There are two populations in the data

The data in the earthquake plots does indeed contain two populations. M2.5+ for the US and M4.5 + for the rest of the world. There seems to have been a lull, a period with fewer earthquakes, at the end of 2009 and in the beginning of 2010. I pieced together some data for different sources the plot looks a little strange. The earthquakes below M 4.0 occurred in the US while those above this magnitude are from mainly outside the US.

I did a fit for the histogram based on the data above for the period from the beginning of 2009 to the present and when logs of the counts are used the curve is fairly linear. The fit yields an empirical equation for the annual number of earthquakes in M 0.25 intervals.

The numbers can be seen in the table below. The first two columns specify the width of the interval, the third column is the expected number of earthquakes in the interval per year and the last column is the expected number of days between earthquakes in the given interval. There are very few M 7+ earthquakes in any given year.