It would be interesting to see the time series on the number of deployed seismometers from 1900 to the present.
Earthquakes can be sensed from basically anywhere, only the time it takes the P/S waves to travel to you are different, especially if the quake is M>5.
More sensors don’t really add more detections, but are useful for pinpointing an origin.
If you don’t have enough sensors how do you distinguish between M<5 far away and M<5 in proximity?
The timing it takes for the P/S waves to reach you. One travels through the center, and the other travels along the surface of the earth.