The data used for this report is extracted from CNSS Earthquake Catalog using the following search criteria for surface quakes:
As you see, I excluded earthquakes Richter 1-3. I did this, because we've heard many comments over past years that earthquakes are not on the increase, and that more earthquakes are detected simply because there are more and more seismic stations installed, which detect more weak earthquakes. By doing this, I think, I avoided this comment. Since the data is from CNSS catalog, which is a union of report from more seismic stations around the world, the Richter 3 earthquake seemes to be a lower limit of what was detectable in 1970 and not missed because of a few seismic stations less than today.
Offered by Kiko.