Geologist didn't have many sensors for locating earthquakes, but then several governments became very interested in tracking any nuclear testing and those sames

Geologist didn't have many sensors for locating earthquakes, but then several governments became very interested in tracking any nuclear testing and those sames sensors are quite good at tracking earthquakes. So much so that earthquakes can now be tracked in 3D anywhere on the planet, even enough to identify the shape and thickness of the various continental plates.