Compare for example the energy released by bolts of lightning during a thunder storm.
One can smooth the data by averaging over time to give a plot similar to the following. The boxy nature of the plot is due to major events contributing to the averaging time of 1/5th of a day. The blue dots are the major earthquakes that occurred in the last week and give an impression of what the peak magnitudes are. The average itself seems to be unstable.