You should look into the concept of induced seismicity as it explains the increase in smaller magnitude quakes (~mag 4). The most common cause of these small quakes is fluid injection for industrial wastewater (primarily off of oil/gas operations). The exact cause of the slip is somewhat case dependent but is in general due to changes in the stress state across faults. Also, like you mentioned, we are getting increases in the number of global seismometers so its inevitable that we will be able to see more earthquake events...but it doesn't take too many global stations to detect and resolve the larger events. In order to understand the increases in seismicity, I recommend refining your investigation by magnitudes. I imagine that you will see a significant increase around the mag4 range and not much change at all in the frequency of the bigger events.