Actually, they didn't need to use a handful instruments spread across the continent. Oklahoma has had its own seismographic instrument network since the mid 1970's. And a significant rise in the number of magnitude 3 and higher has been show to occur in and around Oklahoma City, not exactly the middle of nowhere. Since the activation of Oklahoma's network of instruments well before the period, pinpointing the magnitude and location of earthquakes has no dependency on damage reports or reports of people who felt the quakes. And it didn't depend on the media either. As stated, seismology had a century of development, and 3.0 and higher magnitude events were easy to detect and locate during the 80's, especially since the state operated their own network of seismology instruments. After all, we're talking about the 1980's, not the 1880's.